Kidneys on the Menu<\/h1>\r\n<\/div>\r\nPictured 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.\r\n\r\nLocation\u00a0of the Kidneys<\/h1>\r\n<\/div>\r\nThe two bean-shaped\u00a0[pb_glossary id=\"2988\"]kidneys[\/pb_glossary]<\/strong>\u00a0are located high in the back of the [pb_glossary id=\"5855\"]abdominal cavity[\/pb_glossary], one on each side of the spine. Both kidneys sit just below the [pb_glossary id=\"4292\"]diaphragm[\/pb_glossary], the large\u00a0breathing\u00a0muscle that separates the abdominal and thoracic cavities. As you can see in the\u00a0following\u00a0figure, the right kidney is slightly smaller and lower than the left kidney. The right kidney is behind the [pb_glossary id=\"2989\"]liver[\/pb_glossary], and the left kidney is behind the [pb_glossary id=\"4497\"]spleen[\/pb_glossary]. The\u00a0location\u00a0of the liver explains why the right kidney is smaller and lower than the left.\r\n\r\n[caption id=\"attachment_4737\" align=\"aligncenter\" width=\"455\"]![\"16.4.2](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Gray-Kidneys-2.png\")
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\u2019s main artery (aorta) and vein (inferior vena cava). The ureter exiting each kidney is also shown in the diagram.<\/em>[\/caption]\r\n\r\nKidney Anatomy<\/span>\r\n\r\nThe 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 [pb_glossary id=\"4739\"]hilum[\/pb_glossary]<\/strong>. The entire kidney is surrounded by tough fibrous tissue \u2014 called the\u00a0[pb_glossary id=\"4740\"]renal capsule[\/pb_glossary]<\/strong>\u00a0\u2014 which, in turn, is surrounded by two layers of protective, cushioning fat.\r\n\r\n[caption id=\"attachment_4738\" align=\"aligncenter\" width=\"765\"]![\"16.4.3](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Blausen_0592_KidneyAnatomy_01-2.png\")
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.<\/em>[\/caption]\r\n\r\nInternally, each kidney is divided into two major layers: the outer [pb_glossary id=\"4741\"]renal cortex<\/strong>[\/pb_glossary] and the inner [pb_glossary id=\"4742\"]renal medulla<\/strong>[\/pb_glossary] (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 [pb_glossary id=\"4743\"]renal pyramid<\/strong>[\/pb_glossary]. Within the renal pyramids are the structural and functional units of the kidneys, the tiny [pb_glossary id=\"4718\"]nephrons<\/span>[\/pb_glossary]<\/span>. Between the renal pyramids are projections of cortex called [pb_glossary id=\"4744\"]renal columns<\/strong>[\/pb_glossary]. 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 [pb_glossary id=\"4724\"]renal pelvis[\/pb_glossary], which becomes the ureter as it leaves the kidney.<\/span>\r\n\r\nRenal\u00a0Circulation<\/h3>\r\n<\/div>\r\nThe renal circulation is an important part of the kidney\u2019s primary function of filtering waste products from the blood. [pb_glossary id=\"2702\"]Blood[\/pb_glossary] 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.\r\n\r\nAs 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 [pb_glossary id=\"4385\"]arteries[\/pb_glossary] that extend through the [pb_glossary id=\"4744\"]renal columns[\/pb_glossary] between the [pb_glossary id=\"4743\"]renal pyramids[\/pb_glossary]. These arteries, in turn, branch into arterioles that penetrate the renal pyramids. Blood in the arterioles passes through [pb_glossary id=\"4718\"]nephrons[\/pb_glossary], 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 [pb_glossary id=\"4386\"]veins[\/pb_glossary]. Small veins merge into increasingly larger ones, and ultimately into the renal vein, which carries clean blood away from the kidney to the inferior [pb_glossary id=\"4410\"]vena cava[\/pb_glossary].\r\n\r\n[caption id=\"attachment_4745\" align=\"aligncenter\" width=\"683\"]![\"16.4.4](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Diagram_showing_how_the_kidneys_work_CRUK_138.svg_-2.png\")
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).<\/em>[\/caption]\r\n\r\n\r\nNephron Structure and Function<\/h1>\r\n<\/div>\r\nFigure 16.4.4 gives an indication of the complex structure of a nephron. The [pb_glossary id=\"4718\"]nephron[\/pb_glossary]<\/strong>\u00a0is 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 [pb_glossary id=\"5773\"]metabolism[\/pb_glossary]. At least half of the waste is [pb_glossary id=\"4708\"]urea[\/pb_glossary], a waste product produced by\u00a0[pb_glossary id=\"5813\"]protein[\/pb_glossary]\u00a0[pb_glossary id=\"5721\"]catabolism[\/pb_glossary]. Another important waste is [pb_glossary id=\"4709\"]uric\u00a0acid[\/pb_glossary], produced in\u00a0[pb_glossary id=\"4727\"]nucleic acid[\/pb_glossary]\u00a0catabolism.\r\nComponents of a Nephron<\/h2>\r\nFigure 16.4.5 shows in greater detail the components of a [pb_glossary id=\"4718\"]nephron[\/pb_glossary]. Each nephron is composed of an initial filtering component that consists of a network of capillaries called the [pb_glossary id=\"4747\"]glomerulus[\/pb_glossary]<\/strong>\u00a0(plural, glomeruli), which is surrounded by a space within a structure\u00a0called\u00a0[pb_glossary id=\"4748\"]glomerular capsule[\/pb_glossary] <\/strong>(also known as the Bowman's capsule). Extending from glomerular capsule is the [pb_glossary id=\"4749\"]renal tubule[\/pb_glossary]<\/strong>. The proximal end (nearest glomerular capsule) of the renal tubule is called the [pb_glossary id=\"4750\"]proximal convoluted (coiled) tubule[\/pb_glossary]<\/strong>. From here, the renal tubule continues as a loop (known as the [pb_glossary id=\"4751\"]loop of Henle<\/strong>[\/pb_glossary]) (also known as the loop of the nephron), which in turn becomes the [pb_glossary id=\"4752\"]distal convoluted tubule<\/strong>[\/pb_glossary]. 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 [pb_glossary id=\"4753\"]peritubular capillary network<\/strong>[\/pb_glossary].\r\n\r\n[caption id=\"attachment_4746\" align=\"aligncenter\" width=\"425\"]![\"16.4.5](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Blood_Flow_in_the_Nephron-2.jpg\")
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.<\/em>[\/caption]\r\n\r\n<\/div>\r\n\r\n[caption id=\"attachment_4754\" align=\"alignright\" width=\"257\"]![\"16.4.6](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/1024px-Physiology_of_Nephron-2.png\")
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.[\/caption]\r\nFunction of a Nephron<\/h2>\r\nThe 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\u2019t pass through the glomerulus \u2014 as well as blood after it passes through the glomerular capillaries \u2014 continues on through an arteriole called the efferent arteriole. The efferent arteriole follows the renal tubule of the nephron, where it continues\u00a0playing a role\u00a0in nephron functioning.\r\n\r\n \r\nFiltration<\/h3>\r\nAs 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 \u2014 called [pb_glossary id=\"5471\"]filtrate[\/pb_glossary]<\/strong> \u2014 pass into glomerular capsule, and from there into the proximal end of the [pb_glossary id=\"4749\"]renal tubule[\/pb_glossary].\u00a0 Anything too large to move through the pores in the glomerulus, such as blood cells, large proteins, etc., stay in the cardiovascular system.\u00a0 At this stage, filtrate (fluid in the nephron) includes water, salts, organic solids (such as nutrients), and waste products of metabolism (such as urea).\r\n\r\n[caption id=\"attachment_4756\" align=\"alignright\" width=\"403\"]![\"16.4.7](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Nephron_Secretion_Reabsorption-2.jpg\")
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. Reabsorption is the movement of substance back into the bloodstream and secretion is movement of substances from the blood into the nephron for excretion.<\/em>[\/caption]\r\nReabsorption and Secretion<\/h3>\r\nAs 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 [pb_glossary id=\"4753\"]peritubular capillary network[\/pb_glossary]). 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 [pb_glossary id=\"5451\"]glucose[\/pb_glossary] and [pb_glossary id=\"5707\"]amino acids<\/span>[\/pb_glossary]<\/span>) are reabsorbed from the filtrate by the blood in the peritubular capillary network. [pb_glossary id=\"4757\"]Reabsorption[\/pb_glossary] occurs mainly in the proximal convoluted tubule and the loop of Henle, as seen in Figure 16.4.7.<\/span>\r\n\r\nAt 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\u00a0[pb_glossary id=\"4758\"]secretion[\/pb_glossary]<\/strong>. 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 [pb_glossary id=\"5761\"]homeostasis[\/pb_glossary] 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, [pb_glossary id=\"3467\"]parathyroid hormone[\/pb_glossary] causes more calcium to be reabsorbed into the blood and more phosphorus to be secreted into the filtrate.\r\nCollection of Urine and Excretion<\/h2>\r\n[caption id=\"attachment_4760\" align=\"alignleft\" width=\"219\"]![\"16.4.8](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Weewee-2.jpg\")
