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16.3 Introduction to the Urinary System

Created by CK-12 Foundation/Adapted by Christine Miller

Figure 16.3.1 The surprising uses of pee.

Surprising Uses

What do gun powder, leather, fabric dyes and laundry service have in common?  This may be surprising, but they all historically involved urine.  One of the main components in gun powder, potassium nitrate, was difficult to come by pre-1900s, so ingenious gun-owners would evaporate urine to concentrate the nitrates it contains.  The ammonium in urine was excellent in breaking down tissues, making it a prime candidate for softening leathers and removing stains in laundry.  Ammonia in urine also helps dyes penetrate fabrics, so it was used to make colours stay brighter for longer.

What is the Urinary System?

The actual human  urinary system , also known as the renal system, is shown in Figure 16.3.2. The system consists of the kidneys, ureters, bladder, and urethra. The main function of the urinary system is to eliminate the waste products of metabolism from the body by forming and excreting urine . Typically, between one and two litres of urine are produced every day in a healthy individual.

Organs of the Urinary System

The urinary system is all about urine. It includes organs that form urine, and also those that transport, store, or excrete urine.

Urine is formed by the kidneys , which filter many substances out of the blood , allow the blood to reabsorb needed materials, and use the remaining materials to form urine. The human body normally has two paired kidneys, although it is possible to get by quite well with just one. As you can see in Figure 16.3.3, each kidney is well supplied with blood vessels by a major artery and vein. Blood to be filtered enters the kidney through the renal artery, and the filtered blood leaves the kidney through the renal vein. The kidney itself is wrapped in a fibrous capsule, and consists of a thin outer layer called the cortex, and a thicker inner layer called the medulla.

Blood is filtered and urine is formed by tiny filtering units called nephron s . Each kidney contains at least a million nephrons, and each nephron spans the cortex and medulla layers of the kidney. After urine forms in the nephrons, it flows through a system of converging collecting ducts. The collecting ducts join together to form minor calyces (or chambers) that join together to form major calyces (see Figure 16.3.3 above). Ultimately, the major calyces join the renal pelvis , which is the funnel-like end of the ureter where it enters the kidney.

Ureters, Bladder, Urethra

After urine forms in the kidneys, it is transported through the ureters (one per kidney) via peristalsis to the sac-like urinary bladder, which stores the urine until urination . During urination, the urine is released from the bladder and transported by the urethra to be excreted outside the body through the external urethral opening.

Functions of the Urinary System

Waste products removed from the body with the formation and elimination of urine include many water-soluble metabolic products. The main waste products are urea — a by-product of  protein   catabolism — and uric acid , a by-product of  nucleic acid  catabolism. Excess water and mineral ions are also eliminated in urine.

Besides the elimination of waste products such as these, the urinary system has several other vital functions. These include:

• Maintaining  homeostasis of mineral ions in extracellular fluid : These ions are either excreted in urine or returned to the blood as needed to maintain the proper balance.
• Maintaining homeostasis of blood pH : When pH is too low (blood is too acidic), for example, the kidneys excrete less bicarbonate (which is basic) in urine. When pH is too high (blood is too basic), the opposite occurs, and more bicarbonate is excreted in urine.
• Maintaining homeostasis of extracellular fluids, including the blood volume, which helps maintain  blood pressure:  The kidneys control fluid volume and blood pressure by excreting more or less salt and water in urine.

Control of the Urinary System

The formation of urine must be closely regulated to maintain body-wide homeostasis. Several  endocrine hormones  help control this function of the urinary system, including antidiuretic hormone, parathyroid hormone, and aldosterone.

• Antidiuretic hormone (ADH) , also called vasopressin, is secreted by the posterior pituitary gland. One of its main roles is conserving body water. It is released when the body is dehydrated, and it causes the kidneys to excrete less water in urine.
• Parathyroid  hormone  is secreted by the parathyroid glands. It works to regulate the balance of mineral ions in the body via its effects on several organs, including the kidneys. Parathyroid hormone stimulates the kidneys to excrete less calcium and more phosphorus in urine.
• Aldosterone is secreted by the cortex of the adrenal glands, which rest atop the kidneys, as shown in Figure 16.3.4. Through its effect on the kidneys, it plays a central role in regulating blood pressure. It causes the kidneys to excrete less sodium and water in urine.

