informative essay about respiratory system

The Respiratory System Essay

The respiratory system is the process responsible for the transportation and exchange of gases into and out of the human body. As we breath in, oxygen in the air containing oxygen is drawn into the lungs through a series of air pipes known as the airway and into the lungs. As air is drawn into the lungs and waste gas excreted, it passes through the airway, first through the mouth or nose and through the pharynx, larynx and windpipe – also known as the trachea. At this point it then enters the lungs through the bronchi before finally reaching the air sacs known as alveoli. Within the lungs, through a process known as diffusion, the oxygen is transferred to the blood stream through the alveoli (air ducts) where it is then transported inside …show more content…

Within the alveoli, the oxygen is transferred to the blood whilst simultaneously collecting waste carbon dioxide for excretion as we breath out. This transference is known as diffusion and is linked to the cardiovascular system. Diffusion is an automated process by where the levels of oxygen, water and carbon dioxide pass over a ‘semi-permeable membrane’ between the walls of the cells and blood vessels to create a level environment. This membrane only allows these three elements to pass whilst retaining other elements such as blood cells, hence semi-permeable. The high concentration on one side of the cells transfers through this membrane until the level is equal on both sides. The human body comprises of two respiratory sponges called lungs. The left lung is slightly smaller than the right as it makes room for placement of the heart . Due to this slightly smaller size, it only contains two lobes whereas the right lung has 3 lobes. Both lungs are host to the network of air sacs or alveoli which transport the air from the outside environment to the external and internal respiration processes. As we breathe in, the muscles in the chest wall force the thoracic area, ribs and connective muscles to contract and expand the chest. The diaphragm is contracted and moves down as the area inside the chest increases as air enters the lungs. The lungs are forced open by this expansion and the pressure inside the lungs becomes enough that it pulls

Complicated Respiratory System

The way the Respiratory System works is complicated . It also is surprisingly fast in what it does. First the Diaphragm moves downwards which causes the lungs to expand creating a vacuum for air . Air enters the mouth or nose and goes through the Pharynx and into the Larynx to the vocal cords .The air then goes down the Trachea and into two Bronchi which feed into the lungs .Then air reaches tiny little itty bitty sacs called Alveoli .Which pass oxygen into the bloodstream.When the Diaphragm moves upwards air moves out the lungs up the Trachea through the Pharynx and out the mouth or

Unit 3 Mammals

Air is entered through the mouth and then travels down to the lungs. As gases move between the air within the alveoli and the bloodstream within the capillaries, they must cross a thin barrier of alveolar cells and one layer of capillary cells. As oxygen within the alveolar air is of higher concentration than the oxygen within the bloodstream, oxygen

Emphysema Essay

The main system effected by Emphysema is the respiratory system. Positioned in the left and right sides of the thorax is the lungs which are protected by a membrane which gives the lungs huge elasticity to grow when they gather with air. When looking at the lungs microscopically, the lobes of the lungs are compiled of hundred of little alveoli sacs where the exchanging of oxygen and carbon dioxide happen (Shah, 2010). These alveoli can be located at the closing of the bronchioles and are encompassed by vessels where blood travels. Bronchioles are two tubules that transport oxygen into the lungs and are lined with microscopic cilia hairs.

Alveoli Research Paper

Air is inhaled from the mouth and/or nose and goes through the trachea. The trachea is then split into the bronchial tubes. The bronchial tubes travel through the lungs and eventually split into tiny tubes called the bronchioles, at the end of a bronchiole, there is a sac called the alveoli. The alveoli is surrounded by blood vessels called the capillaries. The initially inhaled air passes through the alveoli walls and into the blood.

Personal Narrative: My Journey Through The Respiratory System

I am an oxygen molecule and will be going on a journey through the respiratory system. I begin my journey in the nose. In the nose I am being inhaled and prickled by tiny little hairs that filter me. I am also being moistened by mucus, green/yellow slimy mucus. Now I am sliding through the adams apple or voice box, into the trachea and the pharynx. Leading me into the bronchi tubes I am being seperated into the tree like branches called brochioles in the lungs. As the tubes get smaller and smaller I make my way down to the alveoli, which are smaller air sacs that fill with air when you breathe. In the alveoli there are tiny blood vessles called capillaries. The walls of the alveoli and capillaries are very thin. Finally by the alveoli

Body Systems In Relation To Energy Metabolism Essay

The diaphragm separates the chest and the abdomen as well as this it has a large role in breathing. The diaphragm moves down when we breathe in which expands the chest cavity making room for air to enter through the nasal cavity or mouth. When we breathe out the diaphragm moves upwards, forcing the chest cavity to reduce in size and pushing the gases in the lungs up and out of either the nose or mouth.

Explain The Structure Of The Respiratory System

This refers to the process of Oxygen and Carbon Dioxide moving between the lungs and blood. Diffusion occurs when molecules move from an area of high concentration to an area of low concentration. This occurs during gaseous exchange as the blood in the capillaries surrounding the alveoli has a lower oxygen concentration of Oxygen than the air in the alveoli which has just been inhaled. Both alveoli and capillaries have walls which are only one cell thick and allow gases to diffuse across them. The same happens with Carbon Dioxide. The blood in the surrounding capillaries has a higher concentration of CO2 than the inspired air due to it being a waste product of energy production. Therefore CO2 diffuses the other way, from the capillaries, into the alveoli where it can then be

Explain The Blood That Travels To The Tissues And Organs In The Body

The blood that travels to the tissues and organs in the body have a high level of oxygen. Diffusion occurs between the capillaries and the body’s tissues and organs. Oxygen diffuses from the blood into the body’s cells and tissues, whereas carbon dioxide is diffused from the body’s tissues and organs into the blood where it then travels to the lungs. Here, it diffuses across the thin walls of the alveoli, where it is removed out of the body by exhalation.

Blood and Digestion

The lungs are sponges that fill with air, and fill every inch the surrounding thoracic cavity. The lungs contain lobes. The lobes divide each lung, and the right lung being bigger has more lobe divisions. The pulmonary veins enter here and exit here as well. Inside the lungs are clusters of air sacks that are called alveoli. The alveoli are part of the bronchioles that enter each lung. The oxygen and carbon dioxide are exchanged in the lungs. The exchange happens from a small blood vassal nod the alveoli. When the oxygen exchange is completed the inhaled air moves into the blood. While the oxygen moves into the blood the carbon dioxide is exhaled by the lungs. Before the blood can be moved properly it has to be made into oxyhemoglobin.

Why Do Mammals Have A Closed Circulatory Gas Exchange

In general, mammals have a closed circulatory system allowing them to ventilate easily. Air is inhaled by mammals through their mouths when their diaphragm contracts before travelling down the trachea. Air enters the lungs through the trachea, which splits into two bronchi and then smaller bronchioles. All of these tubes are held open by rings of cartilage. It is on the bronchioles where alveoli are. Alveoli are the gas exchange surface in mammals. Alveoli provide a moist gas exchange surface in which oxygen diffuses from the water into the alveoli. The oxygen diffuses from the alveoli to the blood capillaries surrounding them before the blood gets transported to the many cells needed because of respiration. The blood supply is extensive, which means that oxygen is carried away to the cells as soon as it has diffused into the blood.

Reabsorbed Lab Report

When we breathe, oxygen is absorbed through the lungs into the blood stream. The heart pumps oxygenated blood around the body through blood vessels (arteries). This oxygenated blood travels to all the body’s tissues and releases the oxygen for the cells to function. Cells then expel waste such as carbon dioxide which is then reabsorbed by the blood. This deoxygenated blood is carried back to the heart via veins this is then pumped back into the lungs the waste products are then exhaled and the blood is oxygenated again to be reabsorbed, carried back to heart, to start the process

Case Study: Malignant Neoplasm Of The Right Lower Lobe

Gas exchange occurs between the air and blood in the lungs. Since the oxygen saturation of air is higher in the lungs than in the blood, oxygen diffuses from air to blood. Carbon dioxide moves from the blood to the air within the lungs by diffusing down its concentration gradient. As a result of this exchange, the inspired air contains more oxygen and less carbon dioxide than the expired air. The lungs provide necessary oxygen to the body, in addition to removing carbon dioxide. More importantly, blood leaving the lungs has a higher oxygen and lower carbon dioxide concentration than the blood delivered to the lungs in the pulmonary arteries. This is because the lungs function to bring the blood into gaseous equilibrium in the air. Gas exchange between the air and blood occurs entirely by diffusion through the lung tissue

Pig Reproductive System Research Paper

The Respiratory System: The respiratory system includes: the lungs, alveoli, nares, glottis, trachea, bronchi, larynx, the esophagus, and diaphragm. Oxygen is inhaled and carried throughout the body through all of these parts of the body and released using the same parts. The location of these parts are primarily between the throat and ribs with veins being used to carry oxygen throughout the body.

Your Lungs Are 2 Wipe Like Organs

Your lungs are 2 wipe like organs in your mid-section. Your right lung has 3 segments, called flaps. Your left lung has 2 flaps. The left lung is littler in light of the fact that the heart takes up additional room on that side of the body. When you take in, air enters through your mouth or nose and goes into your lungs through the trachea (windpipe). The trachea partitions into tubes called bronchi (solitary, bronchus), which enter the lungs and gap into littler bronchi. These gaps to frame littler branches called bronchioles. Toward the end of the bronchioles are small air sacs known as alveoli. The alveoli retain oxygen from the breathed in air into your blood and expel carbon dioxide from the blood. This is removed from the body when

The Respiratory System

The respiratory system consists of different structures that all function together to deliver oxygen to the lungs and expel carbon dioxide from the body. The respiratory system consists of structures such as the nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, alveoli and the lungs. The primary function of the respiratory system is gas exchange but other functions also include sound production, assistance in abdominal compression, and coughing and sneezing. The lungs are the main organ of the respiratory system. The function of the lungs receiving the oxygen from the air we breathe and delivering the oxygen to the red blood cells contained in the blood. Red blood cells carry oxygen around the body to make sure the whole body is

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The Purpose and Function of The Respiratory System

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High school biology

Course: high school biology   >   unit 8.

• Meet the heart!
• Circulatory system and the heart
• The circulatory system review
• Meet the lungs!
• The lungs and pulmonary system

The respiratory system review

• The circulatory and respiratory systems

The respiratory system

Common mistakes and misconceptions.

• Incorrect : Physiological respiration and cellular respiration are the same thing.
• Correct : People sometimes use the word "respiration" to refer to the process of cellular respiration, which is a cellular process in which carbohydrates are used to generate usable energy. Physiological respiration and cellular respiration are related processes, but they are not the same.
• Incorrect : We breathe in only oxygen and breathe out only carbon dioxide.
• Correct : Often the terms "oxygen" and "air" are used interchangeably. It is true that the air we breathe in has more oxygen than the air we breathe out, and the air we breathe out has more carbon dioxide than the air that we breathe in. However, oxygen is just one of the gases found in the air we breathe. (In fact, the air has more nitrogen than oxygen!)
• Incorrect : The respiratory system works alone in transporting oxygen through the body.
• Correct : The respiratory system works directly with the circulatory system to provide oxygen to the body. Oxygen taken in from the respiratory system moves into blood vessels that then circulate oxygen-rich blood to tissues and cells.

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8.1: Introduction to the Respiratory System

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Learning Objectives

By the end of this section, you will be able to:

• List the structures of the respiratory system
• List the major functions of the respiratory system
• Outline the forces that allow for air movement into and out of the lungs
• Outline the process of gas exchange
• Summarize the process of oxygen and carbon dioxide transport within the respiratory system
• Create a flow chart illustrating how respiration is controlled
• Discuss how the respiratory system responds to exercise
• Describe the development of the respiratory system in the embryo

Hold your breath. Really! See how long you can hold your breath as you continue reading. . . . How long can you do it? Chances are you are feeling uncomfortable already. A typical human cannot survive without breathing for more than 3 minutes, and even if you wanted to hold your breath longer, your autonomic nervous system would take control. This is because every cell in the body needs to run the oxidative stages of cellular respiration, the process by which energy is produced in the form of adenosine triphosphate (ATP). For oxidative phosphorylation to occur, oxygen is used as a reactant and carbon dioxide is released as a waste product.

You may be surprised to learn that although oxygen is a critical need for cells, it is actually the accumulation of carbon dioxide that primarily drives your need to breathe. Carbon dioxide is exhaled and oxygen is inhaled through the respiratory system, which includes muscles to move air into and out of the lungs, passageways through which air moves, and microscopic gas exchange surfaces covered by capillaries. The circulatory system transports gases from the lungs to tissues throughout the body and vice versa. A variety of diseases can affect the respiratory system, such as asthma, emphysema, chronic obstruction pulmonary disorder (COPD), and lung cancer. All of these conditions affect the gas exchange process and result in labored breathing and other difficulties.