Figure 16.4.8 Fresh urine is typically yellow or amber in colour.<\/em>[\/caption]\r\n\r\nBy the time the filtrate has passed through the entire renal tubule, it has become the\u00a0liquid\u00a0waste known as [pb_glossary id=\"4717\"]urine[\/pb_glossary]. Urine empties from the distal end of the [pb_glossary id=\"4749\"]renal tubule[\/pb_glossary] into a\u00a0[pb_glossary id=\"4759\"]collecting duct[\/pb_glossary]<\/strong>. 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 [pb_glossary id=\"5533\"]antidiuretic\u00a0hormone\u00a0[\/pb_glossary]from the\u00a0posterior\u00a0[pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. This\u00a0hormone\u00a0makes the collecting ducts permeable to water, allowing water molecules to pass through them into capillaries by\u00a0[pb_glossary id=\"5663\"]osmosis[\/pb_glossary], 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.\r\n\r\nUrine finally exits the largest collecting ducts through the renal papillae. It empties into the renal calyces, and finally into the [pb_glossary id=\"4724\"]renal pelvis[\/pb_glossary]. From there, it travels through the [pb_glossary id=\"4725\"]ureter[\/pb_glossary] to the [pb_glossary id=\"4731\"]urinary bladder[\/pb_glossary] 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<\/span>). The darker the colour, generally speaking, the more concentrated the urine is.<\/span>\r\n\r\n \r\nOther Functions of the Kidneys<\/span><\/div>\r\nBesides filtering blood and forming urine for excretion of soluble wastes, the kidneys have several vital functions in maintaining body-wide\u00a0[pb_glossary id=\"5761\"]homeostasis[\/pb_glossary]. Most of these functions are related to the\u00a0composition\u00a0or volume of urine formed by the kidneys. The kidneys must maintain the proper balance of water and salts in the body, normal\u00a0[pb_glossary id=\"4480\"]blood pressure[\/pb_glossary], and the correct range of blood\u00a0[pb_glossary id=\"4330\"]pH[\/pb_glossary]. 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.\r\nBlood Pressure\u00a0Regulation<\/h2>\r\nThe kidneys do not control homeostasis all alone. As indicated above, endocrine hormones are also involved. Consider the regulation of\u00a0blood pressure\u00a0by 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\u00a0concentration\u00a0of sodium in the blood to control blood pressure.\r\n\r\n[caption id=\"attachment_4761\" align=\"aligncenter\" width=\"736\"]![\"16.4.9](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Renin_Angiotensin_System-01-2.jpg\")
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.<\/em>[\/caption]\r\n\r\nThe renin-angiotensin-aldosterone system is put into play when the\u00a0concentration\u00a0of sodium ions in the blood falls lower than normal. This causes the kidneys to secrete an\u00a0enzyme\u00a0called [pb_glossary id=\"4762\"]renin[\/pb_glossary] into the blood. It also causes the liver to secrete a\u00a0protein\u00a0called angiotensinogen. Renin changes angiotensinogen into a proto-hormone\u00a0called\u00a0angiotensin I. This is converted to angiotensin II by an\u00a0enzyme\u00a0(angiotensin-converting enzyme) in lung capillaries.\r\n\r\nAngiotensin 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\u00a0concentration\u00a0of sodium ions in the blood to normal. The increased water in the blood also increases blood volume and blood pressure.\r\nOther Kidney Hormones<\/h2>\r\nHormones other than renin are also produced and secreted by the kidneys. These include calcitriol and erythropoietin.\r\n\r\n \t- [pb_glossary id=\"4763\"]Calcitriol[\/pb_glossary]<\/strong>\u00a0is 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.<\/li>\r\n \t
- [pb_glossary id=\"4764\"]Erythropoietin[\/pb_glossary]<\/strong> is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of\u00a0 [pb_glossary id=\"4398\"]erythrocytes[\/pb_glossary]\u00a0in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.<\/li>\r\n<\/ul>\r\n\r\n
Feature: Human Biology in the News<\/h1>\r\n<\/div>\r\nKidney failure is a complication of common disorders including [pb_glossary id=\"2771\"]diabetes mellitus[\/pb_glossary] and [pb_glossary id=\"4231\"]hypertension[\/pb_glossary]. It is estimated that approximately 12.5% of Canadians have some form of kidney disease.\u00a0 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.\u00a0 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.\r\n\r\nFor 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\u2019s body in place of the failed kidney. According to Dr. Fissell, the artificial kidney is \u201c... a bio-hybrid device that can mimic a kidney to remove enough waste products, salt, and water to keep a patient off [hemo]dialysis.\u201d\r\n\r\nThe 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\u00a0cells\u00a0that can mimic the natural functions of the kidney. The living cells form a membrane to filter the patient\u2019s blood as a biological kidney would, but with less risk of rejection by the patient\u2019s immune system, because they are embedded within the device. The new kidney doesn\u2019t 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.\r\n\r\nBecause 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.\r\n\r\n\r\n16.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- 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\u00a0circulation.<\/li>\r\n \t
- 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 \u2014 which are the tiny functional units of the kidney \u2014 span the cortex and medulla. The entire kidney is surrounded by a fibrous capsule and protective fat layers.<\/li>\r\n \t
- 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.<\/li>\r\n \t
- In each nephron, the glomerulus and surrounding Bowman\u2019s capsule form the unit that filters blood. From Bowman\u2019s 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.<\/li>\r\n \t
- The kidneys control homeostasis with the help of endocrine hormones.\u00a0The 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\u00a0enzyme\u00a0renin secreted by the kidneys works with hormones from the liver and adrenal gland to stimulate nephrons to reabsorb more sodium and water from urine.<\/li>\r\n \t
- The kidneys also secrete endocrine hormones, including calcitriol \u2014 which helps control the level of calcium in the blood \u2014 and erythropoietin, which stimulates bone marrow to produce red blood\u00a0cells.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n16.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"632\"]<\/li>\r\n \t
- Contrast the renal artery and renal vein.<\/li>\r\n \t
- Identify the functions of a nephron.\u00a0Describe in detail what happens to fluids (blood, filtrate, and urine) as they pass through the parts of a nephron.<\/li>\r\n \t
- Identify two endocrine hormones secreted by the kidneys, along with the functions they control.<\/li>\r\n \t
- Name two regions in the kidney where water is reabsorbed.<\/li>\r\n \t
- Is the blood in the glomerular capillaries more or less filtered than the blood in the peritubular capillaries? Explain your answer.<\/li>\r\n \t
- What do you think would happen if\u00a0blood flow to the kidneys is blocked?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n16.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/FN3MFhYPWWo\r\nHow do your kidneys work? - Emma Bryce, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/es-t8lO1KpA\r\n
Urine Formation, Hamada Abass, 2013.<\/p>\r\nhttps:\/\/youtu.be\/bX3C201O4MA\r\n
Printing a human kidney - Anthony Atala, TED-Ed, 2013.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 16.4.1<\/strong>\r\n\r\nSteak and Kidney Pie<\/a> by Charles Haynes<\/a> on Flickr<\/a> is used under a CC BY-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/2.0\/) license.\r\n\r\nFigure 16.4.2<\/strong>\r\n\r\nGray Kidneys<\/a> by\u00a0Henry Vandyke Carter<\/a>\u00a0(1831-1897) on Wikimedia Commons is in the \u00a0public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain). (Bartleby.com<\/a>:\u00a0Gray\u2019s Anatomy<\/a>, Plate 1120<\/a>).\r\n\r\nFigure 16.4.3<\/strong>\r\n\r\nBlausen_0592_KidneyAnatomy_01<\/a> by BruceBlaus<\/a>\u00a0on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.4<\/strong>\r\n\r\nDiagram_showing_how_the_kidneys_work_CRUK_138.svg<\/a> by Cancer Research UK<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 16.4.5<\/strong>\r\n\r\nBlood_Flow_in_the_Nephron<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.6<\/strong>\r\n\r\n1024px-Physiology_of_Nephron<\/a> by Madhero88<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.7<\/strong>\r\n\r\nNephron_Secretion_Reabsorption<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.8<\/strong>\r\n\r\nUrine<\/a> by User:Markhamilton<\/a> at English Wikipedia<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 16.4.9<\/strong>\r\n\r\nRenin_Angiotensin_System-01<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\nReferences<\/h2>\r\n
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\u00a0<\/span><\/span>25.10<\/span><\/span>\u00a0<\/span>Blood flow in the nephron [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 25.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/25-3-gross-anatomy-of-the-kidney<\/span><\/span><\/p>\r\nBetts, 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\u00a0<\/span><\/span>25.17<\/span><\/span>\u00a0<\/span>Locations of secretion and reabsorption in the nephron [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 25.6). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/25-6-tubular-reabsorption<\/span><\/span><\/span><\/span><\/p>\r\nBetts, 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\u00a0<\/span><\/span>26.14<\/span><\/span>\u00a0<\/span>The renin-angiotensin system [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 26.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/26-3-electrolyte-balance<\/span><\/span><\/span><\/span><\/span><\/p>\r\nBlausen.com Staff. (2014). Medical gallery of Blausen Medical 2014.\u00a0WikiJournal of Medicine<\/i>\u00a01<\/b>\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436<\/p>\r\n
Hamada Abass. (2013). Urine formation. YouTube. https:\/\/www.youtube.com\/watch?v=es-t8lO1KpA&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, February 9). How do your kidneys work? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=FN3MFhYPWWo&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, March 15). Printing a human kidney - Anthony Atala. YouTube. https:\/\/www.youtube.com\/watch?v=bX3C201O4MA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\n![\"16.4.1](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Steak-and-Kidney-Pie-by-Charles-Haynes-on-flickr-2.jpg\")
Figure 16.4.1 Steak and kidney pie!<\/em><\/figcaption><\/figure>\n\nKidneys on the Menu<\/h1>\n<\/div>\n
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.<\/p>\n
\nLocation\u00a0of the Kidneys<\/h1>\n<\/div>\n