Once urine forms, it is excreted from the body in the process of urination , also sometimes referred to as micturition. This process is controlled by both the autonomic and the somatic nervous systems. As the bladder fills with urine, it causes the autonomic nervous system to signal smooth muscle in the bladder wall to contract (as shown in Figure 16.3.5), and the sphincter between the bladder and urethra to relax and open. This forces urine out of the bladder and through the urethra. Another sphincter at the distal end of the urethra is under voluntary control. When it relaxes under the influence of the somatic nervous system, it allows urine to leave the body through the external urethral opening.

16.3 Summary

• The urinary system consists of the kidneys, ureters, bladder, and urethra. The main function of the urinary system is to eliminate the waste products of metabolism from the body by forming and excreting urine .
• Urine is formed by the kidneys, which filter many substances out of blood, allow the blood to reabsorb needed materials, and use the remaining materials to form urine. Blood to be filtered enters the kidney through the renal artery, and filtered blood leaves the kidney through the renal vein.
• Within each kidney , blood is filtered and urine is formed by tiny filtering units called nephrons , of which there are at least a million in each kidney.
• After urine forms in the kidneys, it is transported through the ureters via peristalsis to the urinary bladder . The bladder stores the urine until urination , when urine is transported by the urethra to be excreted outside the body.
• Besides the elimination of waste products (such as urea , uric acid , excess water, and mineral ions), the urinary system has other vital functions. These include maintaining  homeostasis  of mineral ions in extracellular fluid, regulating acid-base balance in the blood, regulating the volume of extracellular fluids, and controlling blood pressure .
• The formation of urine must be closely regulated to maintain body-wide homeostasis. Several endocrine hormones help control this function of the urinary system, including antidiuretic hormone from the posterior  pituitary gland , parathyroid hormone from the parathyroid glands , and aldosterone from the  adrenal glands .
• The process of urination is controlled by both the autonomic and the somatic nervous systems. The autonomic system causes the bladder to empty, but conscious relaxation of the sphincter at the distal end of the urethra allows urine to leave the body.

16.3 Review Questions

• State the main function of the urinary system.
• What are nephrons?
• Other than the elimination of waste products, identify functions of the urinary system.
• How is the formation of urine regulated?
• Explain why it is important to have voluntary control over the sphincter at the end of the urethra.
• In terms of how they affect the kidneys, compare aldosterone to antidiuretic hormone.
• If your body needed to retain more calcium, which of the hormones described in this concept is most likely to increase? Explain your reasoning.

16.3 Explore More

The Urinary System – An Introduction | Physiology | Biology | FuseSchool, 2017.

Maple Syrup Urine Disease, Alexandria Doody, 2016.

How Accurate Are Drug Tests? Seeker, 2016.

Three Ways Pee Could Change the World, Gross Science, 2015.

Attributions

Figure 16.3.1

• File:Pyrodex powder ffg.jpg by Hustvedt on Wikimedia Commons is used under a CC BY SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en).
• Brown leather satchel bag by Álvaro Serrano on Unsplash is used under the Unsplash Licence (https://unsplash.com/license).
• Laundry basket by Andy Fitzsimon on Unsplash is used under the Unsplash Licence (https://unsplash.com/license).
• Tags: Wool Skeins Natural Dyed Colorful Himalayan Weavers by  on Pixabay is used under the Pixabay License (https://pixabay.com/service/license/).

Figure 16.3.2

Urinary_System_(Male) by BruceBlaus on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 16.3.3

2610_The_Kidney  by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 16.3.4

Adrenal glands on Kidney   by Alan Hoofring (Illustrator)/ NCI Visuals Online is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 16.3.5

Urinary_Sphincter by BruceBlaus on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Alexandria Doody. (2016, March 29). Maple syrup urine disease. YouTube. https://www.youtube.com/watch?v=pyMcTUQYMQw&feature=youtu.be

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure  25.8   Left kidney [digital image].  In Anatomy and Physiology (Section 25.3). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/25-3-gross-anatomy-of-the-kidney

FuseSchool. (2017, June 19). The urinary system – An introduction | Physiology | Biology | FuseSchool. YouTube. https://www.youtube.com/watch?v=dxecGD0m0Xc&feature=youtu.be

Gross Science. (2015, September 15). Three ways pee could change the world. YouTube. https://www.youtube.com/watch?v=xt1Tj5eeS0k&feature=youtu.be

Seeker. (2016, January 16). How accurate are drug tests? YouTube. https://www.youtube.com/watch?v=3z-xjfdJWAI&feature=youtu.be

The body system that produces, stores and eliminates urine, the fluid waste excreted by the kidneys. The kidneys make urine by filtering wastes and extra water from blood. Urine travels from the kidneys through two thin tubes called ureters and fills the bladder.

