16.4 Kidneys

Created by CK-12 Foundation/Adapted by Christine Miller

16.4.1 Kidney Pie
Figure 16.4.1 Steak and kidney pie!

Kidneys on the Menu

Pictured in Figure 16.4.1 is a steak and kidney pie; this savory dish is a British favorite. When kidneys are on a menu, they typically come from sheep, pigs, or cows. In these animals (as in the human animal), kidneys are the main organs of excretion.

Location of the Kidneys

The two bean-shaped  are located high in the back of the , one on each side of the spine. Both kidneys sit just below the , the large breathing muscle that separates the abdominal and thoracic cavities. As you can see in the following figure, the right kidney is slightly smaller and lower than the left kidney. The right kidney is behind the , and the left kidney is behind the . The location of the liver explains why the right kidney is smaller and lower than the left.

16.4.2 Classic Kidney Illustration from Gray's Anatomy
Figure 16.4.2 This classic illustration of the abdominal cavity provides a view of the internal organs from the back of the body. It clearly shows the locations of the right and left kidney, as well as the large blood vessels that connect the kidneys to the body’s main artery (aorta) and vein (inferior vena cava). The ureter exiting each kidney is also shown in the diagram.

Kidney Anatomy

The shape of each kidney gives it a convex side (curving outward) and a concave side (curving inward). You can see this clearly in the detailed diagram of kidney anatomy shown in Figure 16.4.3. The concave side is where the renal artery enters the kidney, as well as where the renal vein and ureter leave the kidney. This area of the kidney is called the . The entire kidney is surrounded by tough fibrous tissue — called the  — which, in turn, is surrounded by two layers of protective, cushioning fat.

16.4.3 Kidney Anatomy
Figure 16.4.3 This diagram shows the location and relative size of the two kidneys, as well as the internal structure of each kidney.

Internally, each kidney is divided into two major layers: the outer and the inner (see Figure 16.4.3 above). These layers take the shape of many cone-shaped renal lobules, each containing renal cortex surrounding a portion of medulla called a . Within the renal pyramids are the structural and functional units of the kidneys, the tiny . Between the renal pyramids are projections of cortex called . The tip, or papilla, of each pyramid empties urine into a minor calyx (chamber). Several minor calyces empty into a major calyx, and the latter empty into the funnel-shaped cavity called the , which becomes the ureter as it leaves the kidney.

Renal Circulation

The renal circulation is an important part of the kidney’s primary function of filtering waste products from the blood. is supplied to the kidneys via the renal arteries. The right renal artery supplies the right kidney, and the left renal artery supplies the left kidney. These two arteries branch directly from the aorta, which is the largest artery in the body. Each kidney is only about 11 cm (4.4 in) long, and has a mass of just 150 grams (5.3 oz), yet it receives about ten per cent of the total output of blood from the heart. Blood is filtered through the kidneys every 3 minutes, 24 hours a day, every day of your life.

As indicated in Figure 16.4.4, each renal artery carries blood with waste products into the kidney. Within the kidney, the renal artery branches into increasingly smaller that extend through the between the . These arteries, in turn, branch into arterioles that penetrate the renal pyramids. Blood in the arterioles passes through , the structures that actually filter the blood. After blood passes through the nephrons and is filtered, the clean blood moves through a network of venules that converge into small . Small veins merge into increasingly larger ones, and ultimately into the renal vein, which carries clean blood away from the kidney to the inferior .

16.4.4 Kidney and Nephron
Figure 16.4.4 The renal artery and renal vein carry blood to and from the kidney, respectively. As blood passes through a nephron within the kidney, it is filtered. Substances filtered from the blood are eventually collected in a tubule (collecting duct).

Nephron Structure and Function

Figure 16.4.4 gives an indication of the complex structure of a nephron. The  is the basic structural and functional unit of the kidney, and each kidney typically contains at least a million of them. As blood flows through a nephron, many materials are filtered out of the blood, needed materials are returned to the blood, and the remaining materials form urine. Most of the waste products removed from the blood and excreted in urine are byproducts of . At least half of the waste is , a waste product produced by  . Another important waste is , produced in  catabolism.