The two bean-shaped\u00a0kidneys<\/a><\/strong>\u00a0are located high in the back of the abdominal cavity<\/a>, one on each side of the spine. Both kidneys sit just below the diaphragm<\/a>, the large\u00a0breathing\u00a0muscle that separates the abdominal and thoracic cavities. As you can see in the\u00a0following\u00a0figure, the right kidney is slightly smaller and lower than the left kidney. The right kidney is behind the liver<\/a>, and the left kidney is behind the spleen<\/a>. The\u00a0location\u00a0of the liver explains why the right kidney is smaller and lower than the left.<\/p>\nFigure 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\u2019s main artery (aorta) and vein (inferior vena cava). The ureter exiting each kidney is also shown in the diagram.<\/em><\/figcaption><\/figure>\nKidney Anatomy<\/span><\/p>\nThe 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 hilum<\/a><\/strong>. The entire kidney is surrounded by tough fibrous tissue \u2014 called the\u00a0renal capsule<\/a><\/strong>\u00a0\u2014 which, in turn, is surrounded by two layers of protective, cushioning fat.<\/p>\nFigure 16.4.3 This diagram shows the location and relative size of the two kidneys, as well as the internal structure of each kidney.<\/em><\/figcaption><\/figure>\nInternally, each kidney is divided into two major layers: the outer renal cortex<\/strong><\/a> and the inner renal medulla<\/strong><\/a> (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 renal pyramid<\/strong><\/a>. Within the renal pyramids are the structural and functional units of the kidneys, the tiny nephrons<\/span><\/a><\/span>. Between the renal pyramids are projections of cortex called renal columns<\/strong><\/a>. 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 renal pelvis<\/a>, which becomes the ureter as it leaves the kidney.<\/span><\/p>\n\nRenal\u00a0Circulation<\/h3>\n<\/div>\n
The renal circulation is an important part of the kidney\u2019s primary function of filtering waste products from the blood. Blood<\/a> 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.<\/p>\nAs 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 arteries<\/a> that extend through the renal columns<\/a> between the renal pyramids<\/a>. These arteries, in turn, branch into arterioles that penetrate the renal pyramids. Blood in the arterioles passes through nephrons<\/a>, 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 veins<\/a>. Small veins merge into increasingly larger ones, and ultimately into the renal vein, which carries clean blood away from the kidney to the inferior vena cava<\/a>.<\/p>\nFigure 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).<\/em><\/figcaption><\/figure>\n\nNephron Structure and Function<\/h1>\n<\/div>\n
Figure 16.4.4 gives an indication of the complex structure of a nephron. The nephron<\/a><\/strong>\u00a0is 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 metabolism<\/a>. At least half of the waste is urea<\/a>, a waste product produced by\u00a0protein<\/a>\u00a0catabolism<\/a>. Another important waste is uric\u00a0acid<\/a>, produced in\u00a0nucleic acid<\/a>\u00a0catabolism.<\/p>\nComponents of a Nephron<\/h2>\n
Figure 16.4.5 shows in greater detail the components of a nephron<\/a>. Each nephron is composed of an initial filtering component that consists of a network of capillaries called the glomerulus<\/a><\/strong>\u00a0(plural, glomeruli), which is surrounded by a space within a structure\u00a0called\u00a0glomerular capsule<\/a> <\/strong>(also known as the Bowman’s capsule). Extending from glomerular capsule is the renal tubule<\/a><\/strong>. The proximal end (nearest glomerular capsule) of the renal tubule is called the proximal convoluted (coiled) tubule<\/a><\/strong>. From here, the renal tubule continues as a loop (known as the loop of Henle<\/strong><\/a>) (also known as the loop of the nephron), which in turn becomes the distal convoluted tubule<\/strong><\/a>. 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 peritubular capillary network<\/strong><\/a>.<\/p>\nFigure 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.<\/em><\/figcaption><\/figure>\n<\/div>\nFigure 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.<\/figcaption><\/figure>\nFunction of a Nephron<\/h2>\n
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\u2019t pass through the glomerulus \u2014 as well as blood after it passes through the glomerular capillaries \u2014 continues on through an arteriole called the efferent arteriole. The efferent arteriole follows the renal tubule of the nephron, where it continues\u00a0playing a role\u00a0in nephron functioning.<\/p>\n
<\/p>\n
Filtration<\/h3>\n
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 \u2014 called filtrate<\/a><\/strong> \u2014 pass into glomerular capsule, and from there into the proximal end of the renal tubule<\/a>.\u00a0 Anything too large to move through the pores in the glomerulus, such as blood cells, large proteins, etc., stay in the cardiovascular system.\u00a0 At this stage, filtrate (fluid in the nephron) includes water, salts, organic solids (such as nutrients), and waste products of metabolism (such as urea).<\/p>\nFigure 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. Reabsorption is the movement of substance back into the bloodstream and secretion is movement of substances from the blood into the nephron for excretion.<\/em><\/figcaption><\/figure>\nReabsorption and Secretion<\/h3>\n
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 peritubular capillary network<\/a>). 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 glucose<\/a> and amino acids<\/span><\/a><\/span>) are reabsorbed from the filtrate by the blood in the peritubular capillary network. Reabsorption<\/a> occurs mainly in the proximal convoluted tubule and the loop of Henle, as seen in Figure 16.4.7.<\/span><\/p>\nAt 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\u00a0secretion<\/a><\/strong>. 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 homeostasis<\/a> 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, parathyroid hormone<\/a> causes more calcium to be reabsorbed into the blood and more phosphorus to be secreted into the filtrate.<\/p>\nCollection of Urine and Excretion<\/h2>\nFigure 16.4.8 Fresh urine is typically yellow or amber in colour.<\/em><\/figcaption><\/figure>\nBy the time the filtrate has passed through the entire renal tubule, it has become the\u00a0liquid\u00a0waste known as urine<\/a>. Urine empties from the distal end of the renal tubule<\/a> into a\u00a0collecting duct<\/a><\/strong>. 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 antidiuretic\u00a0hormone\u00a0<\/a>from the\u00a0posterior\u00a0pituitary gland<\/a>. This\u00a0hormone\u00a0makes the collecting ducts permeable to water, allowing water molecules to pass through them into capillaries by\u00a0osmosis<\/a>, 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.<\/p>\nUrine finally exits the largest collecting ducts through the renal papillae. It empties into the renal calyces, and finally into the renal pelvis<\/a>. From there, it travels through the ureter<\/a> to the urinary bladder<\/a> 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<\/span>). The darker the colour, generally speaking, the more concentrated the urine is.<\/span><\/p>\n <\/p>\n
Other Functions of the Kidneys<\/span><\/div>\nBesides filtering blood and forming urine for excretion of soluble wastes, the kidneys have several vital functions in maintaining body-wide\u00a0homeostasis<\/a>. Most of these functions are related to the\u00a0composition\u00a0or volume of urine formed by the kidneys. The kidneys must maintain the proper balance of water and salts in the body, normal\u00a0blood pressure<\/a>, and the correct range of blood\u00a0pH<\/a>. 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.<\/p>\nBlood Pressure\u00a0Regulation<\/h2>\n
The kidneys do not control homeostasis all alone. As indicated above, endocrine hormones are also involved. Consider the regulation of\u00a0blood pressure\u00a0by 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\u00a0concentration\u00a0of sodium in the blood to control blood pressure.<\/p>\nFigure 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.<\/em><\/figcaption><\/figure>\nThe renin-angiotensin-aldosterone system is put into play when the\u00a0concentration\u00a0of sodium ions in the blood falls lower than normal. This causes the kidneys to secrete an\u00a0enzyme\u00a0called renin<\/a> into the blood. It also causes the liver to secrete a\u00a0protein\u00a0called angiotensinogen. Renin changes angiotensinogen into a proto-hormone\u00a0called\u00a0angiotensin I. This is converted to angiotensin II by an\u00a0enzyme\u00a0(angiotensin-converting enzyme) in lung capillaries.<\/p>\nAngiotensin 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\u00a0concentration\u00a0of sodium ions in the blood to normal. The increased water in the blood also increases blood volume and blood pressure.<\/p>\n
Other Kidney Hormones<\/h2>\n
Hormones other than renin are also produced and secreted by the kidneys. These include calcitriol and erythropoietin.<\/p>\n
\n- Calcitriol<\/a><\/strong>\u00a0is 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.<\/li>\n
- Erythropoietin<\/a><\/strong> is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of\u00a0 erythrocytes<\/a>\u00a0in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.<\/li>\n<\/ul>\n\n
Feature: Human Biology in the News<\/h1>\n<\/div>\n
Kidney failure is a complication of common disorders including diabetes mellitus<\/a> and hypertension<\/a>. It is estimated that approximately 12.5% of Canadians have some form of kidney disease.\u00a0 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.\u00a0 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.<\/p>\nFor 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\u2019s body in place of the failed kidney. According to Dr. Fissell, the artificial kidney is \u201c… a bio-hybrid device that can mimic a kidney to remove enough waste products, salt, and water to keep a patient off [hemo]dialysis.\u201d<\/p>\n