A liquid waste product of the body that is formed by the kidneys and excreted by the other organs of the urinary system.

One of a pair of organs of the excretory and urinary systems that filters wastes and excess water out of blood and forms urine.

A body fluid in humans and other animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells. In vertebrates, it is composed of blood cells suspended in blood plasma.

One of the million tiny structural and functional units of the kidney that filters blood and forms urine.

The funnel-like end of a ureter where it enters the kidney and where urine collects before it is transported through the ureter.

A muscular, tube-like organ of the urinary system that moves urine by peristalsis from a kidney to the bladder.

A distinctive pattern of smooth muscle contractions that propels foodstuffs distally through the esophagus and intestines.

The process in which urine leaves the body through the external urethral orifice.

Waste product of protein catabolism that is mainly filtered from blood in the kidneys and excreted in urine.

A class of biological molecule consisting of linked monomers of amino acids and which are the most versatile macromolecules in living systems and serve crucial functions in essentially all biological processes.

The breakdown of larger molecules into smaller ones.

A waste product of nucleic acid catabolism that is mainly filtered from blood by the kidneys and excreted in urine.

Large biomolecules, essential to all known forms of life. The term nucleic acid is the overall name for DNA and RNA. They are composed of nucleotides, which are the monomers made of three components: a 5-carbon sugar, a phosphate group and a nitrogenous base.

The body system which acts as a chemical messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In humans, the major endocrine glands are the thyroid gland and the adrenal glands.

A hormone is a signaling molecule produced by glands in multicellular organisms that target distant organs to regulate physiology and behavior.

also called vasopressin. A hormone made by the hypothalamus in the brain and stored in the posterior pituitary gland. It tells your kidneys how much water to conserve. ADH constantly regulates and balances the amount of water in your blood.

A hormone secreted by the parathyroid gland which helps regulate blood calcium.

The main mineralocorticoid hormone which is responsible for sodium conservation in the kidney, salivary glands, sweat glands and colon.

division of the peripheral nervous system that controls involuntary activities

A division of the peripheral nervous system that controls voluntary activities.

Actions which take place according to the one's desire or are under control.

The chemical processes that occur in a living organism to sustain life.

A sac-like organ that stores urine until it is excreted from the body.

The ability of an organism to maintain constant internal conditions despite external changes.

The measure of the force exerted by circulating blood on the walls of arteries.

A hormone made by the hypothalamus in the brain and stored in the posterior pituitary gland. It tells your kidneys how much water to conserve. ADH constantly regulates and balances the amount of water in your blood. Higher water concentration increases the volume and pressure of your blood.

The master gland of the endocrine system that secretes many hormones, the majority of which regulate other endocrine glands.

One of a pair of small endocrine glands in the neck that secretes hormones that regulate blood calcium.

one of a pair of glands located on top of the kidneys that secretes hormones such as cortisol and adrenaline

A ring of muscles that can contract to close off an opening between structures, such as between the esophagus and stomach.

A tube-like organ of the urinary system that carries urine out of the body from the bladder and, in males, also carries semen out of the body.

Human Biology Copyright © 2020 by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License , except where otherwise noted.

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Module 16: The Urinary System

Introduction to the urinary system, learning objectives.

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

• Describe the composition of urine
• Label structures of the urinary system
• Characterize the roles of each of the parts of the urinary system
• Illustrate the macroscopic and microscopic structures of the kidney
• Trace the flow of blood through the kidney
• Outline how blood is filtered in the kidney nephron
• Provide symptoms of kidney failure
• List some of the solutes filtered, secreted, and reabsorbed in different parts of the nephron
• Describe the role of a portal system in the kidney
• Explain how urine osmolarity is hormonally regulated
• Describe the regulation of major ions by the kidney
• Summarize the role of the kidneys in maintaining acid–base balance

Figure 1. Sewage Treatment Plant (credit: “eutrophication&hypoxia”/flickr.com)

The urinary system has roles you may be well aware of: cleansing the blood and ridding the body of wastes probably come to mind. However, there are additional, equally important functions played by the system. Take for example, regulation of pH, a function shared with the lungs and the buffers in the blood. Additionally, the regulation of blood pressure is a role shared with the heart and blood vessels. What about regulating the concentration of solutes in the blood? Did you know that the kidney is important in determining the concentration of red blood cells? Eighty-five percent of the erythropoietin (EPO) produced to stimulate red blood cell production is produced in the kidneys. The kidneys also perform the final synthesis step of vitamin D production, converting calcidiol to calcitriol, the active form of vitamin D.