Components of a Nephron

Figure 16.4.5 shows in greater detail the components of a . Each nephron is composed of an initial filtering component that consists of a network of capillaries called the  (plural, glomeruli), which is surrounded by a space within a structure called  (also known as the Bowman’s capsule). Extending from glomerular capsule is the . The proximal end (nearest glomerular capsule) of the renal tubule is called the . From here, the renal tubule continues as a loop (known as the ) (also known as the loop of the nephron), which in turn becomes the . The latter finally joins with a collecting duct. As you can see in the diagram, arterioles surround the total length of the renal tubule in a mesh called the .

16.4.5 Nephron
Figure 16.4.5 This model of an individual nephron shows each of the structures that are involved in filtering blood, returning needed materials to blood, or excreting wastes that form urine.
16.4.6 Urine Formation at the Nephron
Figure 16.4.6 This diagram of a nephron shows the parts of the nephron where different stages of nephron function take place. These stages are filtration, reabsorption, secretion, and excretion.

Function of a Nephron

The simplified diagram of a nephron in Figure 16.4.6 shows an overview of how the nephron functions. Blood enters the nephron through an arteriole called the afferent arteriole. Next, some of the blood passes through the capillaries of the glomerulus. Any blood that doesn’t pass through the glomerulus — as well as blood after it passes through the glomerular capillaries — continues on through an arteriole called the efferent arteriole. The efferent arteriole follows the renal tubule of the nephron, where it continues playing a role in nephron functioning.

 

Filtration

As blood from the afferent arteriole flows through the glomerular capillaries, it is under pressure. Because of the pressure, water and solutes are filtered out of the blood and into the space made by glomerular capsule, almost like the water you cook pasta is is filtered out through a strainer. This is the filtration stage of nephron function. The filtered substances — called — pass into glomerular capsule, and from there into the proximal end of the .  Anything too large to move through the pores in the glomerulus, such as blood cells, large proteins, etc., stay in the cardiovascular system.  At this stage, filtrate (fluid in the nephron) includes water, salts, organic solids (such as nutrients), and waste products of metabolism (such as urea).

16.4.7 Nephron Secretion and Reabsorption
Figure 16.4.7 Secretion and reabsorption happen along the length of the renal tubule as the nephron balances blood pH and volume and maintains homeostasis of ions in the blood. Secretion is the movement of substance back into the bloodstream and reabsorption is movement of substances from the blood into the nephron for excretion.

Reabsorption and Secretion

As filtrate moves through the renal tubule, some of the substances it contains are reabsorbed from the filtrate back into the blood in the efferent arteriole (via ). This is the reabsorption stage of nephron function and it is about returning “the good stuff” back to the blood so that it doesn’t exit the body in urine. About two-thirds of the filtered salts and water, and all of the filtered organic solutes (mainly and ) are reabsorbed from the filtrate by the blood in the peritubular capillary network. occurs mainly in the proximal convoluted tubule and the loop of Henle, as seen in Figure 16.4.7.

At the distal end of the renal tubule, some additional reabsorption generally occurs. This is also the region of the tubule where other substances from the blood are added to the filtrate in the tubule. The addition of other substances to the filtrate from the blood is called . Both reabsorption and secretion (shown in Figure 16.4.7) in the distal convoluted tubule are largely under the control of endocrine hormones that maintain of water and mineral salts in the blood. These hormones work by controlling what is reabsorbed into the blood from the filtrate and what is secreted from the blood into the filtrate to become urine. For example, causes more calcium to be reabsorbed into the blood and more phosphorus to be secreted into the filtrate.

Collection of Urine and Excretion

16.4.8 Urine
Figure 16.4.8 Fresh urine is typically yellow or amber in colour.

By the time the filtrate has passed through the entire renal tubule, it has become the liquid waste known as . Urine empties from the distal end of the into a . From there, the urine flows into increasingly larger collecting ducts. As urine flows through the system of collecting ducts, more water may be reabsorbed from it. This will occur in the presence of from the posterior . This hormone makes the collecting ducts permeable to water, allowing water molecules to pass through them into capillaries by , while preventing the passage of ions or other solutes. As much as 75% of the water may be reabsorbed from urine in the collecting ducts, making the urine more concentrated.