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\u00a0cells\u00a0that can mimic the natural functions of the kidney. The living cells form a membrane to filter the patient\u2019s blood as a biological kidney would, but with less risk of rejection by the patient\u2019s immune system, because they are embedded within the device. The new kidney doesn\u2019t 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.<\/p>\n
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.<\/p>\n
\n\n\n16.4 Summary<\/span><\/h1>\n<\/header>\n\n\n
Location\u00a0of the Kidneys<\/h1>\r\n<\/div>\r\nThe two bean-shaped\u00a0[pb_glossary id=\"2988\"]kidneys[\/pb_glossary]<\/strong>\u00a0are located high in the back of the [pb_glossary id=\"5855\"]abdominal cavity[\/pb_glossary], one on each side of the spine. Both kidneys sit just below the [pb_glossary id=\"4292\"]diaphragm[\/pb_glossary], the large\u00a0breathing\u00a0muscle that separates the abdominal and thoracic cavities. As you can see in the\u00a0following\u00a0figure, the right kidney is slightly smaller and lower than the left kidney. The right kidney is behind the [pb_glossary id=\"2989\"]liver[\/pb_glossary], and the left kidney is behind the [pb_glossary id=\"4497\"]spleen[\/pb_glossary]. The\u00a0location\u00a0of the liver explains why the right kidney is smaller and lower than the left.\r\n\r\n[caption id=\"attachment_4737\" align=\"aligncenter\" width=\"455\"] 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\u2019s main artery (aorta) and vein (inferior vena cava). The ureter exiting each kidney is also shown in the diagram.<\/em>[\/caption]\r\n\r\nKidney Anatomy<\/span>\r\n\r\nThe 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 [pb_glossary id=\"4739\"]hilum[\/pb_glossary]<\/strong>. The entire kidney is surrounded by tough fibrous tissue \u2014 called the\u00a0[pb_glossary id=\"4740\"]renal capsule[\/pb_glossary]<\/strong>\u00a0\u2014 which, in turn, is surrounded by two layers of protective, cushioning fat.\r\n\r\n[caption id=\"attachment_4738\" align=\"aligncenter\" width=\"765\"] 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.<\/em>[\/caption]\r\n\r\nInternally, each kidney is divided into two major layers: the outer [pb_glossary id=\"4741\"]renal cortex<\/strong>[\/pb_glossary] and the inner [pb_glossary id=\"4742\"]renal medulla<\/strong>[\/pb_glossary] (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 [pb_glossary id=\"4743\"]renal pyramid<\/strong>[\/pb_glossary]. Within the renal pyramids are the structural and functional units of the kidneys, the tiny [pb_glossary id=\"4718\"]nephrons<\/span>[\/pb_glossary]<\/span>. Between the renal pyramids are projections of cortex called [pb_glossary id=\"4744\"]renal columns<\/strong>[\/pb_glossary]. 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 [pb_glossary id=\"4724\"]renal pelvis[\/pb_glossary], which becomes the ureter as it leaves the kidney.<\/span>\r\n\r\nRenal\u00a0Circulation<\/h3>\r\n<\/div>\r\nThe renal circulation is an important part of the kidney\u2019s primary function of filtering waste products from the blood. [pb_glossary id=\"2702\"]Blood[\/pb_glossary] 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.\r\n\r\nAs 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 [pb_glossary id=\"4385\"]arteries[\/pb_glossary] that extend through the [pb_glossary id=\"4744\"]renal columns[\/pb_glossary] between the [pb_glossary id=\"4743\"]renal pyramids[\/pb_glossary]. These arteries, in turn, branch into arterioles that penetrate the renal pyramids. Blood in the arterioles passes through [pb_glossary id=\"4718\"]nephrons[\/pb_glossary], 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 [pb_glossary id=\"4386\"]veins[\/pb_glossary]. Small veins merge into increasingly larger ones, and ultimately into the renal vein, which carries clean blood away from the kidney to the inferior [pb_glossary id=\"4410\"]vena cava[\/pb_glossary].\r\n\r\n[caption id=\"attachment_4745\" align=\"aligncenter\" width=\"683\"] 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).<\/em>[\/caption]\r\n\r\n\r\nNephron Structure and Function<\/h1>\r\n<\/div>\r\nFigure 16.4.4 gives an indication of the complex structure of a nephron. The [pb_glossary id=\"4718\"]nephron[\/pb_glossary]<\/strong>\u00a0is 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 [pb_glossary id=\"5773\"]metabolism[\/pb_glossary]. At least half of the waste is [pb_glossary id=\"4708\"]urea[\/pb_glossary], a waste product produced by\u00a0[pb_glossary id=\"5813\"]protein[\/pb_glossary]\u00a0[pb_glossary id=\"5721\"]catabolism[\/pb_glossary]. Another important waste is [pb_glossary id=\"4709\"]uric\u00a0acid[\/pb_glossary], produced in\u00a0[pb_glossary id=\"4727\"]nucleic acid[\/pb_glossary]\u00a0catabolism.\r\nComponents of a Nephron<\/h2>\r\nFigure 16.4.5 shows in greater detail the components of a [pb_glossary id=\"4718\"]nephron[\/pb_glossary]. Each nephron is composed of an initial filtering component that consists of a network of capillaries called the [pb_glossary id=\"4747\"]glomerulus[\/pb_glossary]<\/strong>\u00a0(plural, glomeruli), which is surrounded by a space within a structure\u00a0called\u00a0[pb_glossary id=\"4748\"]glomerular capsule[\/pb_glossary] <\/strong>(also known as the Bowman's capsule). Extending from glomerular capsule is the [pb_glossary id=\"4749\"]renal tubule[\/pb_glossary]<\/strong>. The proximal end (nearest glomerular capsule) of the renal tubule is called the [pb_glossary id=\"4750\"]proximal convoluted (coiled) tubule[\/pb_glossary]<\/strong>. From here, the renal tubule continues as a loop (known as the [pb_glossary id=\"4751\"]loop of Henle<\/strong>[\/pb_glossary]) (also known as the loop of the nephron), which in turn becomes the [pb_glossary id=\"4752\"]distal convoluted tubule<\/strong>[\/pb_glossary]. 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 [pb_glossary id=\"4753\"]peritubular capillary network<\/strong>[\/pb_glossary].\r\n\r\n[caption id=\"attachment_4746\" align=\"aligncenter\" width=\"425\"] 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.<\/em>[\/caption]\r\n\r\n<\/div>\r\n\r\n[caption id=\"attachment_4754\" align=\"alignright\" width=\"257\"] 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.[\/caption]\r\nFunction of a Nephron<\/h2>\r\nThe 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\u2019t pass through the glomerulus \u2014 as well as blood after it passes through the glomerular capillaries \u2014 continues on through an arteriole called the efferent arteriole. The efferent arteriole follows the renal tubule of the nephron, where it continues\u00a0playing a role\u00a0in nephron functioning.\r\n\r\n \r\nFiltration<\/h3>\r\nAs 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 \u2014 called [pb_glossary id=\"5471\"]filtrate[\/pb_glossary]<\/strong> \u2014 pass into glomerular capsule, and from there into the proximal end of the [pb_glossary id=\"4749\"]renal tubule[\/pb_glossary].\u00a0 Anything too large to move through the pores in the glomerulus, such as blood cells, large proteins, etc., stay in the cardiovascular system.\u00a0 At this stage, filtrate (fluid in the nephron) includes water, salts, organic solids (such as nutrients), and waste products of metabolism (such as urea).\r\n\r\n[caption id=\"attachment_4756\" align=\"alignright\" width=\"403\"] 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. Reabsorption is the movement of substance back into the bloodstream and secretion is movement of substances from the blood into the nephron for excretion.<\/em>[\/caption]\r\nReabsorption and Secretion<\/h3>\r\nAs 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 [pb_glossary id=\"4753\"]peritubular capillary network[\/pb_glossary]). 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 [pb_glossary id=\"5451\"]glucose[\/pb_glossary] and [pb_glossary id=\"5707\"]amino acids<\/span>[\/pb_glossary]<\/span>) are reabsorbed from the filtrate by the blood in the peritubular capillary network. [pb_glossary id=\"4757\"]Reabsorption[\/pb_glossary] occurs mainly in the proximal convoluted tubule and the loop of Henle, as seen in Figure 16.4.7.<\/span>\r\n\r\nAt 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\u00a0[pb_glossary id=\"4758\"]secretion[\/pb_glossary]<\/strong>. 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 [pb_glossary id=\"5761\"]homeostasis[\/pb_glossary] 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, [pb_glossary id=\"3467\"]parathyroid hormone[\/pb_glossary] causes more calcium to be reabsorbed into the blood and more phosphorus to be secreted into the filtrate.\r\nCollection of Urine and Excretion<\/h2>\r\n[caption id=\"attachment_4760\" align=\"alignleft\" width=\"219\"] Figure 16.4.8 Fresh urine is typically yellow or amber in colour.<\/em>[\/caption]\r\n\r\nBy the time the filtrate has passed through the entire renal tubule, it has become the\u00a0liquid\u00a0waste known as [pb_glossary id=\"4717\"]urine[\/pb_glossary]. Urine empties from the distal end of the [pb_glossary id=\"4749\"]renal tubule[\/pb_glossary] into a\u00a0[pb_glossary id=\"4759\"]collecting duct[\/pb_glossary]<\/strong>. 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 [pb_glossary id=\"5533\"]antidiuretic\u00a0hormone\u00a0[\/pb_glossary]from the\u00a0posterior\u00a0[pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. This\u00a0hormone\u00a0makes the collecting ducts permeable to water, allowing water molecules to pass through them into capillaries by\u00a0[pb_glossary id=\"5663\"]osmosis[\/pb_glossary], 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.