If the kidneys fail, these functions are compromised or lost altogether, with devastating effects on homeostasis. The affected individual might experience weakness, lethargy, shortness of breath, anemia, widespread edema (swelling), metabolic acidosis, rising potassium levels, heart arrhythmias, and more. Each of these functions is vital to your well-being and survival. The urinary system, controlled by the nervous system, also stores urine until a convenient time for disposal and then provides the anatomical structures to transport this waste liquid to the outside of the body. Failure of nervous control or the anatomical structures leading to a loss of control of urination results in a condition called incontinence.

This chapter will help you to understand the anatomy of the urinary system and how it enables the physiologic functions critical to homeostasis. It is best to think of the kidney as a regulator of plasma makeup rather than simply a urine producer. As you read each section, ask yourself this question: “What happens if this does not work?” This question will help you to understand how the urinary system maintains homeostasis and affects all the other systems of the body and the quality of one’s life.

• 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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11: Module 9- The Urinary System

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• 11.1: Introduction to the Urinary System
• 11.2: Physical Characteristics of Urine
• 11.3: Gross Anatomy of Urine Transport
• 11.4: Gross Anatomy of the Kidney
• 11.5: Microscopic Anatomy of the Kidney
• 11.6: Physiology of Urine Formation
• 11.7: Tubular Reabsorption
• 11.8: Regulation of Renal Blood Flow
• 11.9: Endocrine Regulation of Kidney Function
• 11.10: Regulation of Fluid Volume and Composition
• 11.11: The Urinary System and Homeostasis

25.9 The Urinary System and Homeostasis

Learning objectives.

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

• Describe the role of the kidneys in vitamin D activation
• Describe the role of the kidneys in regulating erythropoiesis
• Provide specific examples to demonstrate how the urinary system responds to maintain homeostasis in the body
• Explain how the urinary system relates to other body systems in maintaining homeostasis
• Predict factors or situations affecting the urinary system that could disrupt homeostasis
• Predict the types of problems that would occur in the body if the urinary system could not maintain homeostasis

All systems of the body are interrelated. A change in one system may affect all other systems in the body, with mild to devastating effects. A failure of urinary continence can be embarrassing and inconvenient, but is not life threatening. The loss of other urinary functions may prove fatal. A failure to synthesize vitamin D is one such example.

Vitamin D Synthesis

In order for vitamin D to become active, it must undergo a hydroxylation reaction in the kidney, that is, an –OH group must be added to calcidiol to make calcitriol (1,25-dihydroxycholecalciferol). Activated vitamin D is important for absorption of Ca ++ in the digestive tract, its reabsorption in the kidney, and the maintenance of normal serum concentrations of Ca ++ and phosphate. Calcium is vitally important in bone health, muscle contraction, hormone secretion, and neurotransmitter release. Inadequate Ca ++ leads to disorders like osteoporosis and osteomalacia in adults and rickets in children. Deficits may also result in problems with cell proliferation, neuromuscular function, blood clotting, and the inflammatory response. Recent research has confirmed that vitamin D receptors are present in most, if not all, cells of the body, reflecting the systemic importance of vitamin D. Many scientists have suggested it be referred to as a hormone rather than a vitamin.

Erythropoiesis

Erythropoetin (EPO) is a hormone produced by the kidney that stimulates the formation of red blood cells in the bone marrow. The kidney produces 85 percent of circulating EPO; the liver, the remainder. If you move to a higher altitude, the partial pressure of oxygen is lower, meaning there is less pressure to push oxygen across the alveolar membrane and into the red blood cell. One way the body compensates is to manufacture more red blood cells by increasing EPO production. If you start an aerobic exercise program, your tissues will need more oxygen to cope, and the kidney will respond with more EPO. If erythrocytes are lost due to severe or prolonged bleeding, or under produced due to disease or severe malnutrition, the kidneys come to the rescue by producing more EPO. Renal failure (loss of EPO production) is associated with anemia, which makes it difficult for the body to cope with increased oxygen demands or to supply oxygen adequately even under normal conditions. Anemia diminishes performance and can be life threatening.