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• Anatomy & Physiology. Provided by : OpenStax CNX. Located at : http://cnx.org/contents/[email protected] . License : CC BY: Attribution . License Terms : Download for free at http://cnx.org/contents/[email protected]

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14.2: Organs and Structures of the Respiratory System

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Learning Objectives

• List the structures that make up the respiratory system
• Describe how the respiratory system processes oxygen and CO 2
• Compare and contrast the functions of upper respiratory tract with the lower respiratory tract

The major organs of the respiratory system function primarily to provide oxygen to body tissues for cellular respiration, remove the waste product carbon dioxide, and help to maintain acid-base balance. Portions of the respiratory system are also used for non-vital functions, such as sensing odors, speech production, and for straining, such as during childbirth or coughing (Figure \(\PageIndex{1}\)).

Functionally, the respiratory system can be divided into a conducting zone and a respiratory zone. The conducting zone of the respiratory system includes the organs and structures not directly involved in gas exchange. The gas exchange occurs in the respiratory zone .

Conducting Zone

The major functions of the conducting zone are to provide a route for incoming and outgoing air, remove debris and pathogens from the incoming air, and warm and humidify the incoming air. Several structures within the conducting zone perform other functions as well. The epithelium of the nasal passages, for example, is essential to sensing odors, and the bronchial epithelium that lines the lungs can metabolize some airborne carcinogens.

The Nose and Its Adjacent Structures

The major entrance and exit for the respiratory system is through the nose. When discussing the nose, it is helpful to divide it into two major sections: the external nose, and the nasal cavity or internal nose.

The external nose consists of the surface and skeletal structures that result in the outward appearance of the nose and contribute to its numerous functions (Figure \(\PageIndex{2}\)). The root is the region of the nose located between the eyebrows. The bridge is the part of the nose that connects the root to the rest of the nose. The dorsum nasi is the length of the nose. The apex is the tip of the nose. On either side of the apex, the nostrils are formed by the alae (singular = ala). An ala is a cartilaginous structure that forms the lateral side of each naris (plural = nares), or nostril opening. The philtrum is the concave surface that connects the apex of the nose to the upper lip.

Underneath the thin skin of the nose are its skeletal features (see Figure \(\PageIndex{2}\), lower illustration). While the root and bridge of the nose consist of bone, the protruding portion of the nose is composed of cartilage. As a result, when looking at a skull, the nose is missing. The nasal bone is one of a pair of bones that lies under the root and bridge of the nose. The nasal bone articulates superiorly with the frontal bone and laterally with the maxillary bones. Septal cartilage is flexible hyaline cartilage connected to the nasal bone, forming the dorsum nasi. The alar cartilage consists of the apex of the nose; it surrounds the naris.

The nares open into the nasal cavity, which is separated into left and right sections by the nasal septum (Figure \(\PageIndex{3}\)). The nasal septum is formed anteriorly by a portion of the septal cartilage (the flexible portion you can touch with your fingers) and posteriorly by the perpendicular plate of the ethmoid bone (a cranial bone located just posterior to the nasal bones) and the thin vomer bones (whose name refers to its plough shape). Each lateral wall of the nasal cavity has three bony projections, called the superior, middle, and inferior nasal conchae. The inferior conchae are separate bones, whereas the superior and middle conchae are portions of the ethmoid bone. Conchae serve to increase the surface area of the nasal cavity and to disrupt the flow of air as it enters the nose, causing air to bounce along the epithelium, where it is cleaned and warmed. The conchae and meatuses also conserve water and prevent dehydration of the nasal epithelium by trapping water during exhalation. The floor of the nasal cavity is composed of the palate. The hard palate at the anterior region of the nasal cavity is composed of bone. The soft palate at the posterior portion of the nasal cavity consists of muscle tissue. Air exits the nasal cavities via the internal nares and moves into the pharynx.

Several bones that help form the walls of the nasal cavity have air-containing spaces called the paranasal sinuses, which serve to warm and humidify incoming air. Sinuses are lined with a mucosa. Each paranasal sinus is named for its associated bone: frontal sinus, maxillary sinus, sphenoidal sinus, and ethmoidal sinus. The sinuses produce mucus and lighten the weight of the skull.

The nares and anterior portion of the nasal cavities are lined with mucous membranes, containing sebaceous glands and hair follicles that serve to prevent the passage of large debris, such as dirt, through the nasal cavity. An olfactory epithelium used to detect odors is found deeper in the nasal cavity.

The conchae, meatuses, and paranasal sinuses are lined by respiratory epithelium composed of pseudostratified ciliated columnar epithelium (Figure \(\PageIndex{4}\)). The epithelium contains goblet cells, one of the specialized, columnar epithelial cells that produce mucus to trap debris. The cilia of the respiratory epithelium help remove the mucus and debris from the nasal cavity with a constant beating motion, sweeping materials towards the throat to be swallowed. Interestingly, cold air slows the movement of the cilia, resulting in accumulation of mucus that may in turn lead to a runny nose during cold weather. This moist epithelium functions to warm and humidify incoming air. Capillaries located just beneath the nasal epithelium warm the air by convection. Serous and mucus-producing cells also secrete the lysozyme enzyme and proteins called defensins, which have antibacterial properties. Immune cells that patrol the connective tissue deep to the respiratory epithelium provide additional protection.

The pharynx is a tube formed by skeletal muscle and lined by mucous membrane that is continuous with that of the nasal cavities (see Figure \(\PageIndex{3}\)). The pharynx is divided into three major regions: the nasopharynx, the oropharynx, and the laryngopharynx (Figure \(\PageIndex{5}\)).

The nasopharynx is flanked by the conchae of the nasal cavity, and it serves only as an airway. At the top of the nasopharynx are the pharyngeal tonsils. A pharyngeal tonsil , also called an adenoid, is an aggregate of lymphoid reticular tissue similar to a lymph node that lies at the superior portion of the nasopharynx. The function of the pharyngeal tonsil is not well understood, but it contains a rich supply of lymphocytes and is covered with ciliated epithelium that traps and destroys invading pathogens that enter during inhalation. The pharyngeal tonsils are large in children, but interestingly, tend to regress with age and may even disappear. The uvula is a small bulbous, teardrop-shaped structure located at the apex of the soft palate. Both the uvula and soft palate move like a pendulum during swallowing, swinging upward to close off the nasopharynx to prevent ingested materials from entering the nasal cavity. In addition, auditory (Eustachian) tubes that connect to each middle ear cavity open into the nasopharynx. This connection is why colds often lead to ear infections.

The oropharynx is a passageway for both air and food. The oropharynx is bordered superiorly by the nasopharynx and anteriorly by the oral cavity. The fauces is the opening at the connection between the oral cavity and the oropharynx. As the nasopharynx becomes the oropharynx, the epithelium changes from pseudostratified ciliated columnar epithelium to stratified squamous epithelium. The oropharynx contains two distinct sets of tonsils, the palatine and lingual tonsils. A palatine tonsil is one of a pair of structures located laterally in the oropharynx in the area of the fauces. The lingual tonsil is located at the base of the tongue. Similar to the pharyngeal tonsil, the palatine and lingual tonsils are composed of lymphoid tissue, and trap and destroy pathogens entering the body through the oral or nasal cavities.

The laryngopharynx is inferior to the oropharynx and posterior to the larynx. It continues the route for ingested material and air until its inferior end, where the digestive and respiratory systems diverge. The stratified squamous epithelium of the oropharynx is continuous with the laryngopharynx. Anteriorly, the laryngopharynx opens into the larynx, whereas posteriorly, it enters the esophagus.

The larynx is a cartilaginous structure inferior to the laryngopharynx that connects the pharynx to the trachea and helps regulate the volume of air that enters and leaves the lungs (Figure \(\PageIndex{6}\)). The structure of the larynx is formed by several pieces of cartilage. Three large cartilage pieces—the thyroid cartilage (anterior), epiglottis (superior), and cricoid cartilage (inferior)—form the major structure of the larynx. The thyroid cartilage is the largest piece of cartilage that makes up the larynx. The thyroid cartilage consists of the laryngeal prominence , or “Adam’s apple,” which tends to be more prominent in males. The thick cricoid cartilage forms a ring, with a wide posterior region and a thinner anterior region. Three smaller, paired cartilages—the arytenoids, corniculates, and cuneiforms—attach to the epiglottis and the vocal cords and muscle that help move the vocal cords to produce speech.

The epiglottis , attached to the thyroid cartilage, is a very flexible piece of elastic cartilage that covers the opening of the trachea (see Figure \(\PageIndex{3}\)). When in the “closed” position, the unattached end of the epiglottis rests on the glottis. The glottis is composed of the vestibular folds, the true vocal cords, and the space between these folds (Figure \(\PageIndex{7}\)). A vestibular fold , or false vocal cord, is one of a pair of folded sections of mucous membrane. A true vocal cord is one of the white, membranous folds attached by muscle to the thyroid and arytenoid cartilages of the larynx on their outer edges. The inner edges of the true vocal cords are free, allowing oscillation to produce sound. The size of the membranous folds of the true vocal cords differs between individuals, producing voices with different pitch ranges. Folds in males tend to be larger than those in females, which create a deeper voice. The act of swallowing causes the pharynx and larynx to lift upward, allowing the pharynx to expand and the epiglottis of the larynx to swing downward, closing the opening to the trachea. These movements produce a larger area for food to pass through, while preventing food and beverages from entering the trachea.

Continuous with the laryngopharynx, the superior portion of the larynx is lined with stratified squamous epithelium, transitioning into pseudostratified ciliated columnar epithelium that contains goblet cells. Similar to the nasal cavity and nasopharynx, this specialized epithelium produces mucus to trap debris and pathogens as they enter the trachea. The cilia beat the mucus upward towards the laryngopharynx, where it can be swallowed down the esophagus.

The trachea (windpipe) extends from the larynx toward the lungs (Figure \(\PageIndex{8}\).a). The trachea is formed by 16 to 20 stacked, C-shaped pieces of hyaline cartilage that are connected by dense connective tissue. The trachealis muscle and elastic connective tissue together form the fibroelastic membrane , a flexible membrane that closes the posterior surface of the trachea, connecting the C-shaped cartilages. The fibroelastic membrane allows the trachea to stretch and expand slightly during inhalation and exhalation, whereas the rings of cartilage provide structural support and prevent the trachea from collapsing. In addition, the trachealis muscle can be contracted to force air through the trachea during exhalation. The trachea is lined with pseudostratified ciliated columnar epithelium, which is continuous with the larynx. The esophagus borders the trachea posteriorly.

Bronchial Tree

The trachea branches into the right and left primary bronchi at the carina. These bronchi are also lined by pseudostratified ciliated columnar epithelium containing mucus-producing goblet cells (Figure \(\PageIndex{8}\).b). The carina is a raised structure that contains specialized nervous tissue that induces violent coughing if a foreign body, such as food, is present. Rings of cartilage, similar to those of the trachea, support the structure of the bronchi and prevent their collapse. The primary bronchi enter the lungs at the hilum, a concave region where blood vessels, lymphatic vessels, and nerves also enter the lungs. The bronchi continue to branch into bronchial a tree. A bronchial tree (or respiratory tree) is the collective term used for these multiple-branched bronchi. The main function of the bronchi, like other conducting zone structures, is to provide a passageway for air to move into and out of each lung. In addition, the mucous membrane traps debris and pathogens.

A bronchiole branches from the tertiary bronchi. Bronchioles, which are about 1 mm in diameter, further branch until they become the tiny terminal bronchioles, which lead to the structures of gas exchange. There are more than 1000 terminal bronchioles in each lung. The muscular walls of the bronchioles do not contain cartilage like those of the bronchi. This muscular wall can change the size of the tubing to increase or decrease airflow through the tube.

Respiratory Zone

In contrast to the conducting zone, the respiratory zone includes structures that are directly involved in gas exchange. The respiratory zone begins where the terminal bronchioles join a respiratory bronchiole , the smallest type of bronchiole (Figure \(\PageIndex{9}\)), which then leads to an alveolar duct, opening into a cluster of alveoli.

An alveolar duct is a tube composed of smooth muscle and connective tissue, which opens into a cluster of alveoli. An alveolus is one of the many small, grape-like sacs that are attached to the alveolar ducts.

An alveolar sac is a cluster of many individual alveoli that are responsible for gas exchange. An alveolus is approximately 200 μm in diameter with elastic walls that allow the alveolus to stretch during air intake, which greatly increases the surface area available for gas exchange. Alveoli are connected to their neighbors by alveolar pores , which help maintain equal air pressure throughout the alveoli and lung (Figure \(\PageIndex{10}\)).

The alveolar wall consists of three major cell types: type I alveolar cells, type II alveolar cells, and alveolar macrophages. A type I alveolar cell is a squamous epithelial cell of the alveoli, which constitute up to 97 percent of the alveolar surface area. These cells are about 25 nm thick and are highly permeable to gases. A type II alveolar cell is interspersed among the type I cells and secretes pulmonary surfactant , a substance composed of phospholipids and proteins that reduces the surface tension of the alveoli. Roaming around the alveolar wall is the alveolar macrophage , a phagocytic cell of the immune system that removes debris and pathogens that have reached the alveoli.