\r\n\r\nUrine finally exits the largest collecting ducts through the renal papillae. It empties into the renal calyces, and finally into the [pb_glossary id=\"4724\"]renal pelvis[\/pb_glossary]. From there, it travels through the [pb_glossary id=\"4725\"]ureter[\/pb_glossary] to the [pb_glossary id=\"4731\"]urinary bladder[\/pb_glossary] 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<\/span>). The darker the colour, generally speaking, the more concentrated the urine is.<\/span>\r\n\r\n \r\nOther Functions of the Kidneys<\/span><\/div>\r\nBesides filtering blood and forming urine for excretion of soluble wastes, the kidneys have several vital functions in maintaining body-wide\u00a0[pb_glossary id=\"5761\"]homeostasis[\/pb_glossary]. Most of these functions are related to the\u00a0composition\u00a0or volume of urine formed by the kidneys. The kidneys must maintain the proper balance of water and salts in the body, normal\u00a0[pb_glossary id=\"4480\"]blood pressure[\/pb_glossary], and the correct range of blood\u00a0[pb_glossary id=\"4330\"]pH[\/pb_glossary]. 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.\r\nBlood Pressure\u00a0Regulation<\/h2>\r\nThe kidneys do not control homeostasis all alone. As indicated above, endocrine hormones are also involved. Consider the regulation of\u00a0blood pressure\u00a0by 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\u00a0concentration\u00a0of sodium in the blood to control blood pressure.\r\n\r\n[caption id=\"attachment_4761\" align=\"aligncenter\" width=\"736\"] 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.<\/em>[\/caption]\r\n\r\nThe renin-angiotensin-aldosterone system is put into play when the\u00a0concentration\u00a0of sodium ions in the blood falls lower than normal. This causes the kidneys to secrete an\u00a0enzyme\u00a0called [pb_glossary id=\"4762\"]renin[\/pb_glossary] into the blood. It also causes the liver to secrete a\u00a0protein\u00a0called angiotensinogen. Renin changes angiotensinogen into a proto-hormone\u00a0called\u00a0angiotensin I. This is converted to angiotensin II by an\u00a0enzyme\u00a0(angiotensin-converting enzyme) in lung capillaries.\r\n\r\nAngiotensin 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\u00a0concentration\u00a0of sodium ions in the blood to normal. The increased water in the blood also increases blood volume and blood pressure.\r\nOther Kidney Hormones<\/h2>\r\nHormones other than renin are also produced and secreted by the kidneys. These include calcitriol and erythropoietin.\r\n\r\n \t- [pb_glossary id=\"4763\"]Calcitriol[\/pb_glossary]<\/strong>\u00a0is 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.<\/li>\r\n \t
- [pb_glossary id=\"4764\"]Erythropoietin[\/pb_glossary]<\/strong> is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of\u00a0 [pb_glossary id=\"4398\"]erythrocytes[\/pb_glossary]\u00a0in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.<\/li>\r\n<\/ul>\r\n\r\n
Feature: Human Biology in the News<\/h1>\r\n<\/div>\r\nKidney failure is a complication of common disorders including [pb_glossary id=\"2771\"]diabetes mellitus[\/pb_glossary] and [pb_glossary id=\"4231\"]hypertension[\/pb_glossary]. It is estimated that approximately 12.5% of Canadians have some form of kidney disease.\u00a0 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.\u00a0 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.\r\n\r\nFor 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\u2019s body in place of the failed kidney. According to Dr. Fissell, the artificial kidney is \u201c... a bio-hybrid device that can mimic a kidney to remove enough waste products, salt, and water to keep a patient off [hemo]dialysis.\u201d\r\n\r\nThe 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\u00a0cells\u00a0that can mimic the natural functions of the kidney. The living cells form a membrane to filter the patient\u2019s blood as a biological kidney would, but with less risk of rejection by the patient\u2019s immune system, because they are embedded within the device. The new kidney doesn\u2019t 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.\r\n\r\nBecause 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.\r\n\r\n\r\n16.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- 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\u00a0circulation.<\/li>\r\n \t
- 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 \u2014 which are the tiny functional units of the kidney \u2014 span the cortex and medulla. The entire kidney is surrounded by a fibrous capsule and protective fat layers.<\/li>\r\n \t
- 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.<\/li>\r\n \t
- In each nephron, the glomerulus and surrounding Bowman\u2019s capsule form the unit that filters blood. From Bowman\u2019s 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.<\/li>\r\n \t
- The kidneys control homeostasis with the help of endocrine hormones.\u00a0The 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\u00a0enzyme\u00a0renin secreted by the kidneys works with hormones from the liver and adrenal gland to stimulate nephrons to reabsorb more sodium and water from urine.<\/li>\r\n \t
- The kidneys also secrete endocrine hormones, including calcitriol \u2014 which helps control the level of calcium in the blood \u2014 and erythropoietin, which stimulates bone marrow to produce red blood\u00a0cells.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n16.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"632\"]<\/li>\r\n \t
- Contrast the renal artery and renal vein.<\/li>\r\n \t
- Identify the functions of a nephron.\u00a0Describe in detail what happens to fluids (blood, filtrate, and urine) as they pass through the parts of a nephron.<\/li>\r\n \t
- Identify two endocrine hormones secreted by the kidneys, along with the functions they control.<\/li>\r\n \t
- Name two regions in the kidney where water is reabsorbed.<\/li>\r\n \t
- Is the blood in the glomerular capillaries more or less filtered than the blood in the peritubular capillaries? Explain your answer.<\/li>\r\n \t
- What do you think would happen if\u00a0blood flow to the kidneys is blocked?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n16.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/FN3MFhYPWWo\r\nHow do your kidneys work? - Emma Bryce, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/es-t8lO1KpA\r\n
Urine Formation, Hamada Abass, 2013.<\/p>\r\nhttps:\/\/youtu.be\/bX3C201O4MA\r\n
Printing a human kidney - Anthony Atala, TED-Ed, 2013.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 16.4.1<\/strong>\r\n\r\nSteak and Kidney Pie<\/a> by Charles Haynes<\/a> on Flickr<\/a> is used under a CC BY-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/2.0\/) license.\r\n\r\nFigure 16.4.2<\/strong>\r\n\r\nGray Kidneys<\/a> by\u00a0Henry Vandyke Carter<\/a>\u00a0(1831-1897) on Wikimedia Commons is in the \u00a0public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain). (Bartleby.com<\/a>:\u00a0Gray\u2019s Anatomy<\/a>, Plate 1120<\/a>).\r\n\r\nFigure 16.4.3<\/strong>\r\n\r\nBlausen_0592_KidneyAnatomy_01<\/a> by BruceBlaus<\/a>\u00a0on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.4<\/strong>\r\n\r\nDiagram_showing_how_the_kidneys_work_CRUK_138.svg<\/a> by Cancer Research UK<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 16.4.5<\/strong>\r\n\r\nBlood_Flow_in_the_Nephron<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.6<\/strong>\r\n\r\n1024px-Physiology_of_Nephron<\/a> by Madhero88<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.7<\/strong>\r\n\r\nNephron_Secretion_Reabsorption<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.8<\/strong>\r\n\r\nUrine<\/a> by User:Markhamilton<\/a> at English Wikipedia<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 16.4.9<\/strong>\r\n\r\nRenin_Angiotensin_System-01<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\nReferences<\/h2>\r\n
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\u00a0<\/span><\/span>25.10<\/span><\/span>\u00a0<\/span>Blood flow in the nephron [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 25.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/25-3-gross-anatomy-of-the-kidney<\/span><\/span><\/p>\r\nBetts, 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\u00a0<\/span><\/span>25.17<\/span><\/span>\u00a0<\/span>Locations of secretion and reabsorption in the nephron [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 25.6). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/25-6-tubular-reabsorption<\/span><\/span><\/span><\/span><\/p>\r\nBetts, 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\u00a0<\/span><\/span>26.14<\/span><\/span>\u00a0<\/span>The renin-angiotensin system [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 26.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/26-3-electrolyte-balance<\/span><\/span><\/span><\/span><\/span><\/p>\r\nBlausen.com Staff. (2014). Medical gallery of Blausen Medical 2014.\u00a0WikiJournal of Medicine<\/i>\u00a01<\/b>\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436<\/p>\r\n
Hamada Abass. (2013). Urine formation. YouTube. https:\/\/www.youtube.com\/watch?v=es-t8lO1KpA&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, February 9). How do your kidneys work? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=FN3MFhYPWWo&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, March 15). Printing a human kidney - Anthony Atala. YouTube. https:\/\/www.youtube.com\/watch?v=bX3C201O4MA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\nFigure 16.4.1 Steak and kidney pie!<\/em><\/figcaption><\/figure>\n\nKidneys on the Menu<\/h1>\n<\/div>\n
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.<\/p>\n
\nLocation\u00a0of the Kidneys<\/h1>\n<\/div>\n