Blood Pressure Regulation

Due to osmosis, water follows where Na + leads. In other words, “water follows salt.” Much of the water the kidneys recover from the filtrate follows the reabsorption of Na + . ADH stimulation of aquaporin channels allows for regulation of water recovery in the collecting ducts. Normally, all of the glucose is recovered, but loss of glucose control (diabetes mellitus) may result in an osmotic diuresis severe enough to produce severe dehydration and death. A loss of renal function means a loss of effective vascular volume control, leading to hypotension (low blood pressure) or hypertension (high blood pressure), which can lead to stroke, heart attack, and aneurysm formation.

The kidneys cooperate with the lungs, liver, and adrenal cortex through the renin–angiotensin–aldosterone system (see Chapter 25 Figure 25.4.2 ). The liver synthesizes and secretes the inactive precursor angiotensinogen. When the blood pressure is low, the kidney synthesizes and releases renin. Renin converts angiotensinogen into angiotensin I, and ACE produced in the lung converts angiotensin I into biologically active angiotensin II ( Figure 25.9.1 ). The immediate and short-term effect of angiotensin II is to raise blood pressure by causing widespread vasoconstriction. angiotensin II also stimulates the adrenal cortex to release the steroid hormone aldosterone, which results in renal reabsorption of Na + and its associated osmotic recovery of water. The reabsorption of Na + helps to raise and maintain blood pressure over a longer term.

Regulation of Osmolarity

Blood pressure and osmolarity are regulated in a similar fashion. Severe hypo-osmolarity can cause problems like lysis (rupture) of blood cells or widespread edema, which is due to a solute imbalance. Inadequate solute concentration (such as protein) in the plasma results in water moving toward an area of greater solute concentration, in this case, the interstitial space and cell cytoplasm. If the kidney glomeruli are damaged by an autoimmune illness, large quantities of protein may be lost in the urine. The resultant drop in serum osmolarity leads to widespread edema that, if severe, may lead to damaging or fatal brain swelling. Severe hypertonic conditions may arise with severe dehydration from lack of water intake, severe vomiting, or uncontrolled diarrhea. When the kidney is unable to recover sufficient water from the forming urine, the consequences may be severe (lethargy, confusion, muscle cramps, and finally, death) .

Recovery of Electrolytes

Sodium, calcium, and potassium must be closely regulated. The role of Na + and Ca ++ homeostasis has been discussed at length. Failure of K + regulation can have serious consequences on nerve conduction, skeletal muscle function, and most significantly, on cardiac muscle contraction and rhythm.

pH Regulation

Recall that enzymes lose their three-dimensional conformation and, therefore, their function if the pH is too acidic or basic. This loss of conformation may be a consequence of the breaking of hydrogen bonds. Move the pH away from the optimum for a specific enzyme and you may severely hamper its function throughout the body, including hormone binding, central nervous system signaling, or myocardial contraction. Proper kidney function is essential for pH homeostasis.

Everyday Connection

Stem Cells and Repair of Kidney Damage Stem cells are unspecialized cells that can reproduce themselves via cell division, sometimes after years of inactivity. Under certain conditions, they may differentiate into tissue-specific or organ-specific cells with special functions. In some cases, stem cells may continually divide to produce a mature cell and to replace themselves. Stem cell therapy has an enormous potential to improve the quality of life or save the lives of people suffering from debilitating or life-threatening diseases. There have been several studies in animals, but since stem cell therapy is still in its infancy, there have been limited experiments in humans.

Acute kidney injury can be caused by a number of factors, including transplants and other surgeries. It affects 7–10 percent of all hospitalized patients, resulting in the deaths of 35–40 percent of inpatients. In limited studies using mesenchymal stem cells, there have been fewer instances of kidney damage after surgery, the length of hospital stays has been reduced, and there have been fewer readmissions after release.

How do these stem cells work to protect or repair the kidney? Scientists are unsure at this point, but some evidence has shown that these stem cells release several growth factors in endocrine and paracrine ways. As further studies are conducted to assess the safety and effectiveness of stem cell therapy, we will move closer to a day when kidney injury is rare, and curative treatments are routine.