Urine finally exits the largest collecting ducts through the renal papillae. It empties into the renal calyces, and finally into the . From there, it travels through the to the for eventual excretion from the body. An average of roughly 1.5 litres (a little over 6 cups) of urine is excreted each day. Normally, urine is yellow or amber in colour (see Figure 16.4.8). The darker the colour, generally speaking, the more concentrated the urine is.

 

Other Functions of the Kidneys

Besides filtering blood and forming urine for excretion of soluble wastes, the kidneys have several vital functions in maintaining body-wide . Most of these functions are related to the composition or volume of urine formed by the kidneys. The kidneys must maintain the proper balance of water and salts in the body, normal , and the correct range of blood . Through the processes of absorption and secretion by nephrons, more or less water, salt ions, acids, or bases are returned to the blood or excreted in urine, as needed, to maintain homeostasis.

Blood Pressure Regulation

The kidneys do not control homeostasis all alone. As indicated above, endocrine hormones are also involved. Consider the regulation of blood pressure by the kidneys. Blood pressure is the pressure exerted by blood on the walls of the arteries. The regulation of blood pressure is part of a complex system, called the renin-angiotensin-aldosterone system. This system regulates the concentration of sodium in the blood to control blood pressure.

16.4.9 Regulation of Blood Pressure
Figure 16.4.9 This diagram summarizes the processes that occur in the regulation of blood pressure by the renin-angiotensin-aldosterone system. The final step on the far right occurs in the nephrons and collecting ducts of the kidneys, where aldosterone stimulates increased reabsorption of sodium and water into the blood.

The renin-angiotensin-aldosterone system is put into play when the concentration of sodium ions in the blood falls lower than normal. This causes the kidneys to secrete an enzyme called into the blood. It also causes the liver to secrete a protein called angiotensinogen. Renin changes angiotensinogen into a proto-hormone called angiotensin I. This is converted to angiotensin II by an enzyme (angiotensin-converting enzyme) in lung capillaries.

Angiotensin II is a potent hormone that causes arterioles to constrict. This, in turn, increases blood pressure. Angiotensin II also stimulates the secretion of the hormone aldosterone from the adrenal cortex. Aldosterone causes the kidneys to increase the reabsorption of sodium ions and water from the filtrate into the blood. This returns the concentration of sodium ions in the blood to normal. The increased water in the blood also increases blood volume and blood pressure.

Other Kidney Hormones

Hormones other than renin are also produced and secreted by the kidneys. These include calcitriol and erythropoietin.

  •  is secreted by the kidneys in response to low levels of calcium in the blood. This hormone stimulates uptake of calcium by the intestine, thus raising blood levels of calcium.
  • is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of   in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.

Feature: Human Biology in the News

Kidney failure is a complication of common disorders including and . It is estimated that approximately 12.5% of Canadians have some form of kidney disease.  If the disease is serious, the patient must either receive a donated kidney or have frequent hemodialysis, a medical procedure in which the blood is artificially filtered through a machine. Transplant generally results in better outcomes than hemodialysis, but demand for organs far outstrips the supply. The average time on the organ donation waitlist for a kidney is four years.  There are over 3,000 Canadians on the wait list for a kidney transplant and some will die waiting for a kidney to become available.

For the past decade, Dr. William Fissell, a kidney specialist at Vanderbilt University, has been working to create an implantable part-biological and part-artificial kidney. Using microchips like those used in computers, he has produced an artificial kidney small enough to implant in the patient’s body in place of the failed kidney. According to Dr. Fissell, the artificial kidney is “… a bio-hybrid device that can mimic a kidney to remove enough waste products, salt, and water to keep a patient off [hemo]dialysis.”

The filtration system in the artificial kidney consists of a stack of 15 microchips. Tiny pores in the microchips act as a scaffold for the growth of living kidney cells that can mimic the natural functions of the kidney. The living cells form a membrane to filter the patient’s blood as a biological kidney would, but with less risk of rejection by the patient’s immune system, because they are embedded within the device. The new kidney doesn’t need a power source, because it uses the natural pressure of blood flowing through arteries to push the blood through the filtration system. A major part of the design of the artificial organ was devoted to fine tuning the fluid dynamics so blood flows through the device without clotting.

Because of the potential life-saving benefits of the device, the implantable kidney was given fast-track approval for testing in people by the U.S. Food and Drug Administration. The artificial kidney is expected to be tested in pilot trials by 2018. Dr. Fissell says he has a long list of patients eager to volunteer for the trials.