The simple squamous epithelium formed by type I alveolar cells is attached to a thin, elastic basement membrane. This epithelium is extremely thin and borders the endothelial membrane of capillaries. Taken together, the alveoli and capillary membranes form a respiratory membrane that is approximately 0.5 mm thick. The respiratory membrane allows gases to cross by simple diffusion, allowing oxygen to be picked up by the blood for transport and CO 2 to be released into the air of the alveoli.

Diseases of the Respiratory System: Asthma

Asthma is common condition that affects the lungs in both adults and children. Approximately 8.2 percent of adults (18.7 million) and 9.4 percent of children (7 million) in the United States suffer from asthma. In addition, asthma is the most frequent cause of hospitalization in children.

Asthma is a chronic disease characterized by inflammation and edema of the airway, and bronchospasms (that is, constriction of the bronchioles), which can inhibit air from entering the lungs. In addition, excessive mucus secretion can occur, which further contributes to airway occlusion (Figure \(\PageIndex{11}\)). Cells of the immune system, such as eosinophils and mononuclear cells, may also be involved in infiltrating the walls of the bronchi and bronchioles.

Bronchospasms occur periodically and lead to an “asthma attack.” An attack may be triggered by environmental factors such as dust, pollen, pet hair, or dander, changes in the weather, mold, tobacco smoke, and respiratory infections, or by exercise and stress.

Symptoms of an asthma attack involve coughing, shortness of breath, wheezing, and tightness of the chest. Symptoms of a severe asthma attack that requires immediate medical attention would include difficulty breathing that results in blue (cyanotic) lips or face, confusion, drowsiness, a rapid pulse, sweating, and severe anxiety. The severity of the condition, frequency of attacks, and identified triggers influence the type of medication that an individual may require. Longer-term treatments are used for those with more severe asthma. Short-term, fast-acting drugs that are used to treat an asthma attack are typically administered via an inhaler. For young children or individuals who have difficulty using an inhaler, asthma medications can be administered via a nebulizer.

In many cases, the underlying cause of the condition is unknown. However, recent research has demonstrated that certain viruses, such as human rhinovirus C (HRVC), and the bacteria Mycoplasma pneumoniae and Chlamydia pneumoniae that are contracted in infancy or early childhood, may contribute to the development of many cases of asthma.

Chapter Review

The respiratory system is responsible for obtaining oxygen and getting rid of carbon dioxide, and aiding in speech production and in sensing odors. From a functional perspective, the respiratory system can be divided into two major areas: the conducting zone and the respiratory zone. The conducting zone consists of all of the structures that provide passageways for air to travel into and out of the lungs: the nasal cavity, pharynx, trachea, bronchi, and most bronchioles. The nasal passages contain the conchae and meatuses that expand the surface area of the cavity, which helps to warm and humidify incoming air, while removing debris and pathogens. The pharynx is composed of three major sections: the nasopharynx, which is continuous with the nasal cavity; the oropharynx, which borders the nasopharynx and the oral cavity; and the laryngopharynx, which borders the oropharynx, trachea, and esophagus. The respiratory zone includes the structures of the lung that are directly involved in gas exchange: the terminal bronchioles and alveoli.

The lining of the conducting zone is composed mostly of pseudostratified ciliated columnar epithelium with goblet cells. The mucus traps pathogens and debris, whereas beating cilia move the mucus superiorly toward the throat, where it is swallowed. As the bronchioles become smaller and smaller, and nearer the alveoli, the epithelium thins and is simple squamous epithelium in the alveoli. The endothelium of the surrounding capillaries, together with the alveolar epithelium, forms the respiratory membrane. This is a blood-air barrier through which gas exchange occurs by simple diffusion.

Essay Questions

Q. What happens during an asthma attack. What are the three changes that occur inside the airways during an asthma attack?

A. Inflammation and the production of a thick mucus; constriction of the airway muscles, or bronchospasm; and an increased sensitivity to allergens.

Review Questions

Q. Which of the following anatomical structures is not part of the conducting zone?

B. nasal cavity

Q. What is the function of the conchae in the nasal cavity?

A. increase surface area

B. exchange gases

C. maintain surface tension

D. maintain air pressure

Q. The fauces connects which of the following structures to the oropharynx?

A. nasopharynx

B. laryngopharynx

C. nasal cavity

D. oral cavity

Q. Which of the following are structural features of the trachea?

A. C-shaped cartilage

B. smooth muscle fibers

D. all of the above

Q. Which of the following structures is not part of the bronchial tree?

C. terminal bronchioles

D. respiratory bronchioles

Q. What is the role of alveolar macrophages?

A. to secrete pulmonary surfactant

B. to secrete antimicrobial proteins

C. to remove pathogens and debris

D. to facilitate gas exchange

Critical Thinking Questions

Q. Describe the three regions of the pharynx and their functions.

A. The pharynx has three major regions. The first region is the nasopharynx, which is connected to the posterior nasal cavity and functions as an airway. The second region is the oropharynx, which is continuous with the nasopharynx and is connected to the oral cavity at the fauces. The laryngopharynx is connected to the oropharynx and the esophagus and trachea. Both the oropharynx and laryngopharynx are passageways for air and food and drink.

Q. If a person sustains an injury to the epiglottis, what would be the physiological result?

A. The epiglottis is a region of the larynx that is important during the swallowing of food or drink. As a person swallows, the pharynx moves upward and the epiglottis closes over the trachea, preventing food or drink from entering the trachea. If a person’s epiglottis were injured, this mechanism would be impaired. As a result, the person may have problems with food or drink entering the trachea, and possibly, the lungs. Over time, this may cause infections such as pneumonia to set in.

Q. Compare and contrast the conducting and respiratory zones.

A. The conducting zone of the respiratory system includes the organs and structures that are not directly involved in gas exchange, but perform other duties such as providing a passageway for air, trapping and removing debris and pathogens, and warming and humidifying incoming air. Such structures include the nasal cavity, pharynx, larynx, trachea, and most of the bronchial tree. The respiratory zone includes all the organs and structures that are directly involved in gas exchange, including the respiratory bronchioles, alveolar ducts, and alveoli.

Bizzintino J, Lee WM, Laing IA, Vang F, Pappas T, Zhang G, Martin AC, Khoo SK, Cox DW, Geelhoed GC, et al. Association between human rhinovirus C and severity of acute asthma in children. Eur Respir J [Internet]. 2010 [cited 2013 Mar 22]; 37(5):1037–1042.

Kumar V, Ramzi S, Robbins SL. Robbins Basic Pathology. 7th ed. Philadelphia (PA): Elsevier Ltd; 2005.

Martin RJ, Kraft M, Chu HW, Berns, EA, Cassell GH. A link between chronic asthma and chronic infection. J Allergy Clin Immunol [Internet]. 2001 [cited 2013 Mar 22]; 107(4):595-601.

Contributors and Attributions

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STRUCTURE OF THE RESPIRATORY SYSTEM, RELATED TO FUNCTION

Chapter objectives.

After studying this chapter you should be able to:

• 1. Describe the structures of the upper airway which help it to protect the respiratory system against environmental agents of lung disease.
• 2. Distinguish between the structure of conducting and respiratory airways and relate these structures to the aetiology of restrictive and obstructive lung disease.
• 3. Outline the structure of the bronchial tree and how this is disrupted in disease.
• 4. Describe the histology of the regions of the lung and relate it to function and pathology.
• 5. Explain the special features of the pulmonary circulation and pulmonary hypertension.
• 6. Outline the afferent and efferent innervation of the lungs.
• 7. Describe the gross structure of the chest and thoracic viscera, the way they bring about breathing, and how this is disrupted by pneumothorax.
• 8. Explain the embryological origins of the respiratory system and congenital abnormalities that may arise.
• 9. List the metabolic and non-respiratory functions of the respiratory system.

Introduction

Just as each part of the respiratory system has its particular function, so each part has its particular pathologies. Respiratory structures are disrupted by disease, and the oft-repeated aphorism ‘structure is related to function’ is never more applicable than in the respiratory system in health and disease. Study of its structure considerably eases understanding of how the respiratory system works.

We will first describe the airways of the lung and then the tissues that surround them.

The upper airways

The neck is the part between the face and the trunk. The front part is of gristle and through it speech and respiration take place; it is known as the windpipe. Aristotle, Historia animolium. 4th century bc

The ‘gristle’ (cartilage) that Aristotle describes is important in preventing the collapse of the upper airways, which in turn is vital to lung function because although the gas exchange of respiration takes place deep within the lungs, those parts of the respiratory system outside the chest, which are referred to as the upper airways, allow and effect the process, and are of such clinical importance that they must be considered.

The structures of the upper airways are clearly seen in a paramedial sagittal section of the head and neck ( Fig. 2.1 ).

Paramedial MRI scan of head and neck. The mouth is closed and the subject is breathing through his nose.

Mouth and nose – rhinitis, the common cold and obstructive sleep apnoea

It is unlikely that any of our readers have escaped the unpleasant obstruction to breathing associated with the common cold. The major discomfort of this condition is the result of an inflammation of the nose (rhinitis) and, if more severe, the paranasal sinuses. In about 50% of cases this rhinosinusitis is initially caused by rhinoviruses, 25% by corona viruses and the remainder by other viruses. A transient vasoconstriction of the mucous membrane (see below) is followed by vasodilatation, oedema and mucus production. With secondary bacterial infection the secretions become viscid, contain pus cells and bacteria, and contribute to the obstruction of breathing.

Rhinosinusitis may also be allergic in aetiology or idiopathic (i.e. intrinsic, of no external cause). Idiopathic rhinitis is thought to be a result of an imbalance of the activity of the sympathetic and parasympathetic nerves serving the mucosal blood vessels, and in this type of rhinitis anticholinergic medication often relieves symptoms.

Allergic rhinitis may be seasonal in response to allergens such as pollen, or perennial, where a major cause is the allergen Der pl in the faeces of the house-dust mite Dermatophagoides pteronyssinus .

The mite is just invisible to the unaided eye and lives on shed skin scales, particularly in human bedding. The allergen from this creature is also responsible for much asthma, but the rhinitis it provokes demonstrates the filtering action of the upper airways in trapping it in the nose.

Much more sinister and life-threatening than rhinitis is obstructive sleep apnoea (OSA; apnoea = absence of breathing ). This should not be confused with central sleep apnoea, where the patient ceases to make respiratory efforts while they are sleeping. In OSA the patient's attempts to breathe are physically obstructed by anatomical and physiological peculiarities of the upper airways.

In Figure 2.1 the subject is breathing through his nose because the lips are closed and the tongue lies against the palate. When you breathe through the mouth – for example when you blow out a candle or suck through a straw – the soft palate is arched upward to form a seal against Passavant's ridge at the top of the pharynx. This form of airways obstruction is a normal function. Similarly, under normal circumstances, the genioglossus muscle of the tongue has a high resting tone in conscious subjects, and this holds the tongue forward, preventing it from obstructing the airway. During sleep, and particularly in those suffering from the dangerous condition of obstructive sleep apnoea, the tongue falls against the back wall of the pharynx and obstructs breathing. The muscle tone of the pharynx itself becomes reduced, particularly during REM (rapid eye movement) sleep and in OSA the pharynx collapses under the negative pressure of inspiration. Blocking of the airways by the tongue also and almost inevitably occurs during general anaesthesia and requires immediate attention from the anaesthetist.

Most, but not all, healthy persons breathe through the nose unless exercising. The resistance to breathing of the nose is about twice that of the mouth and nearly half the total resistance of the airways. The disadvantage of this is offset by the advantage obtained by the air-conditioning and filtering activities of the nose, which warm, moisten and filter the air before it comes in contact with the delicate respiratory regions of the lungs. Newborn babies have great difficulty breathing through their mouths: they are almost obligate nose breathers and become very distressed when their nose is blocked. Their predominantly nose breathing may be associated with their ability to suckle and breathe at the same time. On the other hand, many animal species, such as rabbits, manage to eat and breathe at the same time by having lateral food channels on either side of the larynx (see below) that bypass the airway. Marine mammals such as whales have completely separate air and food channels, with the airway ending at the back of the head.

In humans the nose extends from the nostrils (external nares) to the choanae (internal nares), which empty into the nasal part of the pharynx. Each nostril narrows to form its nasal valve, and at this level the total cross-sectional area of the airways is narrower (3 mm 2 ) than anywhere else in the system. This narrowing imposes the majority of the high resistance to airflow found in the nose (see Chapter 5) and, combined with the sharp turn the inspiratory air must make as it enters the wide (140 mm 2 ) lumen of the cavum of the nose, causes turbulence. The walls of the nasal cavum are rigid bone projecting out into the airway from the lateral walls as the turbinates . These have a large surface area (150 cm 2 ) covered by vascular mucosal erectile tissue important in the ‘air-conditioning’ activities of the nose. This mucosal tissue can swell considerably in conditions such as rhinitis (described above), and it is here that nasal decongestants such as oxymetazoline, an agonist of α adrenergic receptors on vascular smooth muscle, act to clear a blocked nose by causing the vascular smooth muscle to contract.

Structure of the respiratory system: 1

Obstructive sleep apnoea.

Mr Sinclair is 50 years old. He is rather overweight for his height: he is 168 cm tall but weighs 102 kg. He also drinks rather heavily and is a smoker.