Chapter Review

The effects of failure of parts of the urinary system may range from inconvenient (incontinence) to fatal (loss of filtration and many others). The kidneys catalyze the final reaction in the synthesis of active vitamin D that in turn helps regulate Ca ++ . The kidney hormone EPO stimulates erythrocyte development and promotes adequate O 2 transport. The kidneys help regulate blood pressure through Na + and water retention and loss. The kidneys work with the adrenal cortex, lungs, and liver in the renin–angiotensin–aldosterone system to regulate blood pressure. They regulate osmolarity of the blood by regulating both solutes and water. Three electrolytes are more closely regulated than others: Na + , Ca ++ , and K + . The kidneys share pH regulation with the lungs and plasma buffers, so that proteins can preserve their three-dimensional conformation and thus their function.

Review Questions

1. Which step in vitamin D production does the kidney perform?

• converts cholecalciferol into calcidiol
• converts calcidiol into calcitriol
• stores vitamin D
• none of these

2. Which hormone does the kidney produce that stimulates red blood cell production?

• thrombopoeitin

3. If there were no aquaporin channels in the collecting duct, ________.

• you would develop systemic edema
• you would retain excess Na +
• you would lose vitamins and electrolytes
• you would suffer severe dehydration

Critical Thinking Questions

1. How does lack of protein in the blood cause edema?

2. Which three electrolytes are most closely regulated by the kidney?

Bagul A, Frost JH, Drage M. Stem cells and their role in renal ischaemia reperfusion injury. Am J Nephrol [Internet]. 2013 [cited 2013 Apr 15]; 37(1):16–29. Available from: http://www.karger.com/Article/FullText/345731

Answers for Review Questions

Answers for Critical Thinking Questions

• Protein has osmotic properties. If there is not enough protein in the blood, water will be attracted to the interstitial space and the cell cytoplasm resulting in tissue edema.
• The three electrolytes are most closely regulated by the kidney are calcium, sodium, and potassium.

This work, Anatomy & Physiology, is adapted from Anatomy & Physiology by OpenStax , licensed under CC BY . This edition, with revised content and artwork, is licensed under CC BY-SA except where otherwise noted.

Images, from Anatomy & Physiology by OpenStax , are licensed under CC BY except where otherwise noted.

Access the original for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction .

Anatomy & Physiology Copyright © 2019 by Lindsay M. Biga, Staci Bronson, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Kristen Oja, Devon Quick, Jon Runyeon, OSU OERU, and OpenStax is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License , except where otherwise noted.

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Anatomy and function of the urinary system, how does the urinary system work.

The body takes nutrients from food and changes them to energy. After the body has taken the food components that it needs, waste products are left behind in the bowel and in the blood.

The kidney and urinary systems help the body to get rid of liquid waste called urea. They also help to keep chemicals (such as potassium and sodium) and water in balance. Urea is made when foods containing protein (such as meat, poultry, and certain vegetables) are broken down in the body. Urea is carried in the blood to the kidneys. This is where it is removed, along with water and other wastes in the form of urine.

The kidneys have other important functions. They control blood pressure and make the hormone erythropoietin. This hormone controls red blood cell production in the bone marrow. The kidneys also control the acid-base balance and conserve fluids.

Kidney and urinary system parts and their functions

Two kidneys.  This pair of purplish-brown organs is located below the ribs toward the middle of the back. They:

Remove waste products and medicines from the body

Balance the body's fluids

Balance a variety of electrolytes

Release hormones to control blood pressure

Release a hormone to control red blood cell production

Help with bone health by controlling calcium and phosphorus

The kidneys remove urea from the blood through tiny filtering units called nephrons. Each nephron consists of a ball formed of small blood capillaries (glomerulus) and a small tube called a renal tubule. Urea, together with water and other waste substances, forms the urine as it passes through the nephrons and down the renal tubules of the kidney.

Two ureters.  These narrow tubes carry urine from the kidneys to the bladder. Muscles in the ureter walls keep tightening and relaxing. This forces urine downward, away from the kidneys. If urine backs up, or is allowed to stand still, a kidney infection can develop. About every 10 to 15 seconds, small amounts of urine are emptied into the bladder from the ureters.