The two bean-shaped\u00a0kidneys<\/a><\/strong>\u00a0are located high in the back of the abdominal cavity<\/a>, one on each side of the spine. Both kidneys sit just below the diaphragm<\/a>, the large\u00a0breathing\u00a0muscle that separates the abdominal and thoracic cavities. As you can see in the\u00a0following\u00a0figure, the right kidney is slightly smaller and lower than the left kidney. The right kidney is behind the liver<\/a>, and the left kidney is behind the spleen<\/a>. The\u00a0location\u00a0of the liver explains why the right kidney is smaller and lower than the left.<\/p>\nFigure 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\u2019s main artery (aorta) and vein (inferior vena cava). The ureter exiting each kidney is also shown in the diagram.<\/em><\/figcaption><\/figure>\nKidney Anatomy<\/span><\/p>\nThe 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 hilum<\/a><\/strong>. The entire kidney is surrounded by tough fibrous tissue \u2014 called the\u00a0renal capsule<\/a><\/strong>\u00a0\u2014 which, in turn, is surrounded by two layers of protective, cushioning fat.<\/p>\nFigure 16.4.3 This diagram shows the location and relative size of the two kidneys, as well as the internal structure of each kidney.<\/em><\/figcaption><\/figure>\nInternally, each kidney is divided into two major layers: the outer renal cortex<\/strong><\/a> and the inner renal medulla<\/strong><\/a> (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 renal pyramid<\/strong><\/a>. Within the renal pyramids are the structural and functional units of the kidneys, the tiny nephrons<\/span><\/a><\/span>. Between the renal pyramids are projections of cortex called renal columns<\/strong><\/a>. 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 renal pelvis<\/a>, which becomes the ureter as it leaves the kidney.<\/span><\/p>\n\nRenal\u00a0Circulation<\/h3>\n<\/div>\n
The renal circulation is an important part of the kidney\u2019s primary function of filtering waste products from the blood. Blood<\/a> 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.<\/p>\nAs 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 arteries<\/a> that extend through the renal columns<\/a> between the renal pyramids<\/a>. These arteries, in turn, branch into arterioles that penetrate the renal pyramids. Blood in the arterioles passes through nephrons<\/a>, 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 veins<\/a>. Small veins merge into increasingly larger ones, and ultimately into the renal vein, which carries clean blood away from the kidney to the inferior vena cava<\/a>.<\/p>\nFigure 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).<\/em><\/figcaption><\/figure>\n\nNephron Structure and Function<\/h1>\n<\/div>\n
Figure 16.4.4 gives an indication of the complex structure of a nephron. The nephron<\/a><\/strong>\u00a0is 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 metabolism<\/a>. At least half of the waste is urea<\/a>, a waste product produced by\u00a0protein<\/a>\u00a0catabolism<\/a>. Another important waste is uric\u00a0acid<\/a>, produced in\u00a0nucleic acid<\/a>\u00a0catabolism.<\/p>\nComponents of a Nephron<\/h2>\n
Figure 16.4.5 shows in greater detail the components of a nephron<\/a>. Each nephron is composed of an initial filtering component that consists of a network of capillaries called the glomerulus<\/a><\/strong>\u00a0(plural, glomeruli), which is surrounded by a space within a structure\u00a0called\u00a0glomerular capsule<\/a> <\/strong>(also known as the Bowman’s capsule). Extending from glomerular capsule is the renal tubule<\/a><\/strong>. The proximal end (nearest glomerular capsule) of the renal tubule is called the proximal convoluted (coiled) tubule<\/a><\/strong>. From here, the renal tubule continues as a loop (known as the loop of Henle<\/strong><\/a>) (also known as the loop of the nephron), which in turn becomes the distal convoluted tubule<\/strong><\/a>. 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 peritubular capillary network<\/strong><\/a>.<\/p>\nFigure 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.<\/em><\/figcaption><\/figure>\n<\/div>\nFigure 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.<\/figcaption><\/figure>\nFunction of a Nephron<\/h2>\n
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\u2019t pass through the glomerulus \u2014 as well as blood after it passes through the glomerular capillaries \u2014 continues on through an arteriole called the efferent arteriole. The efferent arteriole follows the renal tubule of the nephron, where it continues\u00a0playing a role\u00a0in nephron functioning.<\/p>\n
<\/p>\n
Filtration<\/h3>\n
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 \u2014 called filtrate<\/a><\/strong> \u2014 pass into glomerular capsule, and from there into the proximal end of the renal tubule<\/a>.\u00a0 Anything too large to move through the pores in the glomerulus, such as blood cells, large proteins, etc., stay in the cardiovascular system.\u00a0 At this stage, filtrate (fluid in the nephron) includes water, salts, organic solids (such as nutrients), and waste products of metabolism (such as urea).<\/p>\nFigure 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. Reabsorption is the movement of substance back into the bloodstream and secretion is movement of substances from the blood into the nephron for excretion.<\/em><\/figcaption><\/figure>\nReabsorption and Secretion<\/h3>\n
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 peritubular capillary network<\/a>). 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 glucose<\/a> and amino acids<\/span><\/a><\/span>) are reabsorbed from the filtrate by the blood in the peritubular capillary network. Reabsorption<\/a> occurs mainly in the proximal convoluted tubule and the loop of Henle, as seen in Figure 16.4.7.<\/span><\/p>\nAt 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\u00a0secretion<\/a><\/strong>. 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 homeostasis<\/a> 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, parathyroid hormone<\/a> causes more calcium to be reabsorbed into the blood and more phosphorus to be secreted into the filtrate.<\/p>\nCollection of Urine and Excretion<\/h2>\nFigure 16.4.8 Fresh urine is typically yellow or amber in colour.<\/em><\/figcaption><\/figure>\nBy the time the filtrate has passed through the entire renal tubule, it has become the\u00a0liquid\u00a0waste known as urine<\/a>. Urine empties from the distal end of the renal tubule<\/a> into a\u00a0collecting duct<\/a><\/strong>. 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 antidiuretic\u00a0hormone\u00a0<\/a>from the\u00a0posterior\u00a0pituitary gland<\/a>. This\u00a0hormone\u00a0makes the collecting ducts permeable to water, allowing water molecules to pass through them into capillaries by\u00a0osmosis<\/a>, 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.<\/p>\nUrine finally exits the largest collecting ducts through the renal papillae. It empties into the renal calyces, and finally into the renal pelvis<\/a>. From there, it travels through the ureter<\/a> to the urinary bladder<\/a> 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<\/span>). The darker the colour, generally speaking, the more concentrated the urine is.<\/span><\/p>\n <\/p>\n
Other Functions of the Kidneys<\/span><\/div>\nBesides filtering blood and forming urine for excretion of soluble wastes, the kidneys have several vital functions in maintaining body-wide\u00a0homeostasis<\/a>. Most of these functions are related to the\u00a0composition\u00a0or volume of urine formed by the kidneys. The kidneys must maintain the proper balance of water and salts in the body, normal\u00a0blood pressure<\/a>, and the correct range of blood\u00a0pH<\/a>. 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.<\/p>\nBlood Pressure\u00a0Regulation<\/h2>\n
The kidneys do not control homeostasis all alone. As indicated above, endocrine hormones are also involved. Consider the regulation of\u00a0blood pressure\u00a0by 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\u00a0concentration\u00a0of sodium in the blood to control blood pressure.<\/p>\nFigure 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.<\/em><\/figcaption><\/figure>\nThe renin-angiotensin-aldosterone system is put into play when the\u00a0concentration\u00a0of sodium ions in the blood falls lower than normal. This causes the kidneys to secrete an\u00a0enzyme\u00a0called renin<\/a> into the blood. It also causes the liver to secrete a\u00a0protein\u00a0called angiotensinogen. Renin changes angiotensinogen into a proto-hormone\u00a0called\u00a0angiotensin I. This is converted to angiotensin II by an\u00a0enzyme\u00a0(angiotensin-converting enzyme) in lung capillaries.<\/p>\nAngiotensin 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\u00a0concentration\u00a0of sodium ions in the blood to normal. The increased water in the blood also increases blood volume and blood pressure.<\/p>\n
Other Kidney Hormones<\/h2>\n
Hormones other than renin are also produced and secreted by the kidneys. These include calcitriol and erythropoietin.<\/p>\n
\n- Calcitriol<\/a><\/strong>\u00a0is 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.<\/li>\n
- Erythropoietin<\/a><\/strong> is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of\u00a0 erythrocytes<\/a>\u00a0in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.<\/li>\n<\/ul>\n\n
Feature: Human Biology in the News<\/h1>\n<\/div>\n
Kidney failure is a complication of common disorders including diabetes mellitus<\/a> and hypertension<\/a>. It is estimated that approximately 12.5% of Canadians have some form of kidney disease.\u00a0 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.\u00a0 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.<\/p>\nFor 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\u2019s body in place of the failed kidney. According to Dr. Fissell, the artificial kidney is \u201c… a bio-hybrid device that can mimic a kidney to remove enough waste products, salt, and water to keep a patient off [hemo]dialysis.\u201d<\/p>\n