For the past 2 years, Mrs Sinclair has slept in a different room from Mr Sinclair because of his very loud snoring and restlessness at night. Recently, Mr Sinclair has been feeling more and more tired during the day. For some time, he has been regularly falling asleep when he arrives home from work. Over the past month or so, he has found it increasingly difficult to concentrate at work and on one occasion recently, he was caught sleeping at his desk by his manager and he is facing disciplinary action. Mrs Sinclair eventually persuaded her husband to visit his doctor.

Mr Sinclair's doctor referred him to a specialist in sleep medicine. The doctor suggested that he may be suffering from obstructive sleep apnoea (OSA). He explained that during periods of deep sleep, Mr Sinclair's airway was becoming obstructed. During an episode of obstruction, Mr Sinclair's sleep becomes lighter until the obstruction is overcome. These episodes of obstruction and sleep interruption are responsible for Mr Sinclair's daytime sleepiness. The doctor went on to suggest that Mr Sinclair might be treated with a nasal continuous positive airway pressure device.

In this section we will consider:

• 1. What causes obstructive sleep apnoea?
• 2. What are the signs, symptoms and treatment of obstructive sleep apnoea?

Normal physiological swelling of the mucosa and consequent restriction of airflow takes place asymmetrically over a period of time, so that one nasal passage is more constricted than the other. Thus both nasal passages are not uniformly constricted, with the major constriction, and therefore airflow, alternating between nostrils over a period of hours. This oscillation of airflow may help to sustain the nose in its air-conditioning activities by allowing one channel to rest while the other carries out most of the work.

The major function of the upper airway is to air-condition the inspirate. It is not essential to breathe through the nose to do this, and the mouth will make a fairly good job of warming and humidifying inhaled air before it reaches the larynx. However, the mouth has not evolved for that purpose and the unpleasant consequences of using it are well known to anyone who has had to breathe through their mouth because a cold has obstructed their nasal airways.

The larynx – intubation of the airways

A common cause of accidental airway obstruction is the inhalation of food into the trachea. Normally, to prevent this during swallowing, the larynx, a box-like structure at the upper end of the trachea, is elevated (moved towards the head) by the muscles attached to it and the epiglottis folds backward, forming a very effective seal, like the ‘trapdoor’ over the entrance to the larynx. Because the ‘trapdoor’ can only open outwards, increased pressure in the pharynx makes the seal of the epiglottis on the larynx tighter and it can withstand considerable inward pressures of up to 100 kPa.

If this system of preventing solids entering the airways fails, powerful cough reflexes can be provoked by nerves in the lining of the larynx and trachea.

The larynx (see Fig. 2.1 ) is in fact a rather complicated box made up of plates of cartilage. It can be closed off by drawing together the two curtains of muscle which make up the vocal folds across the lumen of the larynx. Effective coughs depend on the closure and rapid opening of these ‘curtains’, which under less extreme circumstances are used to produce and modify the sounds that make up speech. The vocal folds can be drawn together so strongly that they are airtight against the greatest efforts to breathe the subject can make. This is clearly a ‘bad thing’ and can occur accidentally when an anaesthetist is trying to get an endotracheal tube into a patient's trachea. This dangerous closing of the larynx is called laryngospasm . A picture of what an anaesthetist would see when approaching the larynx is shown in Fig. 2.2 .

The vocal folds, as might be seen by an anaesthetist about to intubate a patient.

Bronchoscopy

It is frequently useful to inspect the airways below the larynx. First the trachea (part of which is extrathoracic), and then the intrathoracic airways. The instrument used for this is called a bronchoscope and may be of the rigid ‘open tube’ type through which the airways are inspected, or the flexible fibreoptic variety ( Fig. 2.3 ) which, as well as providing a view of the inside of the airways through its fibreoptic system, contains channels through which a variety of sampling and surgical instruments may be passed. Each type of bronchoscope has its advantages, but 95% of bronchoscopic procedures carried out these days are fibreoptic. Biopsy forceps, brushes and needles, balloon catheters and laser fibres can all now be passed through flexible bronchoscopes to carry out procedures after an initial inspection of even very small intrathoracic airways.

Bronchoscopes. Both fibreoptic flexible (A & C) and rigid (B & D) types are shown. The vast majority of investigations these days are carried out with the flexible type. To insert the rigid bronchoscope the patient's head has to be raised and rotated as shown.

The intrathoracic airways

The trachea is a single tube leading from the extrathoracic environment of the neck, where it is anchored at one end at the larynx, into the intrathoracic environment containing the lungs. It is the first of the conducting airways of the lungs. The conducting airways, as their name implies, conduct air to the respiratory airways , where the exchange of gas that makes up respiration takes place. The structure of these conducting airways differs from that of the respiratory region, mainly in having cartilage and smooth muscle in their relatively thick walls. This cartilage is particularly prominent in the trachea, where it forms horseshoe-shaped incomplete rings, with the two free ends facing backward and closed with a layer of smooth muscle (trachealis) with the oesophagus lying against it.

The airways of the lungs are often referred as the bronchial tree , and casts in which the airways are filled with plastic material and then the tissues dissolved away look like a deciduous tree in winter. The branches of this ‘tree’ can be represented in diagrammatic form as the ‘generations’ of a family tree ( Fig. 2.4 ). In some bronchitic patients secretions sometimes fill small airways, solidify, and are coughed up as small ‘casts’ of part of this ‘tree’.

The naming of airways. There is of course a gradual change in structure from one type of airway to another. One particular type of airway can occur at different distances into the lungs.

The trachea is the first and largest of about 23 generations of airways. The airways of each generation arise from the previous one by a system of irregular dichotomous branching airways. Dichotomous because each ‘mother’ airway gives rise to two ‘daughter’ airways, and irregular because the daughters, although smaller than the mother, are not necessarily of equal size. The naming of these generations is illustrated in Figure 2.4 , from which it may not be obvious that the number of airways (N) in a generation (Z), (counting the single trachea as generation 0) is:

The effect of dichotomous branching of individual airways on the total cross-sectional area (the sum of the cross-sectional areas of all the airways at that level) is remarkable and is shown in Figure 2.5 . Notice that ‘Total cross-sectional area’ is measured on a log scale, and so this value increases much more than it appears to in the figure.

Total cross-sectional area of the human airways. The total cross-sectional area at any level in the bronchial tree is the sum of the cross-sectional areas of all the airways at that level.

The functional consequences of this increase are profound because it causes the velocity of the air to fall rapidly as it moves into the lung. This effect is discussed in more detail in Chapter 5. The dimensions of some of the airways that make up the bronchial tree are given in Table 2.1 .

Dimensions of some of the airways of the human tracheobronchial tree. Note the enormous increase in cross-section and percentage total volume in the last few generations

As you go deeper into the lung the transitional and respiratory generations of the airways bear more and more alveoli until the alveolar sacs are totally made up of them. Alveoli do not look like the bunches of grapes or balloons stylistically represented in many textbooks, but rather pock-marked cavities with holes (pores of Kohn, K in Fig. 2.7C ) between many adjacent alveoli and with macrophages wandering over their surface ready to engulf and digest foreign particles (see Fig. 2.6 , Fig. 2.17 ).

Scanning electron micrograph of an alveolus. A, alveolus; C 1 , C 2 , C 3 , capillaries; E, endothelial cell; P 1 , type I pneumocytes; P 2 type II pneumocyte; L, lamellar bodies.

Airways wall structure. The classification of airways would depend on the characteristics of structure illustrated here. (A) Bronchus; (B) bronchiole; (C) alveolus. RBC, red blood cell; K, pores of Kohn; EP, epithelial nucleus; EN, endothelial nucleus.

Alveolar macrophage. Formed from monocytes produced in the bone marrow, these phagocytic cells contain enzymes destructive to microorganisms. These enzymes can produce emphysema in patients deficient in the protective protein α 1 -antitrypsin. M, macrophage; C, septal capillary; P 1 , type 1 pneumocyte; AP, alveolar pore; BM, basement membrane; Ly, lysosomes; L, lipid droplets.

It is a testament to the remarkable power of evolution that computer models which can analyse a branching system of tubes tell us that the branching angles and the changes in diameter of the airways of the human lungs are just right to cram the maximum alveolar surface into the minimum volume.

• • The airways are divided into upper-above the larynx and lower-below.
• • A major function of the nose is to ‘condition’ the air in terms of temperature and moisture.
• • The larynx protects the lower airways from foreign material.
• • Lower airways can be divided into first the conducting and then the respiratory airways.
• • Lower airways form a bronchial tree of 23 generations.
• • The number of airways increases much faster than their diameter decreases.
• • This means total cross-sectional area increases very rapidly.
• • Air entering the lungs therefore slows down almost to a stop.

Histological structure of the airways

The microscopic structure of the wall of the airways changes as you go deeper into the lungs. Three ‘snapshots’ of airway wall structure are shown in Figure 2.7 but of course the structure changes gradually from generation to generation.

The conducting airways consist of three general layers which vary in proportion depending on airway type:

• • The inner mucosal surface consists of ciliated epithelium and underlying mucus-secreting goblet cells. The activity of the cilia and the secretions of the globlet cells make up the mucociliary escalator (see Air-conditioning, below), which is important in removing inhaled particles in the lungs.
• • Outside the mucosal layer comes a smooth muscle layer in which the fibres are in continuous bundles. This smooth muscle is found in decreasing amounts from the largest airways right down to the entrances to the alveoli.
• • The outermost layer is of connective tissue , which in the large bronchi contains supporting cartilage. As the airways penetrate the lung they first lose their cartilage support and smooth muscle occupies a greater percentage of the airway wall. Then the ciliated epithelium becomes the squamous type, finally forming the respiratory region of the lung.

Bronchitis and the Reid Index

The arrangement representing bronchial structure illustrated in Figure 2.7 and described above is modified in chronic bronchitis in a way that provides a histopathological quantitative diagnosis of the disease. The Reid Index provides a measure of proportion of bronchial glands to total wall thickness ( Fig. 2.8 ). In normal lungs mucous glands occupy less than 40% of total wall thickness. In chronic bronchitis this proportion is altered by hyperplasia of the glands. A characteristic of chronic bronchitis is an increase in the products of these glands.

The Reid Index. The percentage of bronchial wall thickness occupied by gland tissue is known as the Reid Index, and is used as a measure of chronic bronchitis.

The respiratory region

The respiratory regions of the lungs show a wonderful degree of adaptation. They carry out the functions of a respiratory surface while withstanding the assaults of a polluted atmosphere and the mechanical trauma of being stretched and then relaxed about 12 times a minute for the whole of your life as a result of the movements of breathing.

One of the characteristics of the respiratory surface of any animal is that it should be thin, offering minimal separation between the outside medium (air or water) and the blood. This is beautifully demonstrated in the lungs, which are the only place in our body where blood capillaries come into direct contact with the outside air, as a result of the fusion of the type I epithelial cells (which make up about 95% of the lining of the respiratory zone; Fig. 2.6 ) with the pulmonary capillary endothelium. This fusion results in an ultrathin layer ideal for the diffusion of gas but not much good for support. Evolution has resulted in this thinning occurring on only one side of the pulmonary capillaries, whereas the cells on the other side remain separate and more robust, supporting the capillary in its place ( Fig. 2.9 ).

The alveolar–capillary membrane. This diagram of an electron micrograph shows the way the alveolar and capillary cells on one side of the alveolar septum fuse to form an ultrathin layer which offers little barrier to diffusion. The other side of the septum is thicker and provides physical support. RBC, red blood cell.

The junctions between the endothelial cells of the capillaries are ‘leaky’ and allow an easy flux of water and solutes between the plasma and the interstitial space. The junctions between the epithelial cells, however, are sufficiently ‘tight’ to prevent the escape of large molecules such as albumen into the alveoli, which would result in pulmonary oedema. Macrophages can easily push their way through the epithelial junctions to carry out their scavenging activities on the air side of the alveolus.

The rounded type II cells, much less numerous than type I and found at the junctions of alveolar septa, are the stem cells from which type II epithelial cells are formed. They are also important in producing lung surfactant (see Chapter 3).

Blood vessels

The pulmonary circulation only offers one-sixth of the resistance to blood flow that the systemic circulation offers. It is therefore a low-pressure system and this is reflected in the thin walls of its arteries. These arteries follow the airways through the lungs in connective tissue sheaths. The pulmonary arterioles are also very different from systemic arterioles, having very little smooth muscle in their walls. This absence of smooth muscle in the arterioles, and of course the capillaries and venules, persuades many scientists to consider the microcirculation of the lungs as a whole, rather than making a special case of the capillaries, which snake along several alveolar walls, one after the other, before reaching the venules. Venules join to form veins which, unlike the arteries, do not travel with the airways but make their own way along the septa that separate the segments of the lung. The airways and pulmonary blood vessels down as far as the terminal bronchioles receive their nutrition from the bronchial circulation which, as part of the systemic circulation, is distinct from the pulmonary circulation of the lungs. Part of the bronchial circulation returns to the systemic venous system in the normal way, but part drains into the pulmonary veins, ‘contaminating’ their oxygenated blood with deoxygenated blood. This situation constitutes a ‘ shunt ’ (see Chapter 7, p. 97Chapter 7p. 97).