Bladder.  This triangle-shaped, hollow organ is located in the lower belly. It's held in place by ligaments that are attached to other organs and the pelvic bones. The bladder's walls relax and expand to store urine. They contract and flatten to empty urine through the urethra. The typical healthy adult bladder can store up to 2 cups of urine for 2 to 5 hours.

Two sphincter muscles.  These circular muscles help keep urine from leaking by closing tightly like a rubber band around the opening of the bladder.

Nerves in the bladder.  The nerves alert a person when it's time to urinate, or empty the bladder.

Urethra.  This tube allows urine to pass outside the body. The brain signals the bladder muscles to tighten. This squeezes urine out of the bladder. At the same time, the brain signals the sphincter muscles to relax to let urine exit the bladder through the urethra. When all the signals happen in the correct order, normal urination happens.

Facts about urine

Normal, healthy urine is a pale straw or clear yellow color.

Darker yellow or honey-colored urine often means you need more water.

A darker, brownish color may mean a liver problem or severe dehydration.

Pinkish or red urine may mean blood in the urine.

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167 Overview of the Urinary System

Overview of the urinary system.

The urinary system maintains blood homeostasis by filtering out excess fluid and other substances from the bloodstream and secreting waste.

Learning Objectives

Review the urinary system

Key Takeaways

• The renal system eliminate wastes from the body, controls levels of electrolytes and metabolites, controls the osmoregulation of blood volume and pressure, and regulates blood pH.
• The renal system organs include the kidneys, ureter, bladder, and urethra. Nephrons are the main functional component of the kidneys.
• The respiratory and cardiovascular systems have certain functions that overlap with renal system functions.
• Metabolic wastes and excess ions are filtered out of the blood, combined with water, and leave the body in the form of urine.
• A complex network of hormones controls the renal system to maintain homeostasis.
• ureter : These are two long, narrow ducts that carry urine from the kidneys to the urinary bladder.
• osmoregulation : The most important function of the renal system, in which blood volume, blood pressure, and blood osmolarity (ion concentration) is maintained in homeostasis.

The Renal System

The renal system, which is also called the urinary system, is a group of organs in the body that filters out excess fluid and other substances from the bloodstream. The purpose of the renal system is to eliminate wastes from the body, regulate blood volume and pressure, control levels of electrolytes and metabolites, and regulate blood pH. 

The renal system organs include the kidneys, ureters, bladder, and urethra. Metabolic wastes and excess ions are filtered out of the blood, along with water, and leave the body in the form of urine.

Components of the renal system : Here are the major organs of the renal system.

Renal System Functions

The  renal system has many functions. Many of these functions are interrelated with the physiological mechanisms in the cardiovascular and respiratory systems.

• Removal of metabolic waste products from the body (mainly urea and uric acid).
• Regulation of electrolyte balance (e.g., sodium, potassium, and calcium).
• Osmoregulation controls the blood volume and body water contents.
• Blood pressure homeostasis: The renal system alters water retention and thirst to slowly change blood volume and keep blood pressure in a normal range.
• Regulation of acid-base homeostasis and blood pH, a function shared with the respiratory system.

Many of these functions are related to one another as well. For example, water follows ions via an osmotic gradient, so mechanisms that alter sodium levels or sodium retention in the renal system will alter water retention levels as well.

Organs of the Renal System

Kidneys and nephrons.

Kidneys are the most complex and critical part of the urinary system. The primary function of the kidneys is to maintain a stable internal environment (homeostasis) for optimal cell and tissue metabolism. The kidneys have an extensive blood supply from the renal arteries that leave the kidneys via the renal vein.

Nephrons are the main functional component inside the parenchyma of the kidneys, which filter blood to remove urea, a waste product formed by the oxidation of proteins, as well as ions like potassium and sodium. The nephrons are made up of a capsule capillaries (the glomerulus) and a small renal tube. 

The renal tube of the nephron consists of a network of tubules and loops that are selectively permeable to water and ions. Many hormones involved in homeostasis will alter the permeability of these tubules to change the amount of water that is retained by the body.

Urine passes from the renal tube through tubes called ureters and into the bladder.

The bladder is flexible and is used as storage until the urine is allowed to pass through the urethra and out of the body. 

The female and male renal system are very similar, differing only in the length of the urethra.

Human Osmoregulation

The kidneys play a very large role in human osmoregulation by regulating the amount of water reabsorbed from the glomerular filtrate in kidney tubules, which is controlled by hormones such as antidiuretic hormone (ADH), renin, aldosterone, and angiotensin I and II. 