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\u00a0cells\u00a0that can mimic the natural functions of the kidney. The living cells form a membrane to filter the patient\u2019s blood as a biological kidney would, but with less risk of rejection by the patient\u2019s immune system, because they are embedded within the device. The new kidney doesn\u2019t 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.<\/p>\n
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.<\/p>\n
\n\n\n16.4 Summary<\/span><\/h1>\n<\/header>\n\n\n
Renal\u00a0Circulation<\/h3>\r\n<\/div>\r\nThe renal circulation is an important part of the kidney\u2019s primary function of filtering waste products from the blood. [pb_glossary id=\"2702\"]Blood[\/pb_glossary] 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.\r\n\r\nAs 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 [pb_glossary id=\"4385\"]arteries[\/pb_glossary] that extend through the [pb_glossary id=\"4744\"]renal columns[\/pb_glossary] between the [pb_glossary id=\"4743\"]renal pyramids[\/pb_glossary]. These arteries, in turn, branch into arterioles that penetrate the renal pyramids. Blood in the arterioles passes through [pb_glossary id=\"4718\"]nephrons[\/pb_glossary], 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 [pb_glossary id=\"4386\"]veins[\/pb_glossary]. Small veins merge into increasingly larger ones, and ultimately into the renal vein, which carries clean blood away from the kidney to the inferior [pb_glossary id=\"4410\"]vena cava[\/pb_glossary].\r\n\r\n[caption id=\"attachment_4745\" align=\"aligncenter\" width=\"683\"] 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).<\/em>[\/caption]\r\n\r\n\r\nNephron Structure and Function<\/h1>\r\n<\/div>\r\nFigure 16.4.4 gives an indication of the complex structure of a nephron. The [pb_glossary id=\"4718\"]nephron[\/pb_glossary]<\/strong>\u00a0is 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 [pb_glossary id=\"5773\"]metabolism[\/pb_glossary]. At least half of the waste is [pb_glossary id=\"4708\"]urea[\/pb_glossary], a waste product produced by\u00a0[pb_glossary id=\"5813\"]protein[\/pb_glossary]\u00a0[pb_glossary id=\"5721\"]catabolism[\/pb_glossary]. Another important waste is [pb_glossary id=\"4709\"]uric\u00a0acid[\/pb_glossary], produced in\u00a0[pb_glossary id=\"4727\"]nucleic acid[\/pb_glossary]\u00a0catabolism.\r\nComponents of a Nephron<\/h2>\r\nFigure 16.4.5 shows in greater detail the components of a [pb_glossary id=\"4718\"]nephron[\/pb_glossary]. Each nephron is composed of an initial filtering component that consists of a network of capillaries called the [pb_glossary id=\"4747\"]glomerulus[\/pb_glossary]<\/strong>\u00a0(plural, glomeruli), which is surrounded by a space within a structure\u00a0called\u00a0[pb_glossary id=\"4748\"]glomerular capsule[\/pb_glossary] <\/strong>(also known as the Bowman's capsule). Extending from glomerular capsule is the [pb_glossary id=\"4749\"]renal tubule[\/pb_glossary]<\/strong>. The proximal end (nearest glomerular capsule) of the renal tubule is called the [pb_glossary id=\"4750\"]proximal convoluted (coiled) tubule[\/pb_glossary]<\/strong>. From here, the renal tubule continues as a loop (known as the [pb_glossary id=\"4751\"]loop of Henle<\/strong>[\/pb_glossary]) (also known as the loop of the nephron), which in turn becomes the [pb_glossary id=\"4752\"]distal convoluted tubule<\/strong>[\/pb_glossary]. 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 [pb_glossary id=\"4753\"]peritubular capillary network<\/strong>[\/pb_glossary].\r\n\r\n[caption id=\"attachment_4746\" align=\"aligncenter\" width=\"425\"] 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.<\/em>[\/caption]\r\n\r\n<\/div>\r\n\r\n[caption id=\"attachment_4754\" align=\"alignright\" width=\"257\"] 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.[\/caption]\r\nFunction of a Nephron<\/h2>\r\nThe 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\u2019t pass through the glomerulus \u2014 as well as blood after it passes through the glomerular capillaries \u2014 continues on through an arteriole called the efferent arteriole. The efferent arteriole follows the renal tubule of the nephron, where it continues\u00a0playing a role\u00a0in nephron functioning.\r\n\r\n \r\nFiltration<\/h3>\r\nAs 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 \u2014 called [pb_glossary id=\"5471\"]filtrate[\/pb_glossary]<\/strong> \u2014 pass into glomerular capsule, and from there into the proximal end of the [pb_glossary id=\"4749\"]renal tubule[\/pb_glossary].\u00a0 Anything too large to move through the pores in the glomerulus, such as blood cells, large proteins, etc., stay in the cardiovascular system.\u00a0 At this stage, filtrate (fluid in the nephron) includes water, salts, organic solids (such as nutrients), and waste products of metabolism (such as urea).\r\n\r\n[caption id=\"attachment_4756\" align=\"alignright\" width=\"403\"] 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. Reabsorption is the movement of substance back into the bloodstream and secretion is movement of substances from the blood into the nephron for excretion.<\/em>[\/caption]\r\nReabsorption and Secretion<\/h3>\r\nAs 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 [pb_glossary id=\"4753\"]peritubular capillary network[\/pb_glossary]). 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 [pb_glossary id=\"5451\"]glucose[\/pb_glossary] and [pb_glossary id=\"5707\"]amino acids<\/span>[\/pb_glossary]<\/span>) are reabsorbed from the filtrate by the blood in the peritubular capillary network. [pb_glossary id=\"4757\"]Reabsorption[\/pb_glossary] occurs mainly in the proximal convoluted tubule and the loop of Henle, as seen in Figure 16.4.7.<\/span>\r\n\r\nAt 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\u00a0[pb_glossary id=\"4758\"]secretion[\/pb_glossary]<\/strong>. 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 [pb_glossary id=\"5761\"]homeostasis[\/pb_glossary] 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, [pb_glossary id=\"3467\"]parathyroid hormone[\/pb_glossary] causes more calcium to be reabsorbed into the blood and more phosphorus to be secreted into the filtrate.\r\nCollection of Urine and Excretion<\/h2>\r\n[caption id=\"attachment_4760\" align=\"alignleft\" width=\"219\"] Figure 16.4.8 Fresh urine is typically yellow or amber in colour.<\/em>[\/caption]\r\n\r\nBy the time the filtrate has passed through the entire renal tubule, it has become the\u00a0liquid\u00a0waste known as [pb_glossary id=\"4717\"]urine[\/pb_glossary]. Urine empties from the distal end of the [pb_glossary id=\"4749\"]renal tubule[\/pb_glossary] into a\u00a0[pb_glossary id=\"4759\"]collecting duct[\/pb_glossary]<\/strong>. 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 [pb_glossary id=\"5533\"]antidiuretic\u00a0hormone\u00a0[\/pb_glossary]from the\u00a0posterior\u00a0[pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. This\u00a0hormone\u00a0makes the collecting ducts permeable to water, allowing water molecules to pass through them into capillaries by\u00a0[pb_glossary id=\"5663\"]osmosis[\/pb_glossary], 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.\r\n\r\nUrine finally exits the largest collecting ducts through the renal papillae. It empties into the renal calyces, and finally into the [pb_glossary id=\"4724\"]renal pelvis[\/pb_glossary]. From there, it travels through the [pb_glossary id=\"4725\"]ureter[\/pb_glossary] to the [pb_glossary id=\"4731\"]urinary bladder[\/pb_glossary] 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<\/span>). The darker the colour, generally speaking, the more concentrated the urine is.<\/span>\r\n\r\n \r\nOther Functions of the Kidneys<\/span><\/div>\r\nBesides filtering blood and forming urine for excretion of soluble wastes, the kidneys have several vital functions in maintaining body-wide\u00a0[pb_glossary id=\"5761\"]homeostasis[\/pb_glossary]. Most of these functions are related to the\u00a0composition\u00a0or volume of urine formed by the kidneys. The kidneys must maintain the proper balance of water and salts in the body, normal\u00a0[pb_glossary id=\"4480\"]blood pressure[\/pb_glossary], and the correct range of blood\u00a0[pb_glossary id=\"4330\"]pH[\/pb_glossary]. 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.\r\nBlood Pressure\u00a0Regulation<\/h2>\r\nThe kidneys do not control homeostasis all alone. As indicated above, endocrine hormones are also involved. Consider the regulation of\u00a0blood pressure\u00a0by 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\u00a0concentration\u00a0of sodium in the blood to control blood pressure.\r\n\r\n[caption id=\"attachment_4761\" align=\"aligncenter\" width=\"736\"] 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.<\/em>[\/caption]\r\n\r\nThe renin-angiotensin-aldosterone system is put into play when the\u00a0concentration\u00a0of sodium ions in the blood falls lower than normal. This causes the kidneys to secrete an\u00a0enzyme\u00a0called [pb_glossary id=\"4762\"]renin[\/pb_glossary] into the blood. It also causes the liver to secrete a\u00a0protein\u00a0called angiotensinogen. Renin changes angiotensinogen into a proto-hormone\u00a0called\u00a0angiotensin I. This is converted to angiotensin II by an\u00a0enzyme\u00a0(angiotensin-converting enzyme) in lung capillaries.\r\n\r\nAngiotensin 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\u00a0concentration\u00a0of sodium ions in the blood to normal. The increased water in the blood also increases blood volume and blood pressure.\r\nOther Kidney Hormones<\/h2>\r\nHormones other than renin are also produced and secreted by the kidneys. These include calcitriol and erythropoietin.\r\n\r\n \t- [pb_glossary id=\"4763\"]Calcitriol[\/pb_glossary]<\/strong>\u00a0is 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.<\/li>\r\n \t
- [pb_glossary id=\"4764\"]Erythropoietin[\/pb_glossary]<\/strong> is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of\u00a0 [pb_glossary id=\"4398\"]erythrocytes[\/pb_glossary]\u00a0in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.<\/li>\r\n<\/ul>\r\n\r\n