Pulmonary hypertension

Hypertension (high blood pressure) can occur in the pulmonary circulation as well as in the systemic circulation. Pulmonary mean arterial pressure is normally about 15 mm Hg . This means that the limited smooth muscle in the pulmonary system is normally quite adequate to control flow. Pulmonary hypertension can arise for extrapulmonary reasons, such as mitral stenosis or left ventricular failure, both of which prevent the heart pumping away blood returning from the lungs. Congenital defects which allow blood to pass from the left (high-pressure) side of the heart to the pulmonary circulation also produce pulmonary hypertension.

By far the most common causes of pulmonary hypertension are changes in the pulmonary vessels themselves. They may be blocked by emboli, circulating fat, amniotic fluid or cancer cells. They may be obliterated by destruction of the architecture of the capillary beds by emphysema, or the smooth muscle in their walls may be provoked to contract by low oxygen tension, resulting from high altitude or diseases such as bronchitis and emphysema.

The clinical features of pulmonary hypertension are mainly the result of the increased pressure, producing oedema in the lung and imposing a pumping load on the right heart which it has not evolved to cope with. The patient complains of chest pain, dyspnoea and fatigue. Heart sounds are modified and the ECG demonstrates right ventricular hypertrophy.

The lymphatics

The perivascular spaces of the alveolar wall are drained by lymph vessels. The lymph system of the lungs begins as tiny blind-ended vessels just above the alveoli. These join to form lymphatics in close approximation to the blood vessels and airways. They are an important feature in the control of fluid balance in the lung and can contain considerable amounts of lymph, particularly during pulmonary oedema, when they produce the characteristic ‘butterfly shadow’ on the chest X-ray ( Fig. 2.10 ).

X-ray of ‘butterfly shadow’ in pulmonary oedema.

As in other tissue the lymph system plays a key role in immune defence in the lungs. These reactions are more the province of a textbook of immunology, but can be classified in outline as:

• • immediate hypersensitivity
• • antibody-dependent cytotoxicity
• • immune complex reactions
• • cell-mediated immune reactions.

Many immune disorders have the characteristics of asthma, whereas those causing interstitial lung disease are characterized by restrictive patterns (see Chapter 4).

Innervation of the airways of the lungs is separate from that which brings about breathing (see below) and consists of afferent and efferent parts. The most dramatic efferent (motor) effects produced are on bronchomotor tone. The parasympathetic efferent supply is of most importance in this, and arrives at bronchial smooth muscle via preganglionic fibres which course through the jugular and then the nodose ganglia of the vagus nerves. As this is a parasympathetic outflow the fibres synapse in ganglia on the bronchi before sending short postganglionic fibres to the bronchial smooth muscle where they release acetylcholine to produce bronchoconstriction ( Fig. 2.11 ).

Innervation of the diaphragm, intercostal muscles and lungs. The efferent (motor) systems are shown. The afferent (sensory) system is mainly in the vagus nerves.

The sympathetic nervous system, which is anatomically represented, has yet to be proved to have functional importance. The NANC (non-adrenergic non-cholinergic) system, which runs in the vagus nerve, secretes a variety substances that contract and relax bronchial smooth muscle, depending on circumstances.

Afferent nerves from receptors near the alveoli (J receptors), in the smooth muscle of airways (stretch receptors), and free nerve endings between the epithelial cells of airways (rapidly adapting, irritant, receptors) conduct sensation and sensory reflex information from the lungs to the brain, where it influences patterns of breathing (see Chapter 11) and bronchomotor tone.

The pulmonary circulation is innervated by sympathetic and parasympathetic nerves, but unlike the situation in the airways the sympathetic supply is of greater functional importance than the parasympathetic, and even then it only appears to exert a significant effect under conditions requiring ‘fight or flight’.

The limited importance of all these nervous systems is demonstrated by the success of transplanted lungs which are in fact denervated!

• • Conducting airways have relatively thick walls of mucosa, smooth muscle and cartilage.
• • The mucosa is ciliated and forms an ‘escalator’ carrying dust out of the lungs to the mouth.
• • Respiratory airways form a typical respiratory surface, thin, moist and vascular.
• • Blood vessels, nerves and lymph vessels run parallel with the airways.
• • The bronchial circulation nurtures the lung tissues.
• • The pulmonary circulation is involved in gas exchange.
• • The pulmonary circulation is a low-pressure system.
• • Parasympathetic nerves are functionally most important, causing airway smooth muscle contraction.

Gross structure of the respiratory system

As with other organs the general name for the functional tissue of the lungs is parenchyma . The vast majority of the volume of what we see as the lungs when the chest is opened is in fact alveolar tissue surrounding air spaces (see Table 2.1 ). These air spaces make the lungs so insubstantial and light that they are the only organ that floats when placed in water, hence the Middle English name for lungs: lights .

Each lung is anatomically divided into lobes , made up of segments which are subdivided into lobules ( Fig. 2.12 ).

Gross anatomy of the lungs. Each lung is divided into lobes made up of segments subdivided into lobules by fibrous tissue.

The lungs lie on both sides of the mediastinum which contains the trachea, heart, major blood vessels, nerves and oesophagus. The trachea divides into the right and left main bronchi at the carina , which is close to the aortic arch and the division of the pulmonary artery into its left and right branches. The main bronchi, pulmonary arteries and veins penetrate each lung at the hila . The lobes of the lungs are covered, except at their ‘roots’ at the medial surface, by a thin layer of tissue called the visceral pleura . The mediastinum and chest wall are lined by the parietal pleura . It helps some students to visualize the arrangement of the pleurae by thinking of a plastic bag, full of lungs, inside a second plastic bag, the two bags being the visceral and parietal pleurae, respectively ( Fig. 2.13 ).

Schematic diagram of the pleurae. It is important to remember that there is no real ‘space’ between the pleurae, just a few millilitres of slippery fluid.

The pleurae secrete a few millilitres of viscous fluid which lubricates them as they rub against each other during breathing, this fluid constitutes a ‘space’ between the pleurae, but it is important to remember that this tiny space is fluid not air filled. Most animals have pleural space, although it is not essential for life. Elephants are said to lack one, and surgeons sometimes fix the lungs to the chest wall in patients with ruptured lungs, thereby obliterating the space.

Inflammation of the pleura is called pleurisy and may be ‘dry’, where there is no appreciable effusion, or associated with an effusion, which may be of a variety of compositions. The pain of dry pleurisy is the result of the raw plurae moving over each other, and the patient complains of sharp localized pain associated with inspiration or coughing. Dry pleurisy occasionally accompanies pneumonia or carcinoma. Effusions of fluid into the pleural space result from a variety of conditions and can be of sufficient volume to collapse the lungs. If these effusions contain little protein they are known as transudates; if they contain much protein they are called exudates.

The diaphragm and chest wall

The base of the roughly cylindrical container which is the thorax is formed by the diaphragm . This is a sheet of muscle surrounding a large central tendon ( Fig. 2.14 ).

The diaphragm in coronal section. This figure illustrates how far into the chest the diaphragm bulges. This enables it to act very like the piston in a syringe as its muscle fibres shorten when stimulated by the bilateral phrenic nerves.

The diaphragm lies surprisingly high in the thorax, the central tendon being about level with the eighth thoracic vertebra. Muscle fibres attached to the tendon run down obliquely to originate at the xiphisternum (see above), the lower margins of the ribcage and the upper lumbar vertebrae. Innervation of the diaphragm is by the right and left phrenic nerves , each of which serves its half of the diaphragm. The phrenic nerves originate from cervical spinal cord segments C3–C5 (‘C3, 4 and 5 keep the diaphragm alive’), with the major contribution being made by C4. Both nerves run through the thorax in contact with the mediastinum, penetrate the diaphragm, and innervate it from its inferior surface (see Fig. 2.11 ).

The walls of the thorax are made up of the ribcage ( Fig. 2.15 ), which consists of the sternum anteriorly, to which ribs 1–6 are joined at about 45° by the costal cartilages. At the spinal column the ribs articulate by costovertebral joints which may involve more than one vertebra. Ribs 7–10 are joined by costal cartilage to the ribs above, and ribs 11 and 12 are free-‘floating’ at their anterior end.

Structure of the respiratory system: 2

Causes of osa.

In order for efficient gas flow to take place from the mouth to the alveoli, the airways that make up the respiratory system obviously need to be open and patent. The trachea and larger airways are held open by partial rings of cartilage within their walls. The smaller airways and the alveoli are held open by the tension in the lung tissue surrounding them. Above the larynx, the airway is held open by the actions of airway-dilating muscles, including genioglossus and palatopharyngeus. Were it not for the actions of these muscles, the upper airway would collapse, particularly in the supine position. During sleep, the tone in skeletal muscles throughout the body is reduced and this applies equally to the muscles which keep the upper airways patent. It is therefore normal for the upper airway to become narrowed during sleep.

In patients with OSA, the airway narrowing is more pronounced than normal and leads to periods of airway obstruction. There are a number of reasons why this happens, but obesity is the most important. It is thought that in obese patients, the pressure exerted by the fat in the neck tends to cause the airway to collapse. When the tone in the genioglossus and palatopharyngeus is reduced, as during sleep, airway obstruction may result.

The airway may remain obstructed for only a few seconds, or it may be well over a minute before the patient takes his next breath. During this time, the patient may become hypoxic and will begin to make vigorous efforts to try and breathe against the obstructed airway. Furthermore, he will become increasingly aroused from his sleep. Eventually, he regains the tone in his airway-dilating muscles and the airway obstruction is relieved. (Patients do not usually waken.) After the obstruction has been relieved, ventilation resumes and the patient's sleep deepens. This leads to a reduced tone in the airway-dilating muscles and the cycle starts to repeat itself.

Although obesity is probably the most important factor leading to OSA, there are other predisposing factors. These include anatomical variations predisposing to airway narrowing, such as enlarged tonsils, airway tumours and abnormalities of the mandible. Sedative drugs, including alcohol, may also predispose to sleep apnoea, probably by affecting sleep patterns and by reducing muscle tone. A small number of cases of OSA may be explained by abnormalities of neuromuscular function.

The ribcage. This ‘cage’ is much more flexible than prepared specimens or models sometimes suggest. The intercostal muscles stretch between the ribs.

Between the ribs are the three layers of the intercostal muscles :

• 1. External intercostals, running forward and downward
• 2. Internal intercostals, at right-angles to the externals, therefore running downward and posteriorly
• 3. Innermost intercostals, whose fibres run in the same direction as those of the internals.

These muscles are innervated by intercostal nerves from the anterior primary rami of spinal cord segments T1–T11.

Many muscles which do not have a primary role in respiration have their origins on the thorax. They move the head and neck and the upper limbs, for example. These muscles can be enlisted to aid breathing and are therefore called accessory muscles of respiration . The majority of these muscles aid inspiration, with only the flexors of the spine and the muscles of the anterior abdominal wall aiding expiration. Nevertheless, because of the mechanical advantage the accessory expiratory muscles have over the inspiratory muscles, we can blow out more powerfully than we can breathe in.

How breathing is brought about

Anyone who has sucked up fluid with a syringe has demonstrated how inspiration takes place. Before we go into the details of how the two processes are similar, we need to establish two very important facts:

• 1. The lungs do not have muscles that contribute to breathing: the small amount of muscle they contain controls the diameter of the airways.
• 2. Air will only flow from a region of high pressure to a region of low pressure. In inspiration the pressure in the elastic alveoli is made low by stretching them by reducing the pressure around them by expanding the chest. Air is thus sucked into the lungs. During expiration pressure in the lungs is increased by decreasing the size of the chest, thereby compressing the gas in the lungs.

The reduction in pressure around the lungs which brings about inspiration is mainly the result of activity in the phrenic nerves, causing the diaphragm to flatten and descend in the chest like a plunger in a syringe. This draws air into the chest. In quiet breathing inspiration is the only active part of breathing; expiration is largely passive and is the result of the elastic recoil of the lungs pulling them and the diaphragm back into their resting position – something like a balloon deflating when its neck is released.

The central tendon of the diaphragm moves 1–2 cm during breathing at rest, but can move up to about 10 cm during vigorous breathing. Movement of the diaphragm normally accounts for about 75% of the volume of breathing, but is not essential for life: if the diaphragm is paralysed, other respiratory muscles can take over to a large degree. In quiet breathing only some (and not always the same) diaphragmatic muscle fibres contract with each inspiration. This may explain why we rarely suffer from fatigue of the diaphragm.

If we liken the diaphragm to the plunger of a syringe, the ribs can be likened to its walls. The action of the intercostal muscles on the ribs (mainly the second to the tenth) can, however, alter the diameter of the chest and so actively draw air into and expel it from the lungs. This is largely because the ribs are set at an angle, sloping down from the horizontal, and are capable of being raised and lowered (see Fig. 2.14 ).