16.4 Summary

  • The two bean-shaped kidneys are located high in the back of the abdominal cavity on either side of the spine. A renal artery connects each kidney with the aorta, and transports unfiltered blood to the kidney. A renal vein connects each kidney with the inferior vena cava and transports filtered blood back to the circulation.
  • The kidney has two main layers involved in the filtration of blood and formation of urine: the outer cortex and inner medulla. At least a million nephrons — which are the tiny functional units of the kidney — span the cortex and medulla. The entire kidney is surrounded by a fibrous capsule and protective fat layers.
  • As blood flows through a nephron, many materials are filtered out of the blood, needed materials are returned to the blood, and the remaining materials are used to form urine.
  • In each nephron, the glomerulus and surrounding Bowman’s capsule form the unit that filters blood. From Bowman’s capsule, the material filtered from blood (called filtrate) passes through the long renal tubule. As it does, some substances are reabsorbed into the blood, and other substances are secreted from the blood into the filtrate, finally forming urine. The urine empties into collecting ducts, where more water may be reabsorbed.
  • The kidneys control homeostasis with the help of endocrine hormones. The kidneys, for example, are part of the renin-angiotensin-aldosterone system that regulates the concentration of sodium in the blood to control blood pressure. In this system, the enzyme renin secreted by the kidneys works with hormones from the liver and adrenal gland to stimulate nephrons to reabsorb more sodium and water from urine.
  • The kidneys also secrete endocrine hormones, including calcitriol — which helps control the level of calcium in the blood — and erythropoietin, which stimulates bone marrow to produce red blood cells.

16.4 Review Questions

  1. Contrast the renal artery and renal vein.
  2. Identify the functions of a nephron. Describe in detail what happens to fluids (blood, filtrate, and urine) as they pass through the parts of a nephron.
  3. Identify two endocrine hormones secreted by the kidneys, along with the functions they control.
  4. Name two regions in the kidney where water is reabsorbed.
  5. Is the blood in the glomerular capillaries more or less filtered than the blood in the peritubular capillaries? Explain your answer.
  6. What do you think would happen if blood flow to the kidneys is blocked?

16.4 Explore More

How do your kidneys work? – Emma Bryce, TED-Ed, 2015.

Urine Formation, Hamada Abass, 2013.

Printing a human kidney – Anthony Atala, TED-Ed, 2013.

 

Attributions

Figure 16.4.1

Steak and Kidney Pie by Charles Haynes on Flickr is used under a CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/) license.

Figure 16.4.2

Gray Kidneys by Henry Vandyke Carter (1831-1897) on Wikimedia Commons is in the  public domain (https://en.wikipedia.org/wiki/public_domain). (Bartleby.comGray’s Anatomy, Plate 1120).

Figure 16.4.3

Blausen_0592_KidneyAnatomy_01 by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 16.4.4

Diagram_showing_how_the_kidneys_work_CRUK_138.svg by Cancer Research UK on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 16.4.5

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

Figure 16.4.6

1024px-Physiology_of_Nephron by Madhero88 on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

Figure 16.4.7

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

Figure 16.4.8

Urine by User:Markhamilton at English Wikipedia on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 16.4.9

Renin_Angiotensin_System-01 by OpenStax College on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.

References

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 25.10 Blood flow in the nephron [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

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 25.17 Locations of secretion and reabsorption in the nephron [digital image].  In Anatomy and Physiology (Section 25.6). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/25-6-tubular-reabsorption

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 26.14 The renin-angiotensin system [digital image].  In Anatomy and Physiology (Section 26.3). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/26-3-electrolyte-balance

Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436

Hamada Abass. (2013). Urine formation. YouTube. https://www.youtube.com/watch?v=es-t8lO1KpA&feature=youtu.be

TED-Ed. (2015, February 9). How do your kidneys work? – Emma Bryce. YouTube. https://www.youtube.com/watch?v=FN3MFhYPWWo&feature=youtu.be

TED-Ed. (2013, March 15). Printing a human kidney – Anthony Atala. YouTube. https://www.youtube.com/watch?v=bX3C201O4MA&feature=youtu.be

 

License

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

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