A basic example is that a decrease in water concentration of blood is detected by osmoreceptors in the hypothalamus, which stimulates ADH release from the pituitary gland to increase the permeability of the wall of the collecting ducts and tubules in the nephrons. Therefore, a large proportion of water is reabsorbed from fluid to prevent a fair proportion of water from being excreted.

The extent of blood volume and blood pressure regulation facilitated by the kidneys is a complex process. Besides ADH secretion, the renin-angiotensin feedback system is critically important to maintain blood volume and blood pressure homeostasis. 

Boundless Anatomy and Physiology Copyright © by Lumen Learning is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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Review: Introduction to the Urinary System

Here is what we have learned from Introduction to the Urinary System :

• The urinary system rids the body of waste materials, regulates fluid volume, maintains electrolyte concentrations in body fluids, controls blood pH, secretes erythropoietin, and renin.
• The components of the urinary system are the kidneys, ureters, urinary bladder, and urethra.
• The primary organs of the urinary system are the kidneys, which are located retroperitoneally between the levels of the twelfth thoracic and third lumbar vertebrae.
• The cortex and medulla make up the parenchyma of the kidney.
• The central region of the kidney is the renal pelvis, which collects the urine as it is produced.
• The functional unit of the kidney is a nephron, which consists of a renal corpuscle and a renal tubule.
• The ureters transport urine from the kidney to the urinary bladder.
• The urinary bladder is a temporary storage reservoir for urine.
• The urethra is the final passageway for the flow of urine.
• The flow of urine through the urethra is controlled by an involuntary internal urethral sphincter and voluntary external urethral sphincter.

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7 Urinary system

• Published: March 2021
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The ‘Urinary system’ chapter opens with a description of the urinary tract morphology (kidney, ureters, bladder) and its histology. Renal function is considered, including glomerular filtration, the role and regulation of the renal tubules in producing dilute and concentrated urines, and the mechanisms of action of diuretic drugs. The function of the kidney in body fluid homeostasis (extracellular fluid volume and osmolarity, pH) is then discussed, and the regulation of kidney function explored, including bladder control and urinary continence. Finally, renal failure and obstructive uropathy are discussed as examples of renal pathology.

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Chapter Learning Objectives

• Name the organs of the urinary system
• Explain the major function(s) of each organ of the urinary system
• Describe the macroscopic and microscopic structures of the kidney
• Trace the flow of blood and urine through the kidney
• Trace the flow of urine through the urinary tract

This chapter will help you to understand the anatomy of the urinary system: kidneys, ureters, urinary bladder, and urethra. You will trace the blood flow through the kidney; think of the kidney as a regulator of plasma makeup rather than simply a urine producer. Knowing the anatomy will help you to understand how the urinary system maintains homeostasis and affects all the other systems of the body when you take physiology.

Interactive Link

Watch this video Kidney Function from the Howard Hughes Medical Institute for an introduction to the urinary system.

The Urinary or Renal System consists of two kidneys and the pathway, two ureters, urinary bladder, and one urethra, for urine to exit the body. The kidneys' primary function is to make urine by first filtering out many of the electrolytes, solutes, and water in the blood, forming the filtrate. As the filtrate passes through microscopic units called nephrons, certain solutes and water molecules are reabsorbed back to the blood because they are too precious to waste and other molecules are secreted because the body does not want them. As the filtrate becomes more concentrated, because water has been reabsorbed back to the blood, as it moves through the nephron, the end product is urine containing the waste products that the body can spare. Another perspective on the function of the kidneys is to monitor the level of certain solutes in the blood to maintain homeostasis. The kidneys' secondary function is to make hormones. Please review the Endocrine chapter for details.

The two ureters will carry urine from the kidneys down to the urinary bladder by peristalsis for storage. As the urinary bladder fills with urine, nerve endings will detect the stretched bladder to signal emptying. With sphincters relaxing, urine will flow out of the urinary bladder through the urethra to be expelled.

Figure 22.1.1:  Organs of the Urinary System.  The male and female human urinary system, including kidneys, bladder, ureter and urethra, pelvic floor muscles and the prostate.   (Image credit: " Organs of the Urinary System " by NIH Medial Arts is licensed under CC-BY-NC 2.0 .)

Contributors and Attributions

OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at  https://openstax.org/books/anatomy-and-physiology