The external intercostal muscles cause two types of movement during inspiration:

• 1. ‘Pump-handle’ movements, in which the anterior end of each rib is elevated like the action of an old-fashioned water pump.
• 2. ‘Bucket-handle’ movements, in which the diameter of the chest increases, each rib on either side acting like the raising of the handle of a bucket from the horizontal position.

Both these types of action increase the diameter of the chest and thus draw air into the lungs by reducing the pressure in the chest. Not only do the external intercostal muscles help to bring about this reduction in pressure, but by stiffening the chest wall during inspiration they prevent a ‘sucking-in’ of the chest (just as you can suck in your cheeks) that would take place if they did not contract. The action of the intercostal muscles accounts for about 25% of maximum voluntary ventilation. The importance of the ribs and intercostal muscles to breathing is seen in patients whose ribs are broken and who exhibit what is known as ‘flail chest’ in which the chest wall moves in during inspiration and out during expiration.

Although expiration is largely passive during quiet breathing (resulting from the elastic recoil of the lungs–like a balloon collapsing) expiratory muscles can contract actively during high levels of breathing or if the airways are obstructed by disease. Under these conditions the abdominal muscles are the most important muscles of expiration. By squeezing the contents of the abdomen up against the diaphragm they force it up into the chest, thereby expelling air from the lungs. These abdominal muscles are especially active during a cough or a sneeze, as will be apparent if you press your fingers into your abdomen and cough. The internal and innermost intercostal muscles, like the external intercostals, occupy the spaces between the ribs and are innervated by segmental nerves. They pull the ribs down, reduce the diameter of the chest, and so contribute to expiration. Like the external intercostals, they reinforce the spaces between the ribs and prevent the chest from bulging out during expiration. The changes in size and shape of the chest brought about by the activity of the diaphragm, intercostals and accessory muscles are transmitted to the outer surface of the lungs. Because the lungs are so flexible, any change in pressure on their surface is rapidly transmitted to the air within the alveoli. This does not mean that the actual pressure in the fluid between the layers of pleura that form the covering of the lungs and the lining of the chest is the same as the pressure in the alveoli (see Chapter 5): in fact, it is important for the student to realize that they are very different.

A knowledge of the embryological origins of anatomical structures is often of use in understanding their physiological function, and many clinical situations. For example, the phenomenon of referred pain can be explained on the basis of common embryological origins of structures. Development of the fetal and neonatal lung can explain many differences in the function of the immature and the adult lung.

Prematurity, particularly with birthweights less than 2500 g, can result in respiratory distress in the infant because of the immaturity of type II pneumocytes, which produce surfactant (pp. 18, 36). This respiratory distress syndrome , also called hyaline membrane disease, develops within minutes or hours of birth and is characterized by high breathing rates requiring great effort owing to the reduced compliance of the lungs. When premature delivery of an infant is threatened its ability to secrete surfactant is estimated by measuring the ratio of lecithin to sphingomyelin in its amniotic fluid. If necessary, the activity of the type II pneumocytes can be enhanced by the administration of corticosteroids, and after birth exogenous surfactant can be administered as an aerosol. Nevertheless, mortality can be as high as 40%, which demonstrates how, even in the full-term baby, the development of the respiratory system is only just sufficiently complete.

In the 4-week-old human embryo the beginnings of the respiratory system are first seen as an outpouching, the laryngotracheal bud , on the ventral surface of the endoderm of the digestive tract ( Fig. 2.16 ). As the bud elongates the proximal portion forms the trachea and the distal end bifurcates to form first the two main bronchi and then the more distal parts of the bronchial tree, eventually forming a limited number of alveoli. The whole of the epithelium lining the entire respiratory tract is therefore derived from endoderm . The cartilage, muscle and connective tissue which make up much of the structure of the lungs develop from embryonic mesoderm that becomes associated with the laryngotracheal bud.

Lateral view of a 4-week human embryo. The laryngotracheal bud is beginning to divide to form the two lungs.

The lung undergoes five overlapping phases of development:

• • pseudoglandular
• • canalicular
• • saccular
• • alveolar
• • microvascular maturation.

In the pseudoglandular phase, which lasts from the fifth to the 17th week, the lung resembles a primitive compound gland, with the airways down to terminal bronchioles becoming visible. From week 16 to week 26 is the canalicular stage, with the airway generations of the future respiratory regions being formed. At the same time the airways are pushing through the surrounding mesenchyme, picking up a sleeve of capillaries which forms a local network that grows with the airway. From week 25 to birth the future alveolar ducts and alveolar sacs are produced by growth and branching of the irregularly shaped saccules at the ends of the prospective respiratory bronchioles. Although alveolar formation has started as early as week 36 of gestation, at birth there are only 50 million alveoli present, compared with 300 million in the adult lung. Alveolization continues for about 2 years after birth. The maturation of the microvasculature of the lung parallels alveolization during the first 2 years of independent life. From then onwards, the lung compartments grow in proportion to each other and to body weight.

• • There are no muscles in the lungs that bring about breathing.
• • Inspiration is brought about mainly by the diaphragm descending, like the plunger in a syringe.
• • Expiration is largely passive due to the elasticity of the lungs, like a balloon deflating.
• • Active expiration, as in exercise, involves the muscles of the abdomen.

Air-conditioning

The characteristics of all respiratory surfaces do not lead to their physical robustness (thin walls, vascular, moist). Therefore, it is of evolutionary advantage to have the respiratory surface of the lungs protected from damage by the air, or anything in the air, that must be moved over them during respiration. Even under the most congenial conditions the air around us is cold and dry compared with the respiratory surface of the lungs.

Structure of the respiratory system: 3

Signs and symptoms of osa.

Often, the first person to complain about a patient's OSA is his or her spouse! OSA is invariably associated with loud snoring as the airway becomes narrowed and this combined with the cycles of obstruction and arousal can lead to a very poor night's sleep for anyone in the same room as the patient. By the time a patient presents for treatment, their spouse has often resorted to sleeping alone.

The main symptom that the patient complains of is daytime drowsiness. Because their sleep patterns are so disrupted by cycles of apnoea and arousal, these patients are very tired and sleepy during the day. This somnolence may begin to impinge upon the patient's work and home life as their ability to concentrate for long periods of time begins to diminish. At worst, the patient may have a tendency to lose concentration or even fall asleep at the wheel of their car – motor accidents are more common in patients with OSA.

Other symptoms that the patient may complain of include morning headaches and night sweating and relatives may notice personality changes. For reasons that are not fully understood, patients often complain of having to get up to urinate during the night, sometimes on a few occasions.

Treatment is aimed at reducing the incidence of airway obstruction. The patient is advised to lose weight and to limit alcohol consumption, particularly before retiring to bed.

The most effective form of treatment, and the one tried by Mr Sinclair, is nasal continuous positive airway pressure (NCPAP). The patient wears the small mask strapped over his nose at night. The mask forms an airtight seal around the patient's nose. A continuous positive pressure, generated by a small pump, is applied to the mask. This pressure is transmitted to the upper airways and tends to prevent them from collapsing.

Other treatments are available – a surgical treatment of the condition that was at one time popular is the uvulopalatopharyngoplasty (UPPP). This operation involves removing the uvula and part of the soft palate. It has only a limited success rate and is associated with complications including fluid refluxing into the nose during drinking. It is therefore infrequently performed today.

Heat and water

Because temperature and water vapour gradients between mucosa and inspired air are greatest in the nose and upper airways, these regions carry a large portion of the air-conditioning burden. This burden is, however, shared with the lower airways. During nose breathing at rest air transit through the nose takes < 0.1 s. During that time temperature is raised (if in comfortable room air) from 20°C to 31°C by the time the air leaves the internal nares and to 35°C by the time it reaches the mid trachea. Humidification takes place equally rapidly, inspired air being close to saturation by the time it reaches the pharynx. Humidification of inspired air places a thermal demand on the body because of the high latent heat of vaporization of water. Five times as much heat is used to vaporize water to saturate inspired air than is used to warm that air. The air-conditioning process is a metabolic ‘expense’, and up to 40% of this cost is recovered from the expired air which warms and moistens the nasal mucosa as we breathe out. Desert animals such as camels and gerbils have highly developed turbinate systems in their noses which recover more heat and water than do ours.

This countercurrent exchange of heat and water in our nose is well demonstrated under cold conditions, when the mucosa of the nose is much colder than the exhaled air from deep in the lungs. Under these conditions sufficient water may condense to form a drop on the end of the nose. This is a purely natural physical phenomenon, not ‘a cold’ or other pathological condition.

At rest most people breathe through their nose, although 15% of the population are habitual mouth breathers. We all resort to mouth breathing during heavy exercise. The mouth is surprisingly good at air-conditioning, and by the time the air reaches the glottis conditions are very similar whether you are breathing through nose or mouth. The disadvantage of mouth breathing is in expiration, when much less heat and water is recovered. We have all experienced the discomfort of the dry mouth which often accompanies the nasal obstruction of a cold.

Particles and vapours

The respiratory system is threatened by many of the particles and chemical vapours in the air. The upper and conducting airways are much more robust than the respiratory surface, and they bear the brunt of protecting the respiratory surface by filtering these particles and vapours out of the inspired air.

Particles must be relatively small to penetrate the respiratory tract to any depth, and it is their size and shape that determine where they land. Where they land determines how they are dealt with.

An aerosol is a cloud of particles or droplets that remains stable and suspended in the air for some time. Because the volume (and hence the mass) of a drop is related to the cube of its diameter and its surface area is related to the square of its diameter, large drops fall faster than smaller ones. A shower of rain falls to the ground; a mist remains suspended for some time (Stoke's Law tells us that the terminal velocity of a falling sphere is proportional to the square of its radius). Scientists interested in the way aerosols behave in the lungs often convert the weight and shape of the particles into the size of the aerodynamically equivalent spheres. The mass median diameter of an aerosol is the diameter about which 50% of the total particle mass resides. The mass median aerodynamic diameter (MMAD) is the product of the mass median diameter and the square root of the particle density. Using this system we see 95% of particles > 5 mm MMAD impact on the walls of the nose and pharynx, where they are trapped by the sticky mucus. This impaction is the result of turbulence and the particles' momentum throwing them out of the airstream, when they rapidly change direction. In our noses the mucus that has trapped the dust is wafted to the pharynx by cilia. It is then swallowed. In dogs the cilia beat toward the outside and this contributes to the wet nose of a healthy dog. Slightly smaller particles (1–5 mm) survive the twists and turbulence of the upper airways and are removed by sedimentation in the small airways. Sedimentation is the settling of particles' under gravity, and this slow process is only effective in the small airways because their diameter is so small that the particles have only a little way to fall. Small though they are, these particles are too massive to be much affected by the buffeting of the gas molecules around them that constitutes the phenomenon of diffusion. Particles which reach the wall of the small airways are trapped in the mucus there and travel up the mucociliary escalator at a rate of about 2 mm s −1 to the pharynx in considerable amounts, to be swallowed. The mucus blanket which traps the particles is 5–10 mm thick and in two layers. The outer gel layer rests on a less viscous layer in which the cilia beat toward the mouth at a frequency of about 20 Hz.

The smallest particles of all (<0.1 mm) are deposited by diffusion of gas molecules producing Brownian motion. The particles are ‘jostled’ until they bump into the wall of a small airway or alveolus. In this region particles are stuck to the walls by surface tension because there is no secretion of mucus. They are also beyond the end of the ciliary escalator. In the alveolar region amoeboid macrophages ( Fig. 2.17 ) engulf particles and carry them to the escalator, or take them into the blood or lymph. When the dust load is large the macrophages dump their load around the respiratory, bronchioles, and any pathologist from a coal-mining area will have seen the black ‘halos’ so formed. Bacteria are particularly susceptible to the attentions of macrophages, which kill them with enzymes and oxygen-based free radicals (see Metabolic activity, p. 26) or transport them out of the lungs. The activities of these phagocytic cells ensure that the alveolar region of the lung is effectively sterile.

The free radicals and proteases produced by macrophages to deal with foreign material have the potential to damage the lung itself; how these dangerous substances are neutralized is described below (Metabolic activity, p. 36).

The influence of impaction, sedimentation and diffusion on particles of different aerodynamic diameters is illustrated in Figure 2.18 .

Deposition of particles in the lung. Notice that the aerodynamic diameter is a log scale and that there is a point at about 0.5 mm diameter where deposition is minimal.

The majority of particles of 0.5 mm aerodynamic diameter are not deposited: they ride the airflow into the lungs and back out again with expiration. Figure 2.18 represents the case during quiet breathing. Treatment of disease with therapeutic aerosols requires slow deep breathing to ensure deep penetration and sufficient time for diffusion. The increased ventilation of exercise enhances impaction and the danger of heavy work in dusty environments.

Particles account for only a small fraction by weight of the pollutants we breathe ( Fig. 2.19 ).

Air pollutants and their sources. Note the contribution made by automobiles.

Many gases and vapours also pose a serious threat, augmented by the self-abuse of tobacco smoking. Oxides of sulphur and nitrogen, hydrocarbons and chemicals produced by the action of sunlight on these substances inflame the respiratory tract. More than 1000 harmful constituents are inhaled in tobacco smoke. Smoking was identified by no less an authority than King James (VI of Scotland) I of England (1603–1625) as ‘ a custom loathsome to the eye, hateful to the nose, harmful to the brain and dangerous to the lungs ’. Little wonder he was known as ‘the wisest fool in Christendom’. Many harmful substances are produced by internal combustion engines, but the introduction of catalytic converters has significantly reduced production of carbon monoxide which has a particularly deleterious effect on the carriage of oxygen by the blood (see Chapter 8).

Metabolic activity

The metabolism of the tissues of the lung itself is unremarkable, with a metabolic rate only slightly higher than average for the whole body. Although it is the major extrahepatic site for mixed function oxidation by the cytochrome P450 systems, gram for gram it is much less active than the liver and much less tissue is involved. The major role of the P450 system in the lungs may therefore be in detoxification of inhaled foreign substances. Bloodborne toxic substances are extensively sequestered or detoxified in the lungs, with basic substances being particularly well processed. This protective activity on the part of the lung can be ‘heroic’ to a degree that causes fatal local damage: for example, the accumulation of oxygen-derived free radicals (useful in moderate concentrations to attack bacteria) is enhanced by the weedkiller paraquat (Weedol or Gramoxone), a dose of 1.5 g of which may be fatal because of its selective uptake by the lung. Although the initial clinical features of paraquat poisoning include dramatic ulceration of the mouth and oesophagus, diarrhoea and vomiting, it is usually the diffuse pulmonary fibrosis produced by the excess of free radicals that causes death. As well as free radicals the proteases , particularly elastase and trypsin, released by phagocytes in their normal defensive roles have to be neutralized or removed after they have carried out their function or they will attack the lung itself. Any of these substances caught up in the mucus of the mucociliary escalator will be carried out of the lung. In addition, their activity is terminated by conjugation with α 1 -antitrypsin in the plasma. The importance of this mechanism is demonstrated by the high incidence and severity of pulmonary emphysema in people who lack antitrypsin because of a genetic deficiency.

Metabolism of circulating biologically active substances

As they are in series with the systemic circulation and receive the whole of cardiac output, the lungs are ideally situated to rapidly control levels of substances circulating in the blood. This they do by utilizing the enormous surface area of endothelium (100 m 2 ) to remove or degrade substances whose effects need to be rapidly terminated once they have carried out their function:

• • Noradrenaline (norepinephrine)
• • ATP, ADP, AMP
• • Bradykinin
• • 5HT
• • Leukotrienes
• • PGE 1 , PGE 2 , PGF 2 α .

Substances which are more generally active and sustained in their actions pass through the pulmonary circulation unchanged, and include:

• • Adrenaline (epinephrine)
• • Angiotensin II
• • Dopamine
• • Histamine
• • Salbutamol
• • PGI 2 , PGA 2 .

Of particular interest, as the only example of activation of a bloodborne substance by the lung is the transformation of angiotensin I in the plasma to the powerful vasoconstrictor substance angiotensin II by angiotensin-converting enzyme (ACE). Although this is not restricted to the lung, being found in plasma and endothelium, pulmonary vasculature does seem to be most plentifully supplied with this enzyme and 80% of plasma angiotensin I is converted in a single pass though the lungs. ACE is also responsible for the removal of bradykinin by the lung. The lung endothelium is also responsible for a tonic production of NO.

The vasoactive and bronchoactive leukotrienes and prostaglandins , which are released into the circulation under certain conditions, are metabolized from arachadonic acid (apparently so-called because its crystals look like hairy spiders) by the pulmonary capillary endothelium.

As well as modifying the blood the lung also produces mucopolysaccharides as part of the production of bronchial mucus and secretes immunoglobulins (Ig) into the airways to defend against infection.

The production of surfactant by type II pneumocytes is discussed on p. 36.

Non-respiratory functions

The blood filtering function of the lungs, protecting the vulnerable cerebral and coronary circulations, is frequently and justifiably mentioned. However, the capillary diameter of the pulmonary circulation (about 7 mm) can not be regarded as the overall pore size of the filter. Many studies have shown that particles up to 400 mm diameter can pass through the pulmonary circulation. The effective filter size depends, in part, on the level of exercise the subject is undertaking, and may be affected by normally closed arteries opening to ‘shunt’ blood across the lungs. The particles filtered by the lungs include agglutinated white and red blood cells, fat droplets, and droplets of amniotic fluid during pregnancy. Tumour cells may lodge and grow in the lungs, but it is blood clots from the systemic circulation that form the major filtered load and interfere with the fluidity of the blood.

Blood fluidity

As well as trapping blood clots the lung contributes to blood fluidity by being the richest source of factors that promote (thromboplastin) or inhibit (heparin) clotting. The balance between their effects maintains the fluidity of the blood. Any blood clots already formed are broken down by the proteolytic enzyme plasmin , activated from its inactive precursor in the plasma by factors found in large quantities in pulmonary endothelium.

Blood capacity

Pulmonary blood volume is about 500 mL in a recumbent man. This volume can be halved by increases in pressure within the chest, such as forced expiration against a closed larynx. On the other hand, the volume of blood in the chest can be doubled by a forced inspiration. This phenomenon allows the pulmonary circulation to act as a reservoir, for example at the start of exercise, when the output of the left ventricle rapidly increases. Activity of the sympathetic nervous system may influence the capacity of the system by triggering contraction of smooth muscle in the blood vessel walls.

The high latent heat of vaporization of water makes its evaporation from the respiratory surface a useful mechanism for cooling in small furry animals. This mechanism is less evident in humans, perhaps because we use evaporation from our particularly hairless skin. However, a residue of this mechanism can be seen if you stay too long in a very hot bath, or if you have a fever – you will notice you begin reflexly to breathe through your mouth.

Breathing is unique among the major functions of the body in that it is both voluntarily and involuntarily controlled. For example, our hearts and kidneys pump and filter our blood without our being aware of it. We cannot, however, consciously control the rate at which they work. Breathing goes on unconsciously for most of the time (except for those unfortunate individuals suffering from Ondin's Curse, see p. 135), but in an instant we can take control of our breathing, for example to allow us to speak.

Our respiratory muscles help other systems of the body in many non-respiratory ways. When lifting a heavy weight our breathing stops, the muscles of the chest contract and it forms a rigid cage against which the muscles of the arms can act.

The diaphragm and abdominal muscles contract simultaneously to raise intra-abdominal pressure during vomiting, defecation and childbirth. Conversely, inspiration is switched off while you swallow food or drinks, to prevent their inhalation (have you noticed that each swallow is followed by an expiration?).

Changes in patterns of breathing can signal emotion, amicable or otherwise, and above all we use our respiratory system to power speech and vocalization.

• • Relatively large particles are deposited in the nose by impaction.
• • Smaller particles are deposited in the airways by sedimentation.
• • Sedimented particles are removed to the mouth by the mucociliary escalator.
• • Macrophages deal with particles that reach the alveoli.
• • The pulmonary circulation forms an important filter of the blood, particularly of blood clots.

Further reading

• Horsfield K. Morphometry of the lungs. In: Macklem P.T., Mead J., editors. Handbook of Physiology. Section 3, The Respiratory System. Vol III Mechanics of Breathing, Part I. American Physiological Society; Bethesda, MD: 1986. p. 75. [ Google Scholar ]
• Murray J.F. second ed. WB Saunders; Philadelphia: 1986. The Normal Lung. [ Google Scholar ]
• Silverman E.S., Gerritsen M.E., Collins T. Metabolic function of the pulmonary endothelium. In: Crystal R.G., West J.B., Barnes P.J., Weibel E.R., editors. The Lung: Scientific Foundations. second ed. Raven Press; New York: 1997. [ Google Scholar ]
• Weibel E.R. Academic Press; New York: 1963. Morphometry of the Human Lung. [ Google Scholar ]
• Weibel E.R. Design and morphometry of the pulmonary gas exchanger. In: Crystal R.G., West J.B., Barnes P.J., Weibel E.R., editors. The Lung: Scientific Foundations. second ed. Raven Press; New York: 1997. [ Google Scholar ]
• Young B., Heath J.W. Churchill Livingstone; Edinburgh: 2000. Wheater's Functional Histology: A Text and Colour Atlas. [ Google Scholar ]

Lungs – Five Paragraph Essay on the Respiration System

Students will blog a five paragraph essay (complete with an introductory, three body paragraphs, and a concluding paragraph) about what happens when people inhale and exhale. A clear thesis statement should be presented and a third body paragraph should detail cellular respiration.

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    Breathing me in, I come in through the nose. Quickly, I go through th throat and into a tube. In one of the lungs I go through the bronchi. I get absorb by cell bodies with nutrients and turns into energy. I get used by a human body so they won’t die. The respiratory system makes the heart pump blood and oxygen and carbon dioxide exchange, and exhaled while getting rid of waste.     The steps of the air entering our lungs. First, the air enters our body through our nose or mouth. Next, the air passes through our throat, in our throat there are the pharynx and the larynx (voice box). Finally, air passes into the trachea (strong tube) witch divides into two branches (bronchi). Before the air enters your body it has to go through three steps.      The air is now in the lungs. First, the bronchi leads the air to a air sac called alveoli. Next, the alveoli is surrounded by tiny blood vessels called capillaries. Then, in the capillaries, oxygen in the air enters the blood, in the capillaries carbon dioxide leaves the blood then it enters alveoli. Finally, CO2 is exhaled.     Cellular respiration happens in animal cell in the whole body. First, in the capillaries blood cells absorbs oxygen from the air in the alveoli. Secondly, nutrients and oxygen exits out of the body cell and goes into other body cells. Third, carbon dioxide and HO2 exits from the body cells, then it goes back into the blood cells in the capillaries. To be oxygenated the capillaries takes carbon dioxide-rich cells back to the lungs. In the whole body is where cellular respiration happens.     The body needs oxygen to do things to make it survive. The body need oxygen and nutrients to make energy. Without oxygen the heart won’t pump blood. What else would happen if the heart won’t pump blood?

Frightningly I get sucked into somebodys nose or mouth. Then I get filtered and get warmed up and flow through tubes in your throat than arrive at the at the lungs. Quickly I travel to the trachea and then flow to the aveoli. I am what you breath evryday. The respretory system is important to get you oxegen in your body. The respretory system is what helps you breath. When you breath in air through your mouth or nose it gets filtered and warmed up. Then,the air travels through throat passed the larynx and pharynx. Soon I enter the trachea. Breathing is how you get oxyegen. The lungs are necessary for the respretory system. Once you enter the lungs,you than come upon the trachea. The trachea splits into two branches. Quickly you go through the branches into the aveoli. Lungs are apart of the respretory system.      Cellurlar resperation happens in the lungs of the respretory system. When the air enters the aveoli the oxygen passes through by diffusion. The aveoli is surrounded by capillaries. Blood cells in the capillaries have a little amount of oxygen. Then cells in the capillaries have more carbon dioxide than air,so carbon dioxide diffuses from the blood cells to the air in the aveoli,then carbon dioxide is exhaled out of the organs of the respretory system. Oxygen has to be in your body at all times.     The respretory system helps you get oxygen in your body. It is what helps keep you alive. Seeing people pulute our home is very sad. They are trashing their lungs by smokeing and breahting it out into the air that we breathe. The more this happens the harder it gets to get oxygen in our lungs. We need pure oxygen.

        Having me sucked in her nostrils drawn me inside the nose. Safely,I enter the throat and then though a tube,the trachea. .The left side of the lungs I go though one of the bronchi and then go with my friend oxygen molecules to get absorbed by the blood in the alveoli. Getting used by a person body keeps them living. The restpitorary system is useful so that the body can get enough oxygen it wants.

    Floating in the air I get suck in through the nose to your throat. Quickly, I pass the throat to the trachea. Then the trachea divides into two tubes leading to the lungs. When I’am inside the lungs the tubes branch out into smaller branches like trees. The respiratory system help the oxygen in the body get rid of the waste.     The first stage of the respiratory system is simple. First of all, the air enters into the nose. Secondly, the air passes the throat and then past the pharynyx and larynx. Lastly, the air passes into the trachea and divides into the two branches. The respiratory system is easy.     The second stage about the respiratory system happens in the lungs. First, the lungs and the two bronchi leads to the air sacs alveoil. Second, alveoil is surronded by tiny blood vessel that are called capillaries. Then, in the capillaries

         Entering through your nose or mouth is where i get inhaled into your throat. Quickly I go down a dark tube called the trachea. Going to the branches from the bronchi empties me into very thin walled air sacs called alveoli. The respiratory system helps your body get oxygen

The first stage of the respiratory system is the air goes in your body in a couple steps. First, air enters through your nose or mouth. Your nose warmths the air and filters the dirt. Second, air passes your throat. While passing your throat the air passes the pharynx and the larynx (voice box). Third, air passes into the trachea which divides into to branches (Bronchi). In a couple steps the air goes in your body.

The second stage in the respiratory is something that keeps us alive. First, bronchi leads to the air sacs called alveoli. Second , the alveoli is surrounded by tiny blood vessels called capillaries. Third in the capillaries oxygen in air enters the blood in capillaries carbon dioxide leaves the blood and enters the alveoli where co2 is exhaled. Something important is the respiratory system for us.

    Quickly, I get inhaled into someone’s nostril and i enter the throat. In the tracea, i go into the right tube leading to the lungs. To enter the bronchi, I get emptied into the alveoli. The respiratory system is vital and helps you live.

    The first stage in the respiratory system is breathing in. First, air enters the nose and mouth, the nose warms air filters it. Next, air passes your throat and in the throat there are two things, the larynx and the pharnx. Then, air passes into the tracea which divides into two bronchi. The first step in the respiratory system is inhaling.

    The second stage in the respiratory system accures in the lungs. First, the bronchi

    Flaoting in the air, i could feel someone inhaling me in their mouth.Quickly, i go through a tube called the trachea. When i go to the the right side , i go to the bronchi. To absorb blood to the alveoli is valuable. I go through a body so that its life can continue. The respetory system is important to the body so it can give away of the waste that the body has.

     There are two ways for air to enter . It is used in a procces. It enters in the mouth or nose. Also, it passes throughthe throat then the trachea. They are called phaynk and laynk. There are vessels that are aroubd the alveoli.

    Floating in the air I suddenly get sucked in through somebody’s nose. In the throat I enter the trachea. Quickly, I go to the right side of the lung and into the bronchi. Now with more oxygen in the alveoli we will get absorbed by blood. The respitory system gives your body oxygen, blood absorbs oxygen and releases carbon dioxide.

    You need the respiratory system to breathe. First, air enters your mouth and nose. Your nos warms and filters the air. Then, air passes your throat through the pharynx and larynx. Finally, air passes into the trachea witch divide into bronchi. The respiratory system is how you breathe.

The respiratory system continues in the lungs. First, in the lungs two bronchi lead to the alveoli. Alveoli are surrounded by tiny blood vessels called capillaries. Finally, in the capillaries, oxygen in air enters blood in the capillaries and carbon dioxide leaves the blood in the capillaries and carbon dioxide is exhaled.

    Cellular respiration is the last stage in the respiratory system. First, animal cells need oxygen to carry out cellular respiration. The capillaries collect oxygen from air in the alveoli and nutrients from the small intestine. Then, the mitochondria break down the sugar with oxygen, releasing the energy stored in the sugar. Finally, carbon dioxide and water diffuse from cells and back into the capillaries. The last stage of the respiratory system is cellular respiration. INCOMPLETE

         Floating in air, I feel someones nose breathing in then I got sucked up. Quickly, I slide down through the throat and entered the trachea. Into the right side of the lung I travel through one of the bronchi. In the alveoli I awaited with my fellow molecules to be absorbed by the alveoli. I got used by a human body so it can continue it’s journey. Being a respiratory system is kind of hard.

        The first stage of the respiratory system is important. First, the air enters to the nose and the mouth. The nose warms the air up and filters any dirt or dust. Next, the air passes your throat it’s called pharynx and larynx. Finally, the air passes into the trachea which divides into two branches. The trachea is a strong tube. Respiratory system is important to our whole body.

        The second stage of the respiratory system is the lungs. First, the bronchi leads to air sacs called alveoli. The alveoli is surrounded by tiny blood vessels called the capillaries. In the capillaries, oxygen in the air enters blood in the capillaries. Carbon dioxide leaves the blood and enters the alveoli. Then carbon dioxide is exhaled. The lungs in the respiratory system is very important to us so we can breathe.

        The third stage of the respiratory system is the Cellular Respiratory system. First INCOMPLETE

         I am floating, now inhaled by someones nose. Passing the throat. Quickly I pass the trachea. Into one of the bronchi. To wait with oxygen molecules to be absorbed by the blood from the alveoli. Getting to be used by someone to make it stay alive. To have the respiratory system is seriously really needed because you need it enable to live and breathe, your body also needs the oxygen to function while it is getting rid of waste.         The first stage in the respiratory system is moving air. First, air enters the nose and the mouth. It is better to breath in through your nose because it warms and filters the air too. Secondly, air passes your throat, In your throat there are two things called the pharynx and your larynx, the larynx is your voice box. Finally, air passes through the trachea witch will divide into two separate branches, then will become bronchi.The respiratory system is moving air in the first stage.         The second stage of the respiratory system is inside the lungs. First, bronchi witch is in the lungs leads to air sacs called alveoli. Secondly, the alveoli is surrounded by tiny blood vessels called capillaries. Finally, in the capillaries the oxygen in the air enters the blood in the capillaries . carbon dioxide will then leave the blood and alveoli is then exhaled. In the lungs is where the second step in the respiratory system happends.         Animal cells and plant cells need oxygen to carry out cellular respiration. First, blood cells that are in the capillaries collect the oxygen from the alveoli,the blood cells will also be collecting nutrients from the small intestine. Second, the capillaries throughout the body come with other body cells. Finally , the oxygen and the nutrients diffuse out of the blood cells and into other blood cells. Both animal and plant cells need oxygen to carry out cellular respiration.         Having the respiratory system is important because you need it to inhale oxygen and exhale carbon dioxide. To breathe in oxygen and breathe out carbon dioxide is important because if I weren’t able to breathe in or out I would probably not even be alive. With the amount of pollution the world has, it is getting more and more dangerous to breathe in air/oxygen.

         Floating in the air, I see a person just then he inhaled then I went inside his nostrils in his nostrils I feel something tickling me it hair. I pass through his throat then I enter a tube known as the trachea. As I enter the right side of the lung I travel through one of the bronchi. As I await with my fellow oxygen to be recycled by the blood in the alveoli is fun. I get to be used all up by a humans body to continue , its life. The respiratory system, is so the body gets the oxygen it needs to function while getting rid of the waste.

        The first stage is about the respiratory system moving air. First, air enters your body through your nose and your mouth. Then, air passes your throat where your pharynx and your larynx are located. Finally, air gos into your trachea which divides into two branches which is called the bronchi. In the respiratory system air is moving around.

         The second stage is about the respiratory system in the lungs. First, the trachea splits into two. Then, it leads into air sacs called the alveoli. Next, there is blood vessels that surround the alveoli called capillaries. Finally in the capillaries oxygen in the air enters the blood. In the capillaries carbon dioxide leaves the blood then enters the alveoli where CO2 is exhaled. In the lungs the second step of the respiratory system is how the lungs work.

        This is what happens to the oxygen after it absorbs by your blood. First, oxygen travels inside red blood cells along with nutrients to cell bodies throughout your body. Then, in the capillaries , a cell body absorbs the oxygen and the food then turns it into energy.

        Getting sucked in the nose, I rapidly go down the throat. In the throat, I quickly pass the trachea. Quickly, I enter the bronchi. Next I reach the alveoli. Your body needs the respiratory system so you get the oxygen you need and the waste you need to release.

        The first stage in the respiratory system is about how you breathe in. First, the air enters the nose and mouth.

        Suddenly,i got sucked up by a human nose. Traviling,to the blood cells i went down alot of stuff.The first thing that i went down was the throat.In the throat i knew that i needed to go down even further so i went down and i was in the trachea as soon as i entered it i had to go even more so i was at the aveoli. When was in the aveoli i saw some capillaries (capillaries is a tiny blood vessel)the capillaries will meet with a cell and the cell will adsorb me. everyone needs a respiratory system so you can get air we need and get rid of waste.

Entering through your nose or mouth is where i get inhaled into your throat. Quickly I go down a dark tube called the trachea. Going to the branches from the bronchi empties me into very thin walled air sacs called alveoli. The respiratory system helps your body get oxygen

         In the animal cells is where cellular respiration is. First blood cells in the capillaries collect oxygen from the air in the alveoli and nutrients from the small intestine. Second capillaries throughout your body contacts with other body cells Third between sugar and oxygen produces carbon dioxide and water. Lastly the capillaries take carbon dioxide-rich cells back to the lungs so they can be oxygenated. Animal cells have cellular respiration in them.

         The respiratory system needs the body to function to get rid of waste. You can not live without your respiratory system. Why is it important to breathe?

        Floating in air, I feel someones nose breathing in then I got sucked up. Quickly, I slide down through the throat and entered the trachea. Into the right side of the lung I travel through one of the bronchi

Floating in air, I feel someones nose breathing in then I got sucked up. Quickly, I slide down through the throat and entered the trachea. Into the right side of the lung I travel through one of the bronchi. In the alveoli I awaited with my fellow molecules to be absorbed by the alveoli. I got used by a human body so it can continue it’s journey. Being a respiratory system is kind of hard.

The first stage of the respiratory system is important. First, the air enters to the nose and the mouth. The nose warms the air up and filters any dirt or dust. Next, the air passes your throat it’s called pharynx and larynx. Finally, the air passes into the trachea which divides into two branches. The trachea is a strong tube. Respiratory system is important to our whole body.

The second stage of the respiratory system is the lungs. First, the bronchi leads to air sacs called alveoli. The alveoli is surrounded by tiny blood vessels called the capillaries. In the capillaries, oxygen in the air enters blood in the capillaries. Carbon dioxide leaves the blood and enters the alveoli. Then carbon dioxide is exhaled. The lungs in the respiratory system is very important to us so we can breathe.

The third stage of the respiratory system is the Cellular Respiratory system. First, the red blood in the capillaries collect oxygen from the air in the alveoli, and nutrients form in the small intestine. Secondly, as the capillaries throughout the body comes in contact with other body cells, the oxygen and the nutrients diffuse out of the blood cells and into other body cells. Next, the body cell makes oxygen and sugar enters the Mitochondria. Then, the milochondria breaks down the sugar with oxygen, releasing the energy stored in the sugar. The Cellular Respiration system is like any other system.

        A Respiratory System carries oxygen into the lungs. The lungs is an important body part. When the oxygen goes through the lungs to the bronchi, the oxygen can go into any of the two branches. Then when the oxygen goes to the alveoli it is surrounded by capillaries. Do you know what is inside your body or how your body works.

4th stage is about how is the respiratary system.You need your respiratary system.Because air goes though your nose or mouth.Second ,the epiglottis and go though the throat and the trachea.Next,air goes in the brronchi and the right lung and the left lung.But not the alveoli or diaphragm. 1st stage is about in the lungs.First,in the lungs brronchi lead to air sacs-alveoli.Second,alveoli surrounded by ting blood vessels-capillaries.Next,in capillaries,oxygen in air enters blood in capillaries.Third,carbon dioxide leaves the blood enters alveoli carbon dioxide is exbaled. 2nd stage is about the respiratary system.First,air got in the noes though the epiglottis though the throat though the trocheainto the bronchi though the lungs into alveoli through the diaphragm. 3rd stage is about the circulatory.First,artery goes though the vein.Second,then the veins goes to the heart.Third,the blood in the heart goes to the capillary. INCOMPLETE

4th stage is about how is the respiratary system.You need your respiratary system.Because air goes though your nose or mouth.Second ,the epiglottis and go though the throat and the trachea.Next,air goes in the brronchi and the right lung and the left lung.But not the alveoli or diaphragm. First stage is about in the lungs.First,in the lungs brronchi lead to air sacs-alveoli.Second,alveoli surrounded by ting blood vessels-capillaries.Next,in capillaries,oxygen in air enters blood in capillaries.Third,carbon dioxide leaves the blood enters alveoli carbon dioxide is exbaled. Second stage is about the respiratary system.First,air got in the noes though the epiglottis though the throat though the trocheainto the bronchi though the lungs into alveoli through the diaphragm Third stage is about the circulatory.First,artery goes though the vein.Second,then the veins goes to the heart.Third,the blood in the heart goes to the capillary. incompete

Having me sucked in her nostrils drawn me inside the nose. Safely,I enter the throat and then though a tube,the trachea. .The left side of the lungs I go though one of the bronchi and then go with my friend oxygen molecules to get absorbed by the blood in the alveoli. Getting used by a person body keeps them living. The restpitorary system is useful so that the body can get enough oxygen it wants. Air passing and entering the lungs is important. First, air enters from the nose and mouth. The air is warmed and cleaned when it passes though the nose. Secondly, air passes though the throat which we use to talk, there are two the larynx which is the voice box and the other one is called the pharyny. Fanilly, air passes in the treachea which is a strong tube that divides into two branches which is now called the bronchi..Inportantly, air going though and entering the lungs is one part that is important.

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Respiratory System Essay Examples

How usa and china nations destroy humans respiratory system.

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Application of Various Measurements of Respiratory System Functioning in Modern Clinical Practice

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Cystic Fibrosis' Description, Symptoms & Treatment

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Based on the schedule, I assigned to go to a hospital tour at 9:00 AM. We went to the hospital, and we were lost. We did not know where should we go and who would take and guide us in the tour. Then, there was...

The Airway Surface Layer (asl) as an Important Part of Respiratory Function

The airway surface layer (ASL) is an important part of the respiratory function as it functions to defend against inhaled pathogens which are introduced to the body via the lungs. The body's primary defense mechanism against pathogens and infection is mucociliary clearance (MCC). The structure...

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