Feature: Human Biology in the News<\/h1>\r\n<\/div>\r\nKidney failure is a complication of common disorders including [pb_glossary id=\"2771\"]diabetes mellitus[\/pb_glossary] and [pb_glossary id=\"4231\"]hypertension[\/pb_glossary]. It is estimated that approximately 12.5% of Canadians have some form of kidney disease.\u00a0 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.\u00a0 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.\r\n\r\nFor 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\u2019s body in place of the failed kidney. According to Dr. Fissell, the artificial kidney is \u201c... a bio-hybrid device that can mimic a kidney to remove enough waste products, salt, and water to keep a patient off [hemo]dialysis.\u201d\r\n\r\nThe 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\u00a0cells\u00a0that can mimic the natural functions of the kidney. The living cells form a membrane to filter the patient\u2019s blood as a biological kidney would, but with less risk of rejection by the patient\u2019s immune system, because they are embedded within the device. The new kidney doesn\u2019t 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.\r\n\r\nBecause 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.\r\n\r\n\r\n16.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- 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\u00a0circulation.<\/li>\r\n \t
- 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 \u2014 which are the tiny functional units of the kidney \u2014 span the cortex and medulla. The entire kidney is surrounded by a fibrous capsule and protective fat layers.<\/li>\r\n \t
- 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.<\/li>\r\n \t
- In each nephron, the glomerulus and surrounding Bowman\u2019s capsule form the unit that filters blood. From Bowman\u2019s 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.<\/li>\r\n \t
- The kidneys control homeostasis with the help of endocrine hormones.\u00a0The 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\u00a0enzyme\u00a0renin secreted by the kidneys works with hormones from the liver and adrenal gland to stimulate nephrons to reabsorb more sodium and water from urine.<\/li>\r\n \t
- The kidneys also secrete endocrine hormones, including calcitriol \u2014 which helps control the level of calcium in the blood \u2014 and erythropoietin, which stimulates bone marrow to produce red blood\u00a0cells.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n16.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"632\"]<\/li>\r\n \t
- Contrast the renal artery and renal vein.<\/li>\r\n \t
- Identify the functions of a nephron.\u00a0Describe in detail what happens to fluids (blood, filtrate, and urine) as they pass through the parts of a nephron.<\/li>\r\n \t
- Identify two endocrine hormones secreted by the kidneys, along with the functions they control.<\/li>\r\n \t
- Name two regions in the kidney where water is reabsorbed.<\/li>\r\n \t
- Is the blood in the glomerular capillaries more or less filtered than the blood in the peritubular capillaries? Explain your answer.<\/li>\r\n \t
- What do you think would happen if\u00a0blood flow to the kidneys is blocked?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n16.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/FN3MFhYPWWo\r\nHow do your kidneys work? - Emma Bryce, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/es-t8lO1KpA\r\n
Urine Formation, Hamada Abass, 2013.<\/p>\r\nhttps:\/\/youtu.be\/bX3C201O4MA\r\n
Printing a human kidney - Anthony Atala, TED-Ed, 2013.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 16.4.1<\/strong>\r\n\r\nSteak and Kidney Pie<\/a> by Charles Haynes<\/a> on Flickr<\/a> is used under a CC BY-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/2.0\/) license.\r\n\r\nFigure 16.4.2<\/strong>\r\n\r\nGray Kidneys<\/a> by\u00a0Henry Vandyke Carter<\/a>\u00a0(1831-1897) on Wikimedia Commons is in the \u00a0public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain). (Bartleby.com<\/a>:\u00a0Gray\u2019s Anatomy<\/a>, Plate 1120<\/a>).\r\n\r\nFigure 16.4.3<\/strong>\r\n\r\nBlausen_0592_KidneyAnatomy_01<\/a> by BruceBlaus<\/a>\u00a0on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.4<\/strong>\r\n\r\nDiagram_showing_how_the_kidneys_work_CRUK_138.svg<\/a> by Cancer Research UK<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 16.4.5<\/strong>\r\n\r\nBlood_Flow_in_the_Nephron<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.6<\/strong>\r\n\r\n1024px-Physiology_of_Nephron<\/a> by Madhero88<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.7<\/strong>\r\n\r\nNephron_Secretion_Reabsorption<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.8<\/strong>\r\n\r\nUrine<\/a> by User:Markhamilton<\/a> at English Wikipedia<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 16.4.9<\/strong>\r\n\r\nRenin_Angiotensin_System-01<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\nReferences<\/h2>\r\n
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\u00a0<\/span><\/span>25.10<\/span><\/span>\u00a0<\/span>Blood flow in the nephron [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 25.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/25-3-gross-anatomy-of-the-kidney<\/span><\/span><\/p>\r\nBetts, 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\u00a0<\/span><\/span>25.17<\/span><\/span>\u00a0<\/span>Locations of secretion and reabsorption in the nephron [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 25.6). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/25-6-tubular-reabsorption<\/span><\/span><\/span><\/span><\/p>\r\nBetts, 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\u00a0<\/span><\/span>26.14<\/span><\/span>\u00a0<\/span>The renin-angiotensin system [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 26.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/26-3-electrolyte-balance<\/span><\/span><\/span><\/span><\/span><\/p>\r\nBlausen.com Staff. (2014). Medical gallery of Blausen Medical 2014.\u00a0WikiJournal of Medicine<\/i>\u00a01<\/b>\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436<\/p>\r\n
Hamada Abass. (2013). Urine formation. YouTube. https:\/\/www.youtube.com\/watch?v=es-t8lO1KpA&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, February 9). How do your kidneys work? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=FN3MFhYPWWo&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, March 15). Printing a human kidney - Anthony Atala. YouTube. https:\/\/www.youtube.com\/watch?v=bX3C201O4MA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\nFigure 16.4.1 Steak and kidney pie!<\/em><\/figcaption><\/figure>\n\nKidneys on the Menu<\/h1>\n<\/div>\n
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.<\/p>\n
\nLocation\u00a0of the Kidneys<\/h1>\n<\/div>\n
The two bean-shaped\u00a0kidneys<\/a><\/strong>\u00a0are located high in the back of the abdominal cavity<\/a>, one on each side of the spine. Both kidneys sit just below the diaphragm<\/a>, the large\u00a0breathing\u00a0muscle that separates the abdominal and thoracic cavities. As you can see in the\u00a0following\u00a0figure, the right kidney is slightly smaller and lower than the left kidney. The right kidney is behind the liver<\/a>, and the left kidney is behind the spleen<\/a>. The\u00a0location\u00a0of the liver explains why the right kidney is smaller and lower than the left.<\/p>\nFigure 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\u2019s main artery (aorta) and vein (inferior vena cava). The ureter exiting each kidney is also shown in the diagram.<\/em><\/figcaption><\/figure>\nKidney Anatomy<\/span><\/p>\nThe 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 hilum<\/a><\/strong>. The entire kidney is surrounded by tough fibrous tissue \u2014 called the\u00a0renal capsule<\/a><\/strong>\u00a0\u2014 which, in turn, is surrounded by two layers of protective, cushioning fat.<\/p>\nFigure 16.4.3 This diagram shows the location and relative size of the two kidneys, as well as the internal structure of each kidney.<\/em><\/figcaption><\/figure>\nInternally, each kidney is divided into two major layers: the outer renal cortex<\/strong><\/a> and the inner renal medulla<\/strong><\/a> (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 renal pyramid<\/strong><\/a>. Within the renal pyramids are the structural and functional units of the kidneys, the tiny nephrons<\/span><\/a><\/span>. Between the renal pyramids are projections of cortex called renal columns<\/strong><\/a>. 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 renal pelvis<\/a>, which becomes the ureter as it leaves the kidney.<\/span><\/p>\n\nRenal\u00a0Circulation<\/h3>\n<\/div>\n
The renal circulation is an important part of the kidney\u2019s primary function of filtering waste products from the blood. Blood<\/a> 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.<\/p>\nAs 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 arteries<\/a> that extend through the renal columns<\/a> between the renal pyramids<\/a>. These arteries, in turn, branch into arterioles that penetrate the renal pyramids. Blood in the arterioles passes through nephrons<\/a>, 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 veins<\/a>. Small veins merge into increasingly larger ones, and ultimately into the renal vein, which carries clean blood away from the kidney to the inferior vena cava<\/a>.<\/p>\nFigure 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).<\/em><\/figcaption><\/figure>\n\nNephron Structure and Function<\/h1>\n<\/div>\n
Figure 16.4.4 gives an indication of the complex structure of a nephron. The nephron<\/a><\/strong>\u00a0is 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 metabolism<\/a>. At least half of the waste is urea<\/a>, a waste product produced by\u00a0protein<\/a>\u00a0catabolism<\/a>. Another important waste is uric\u00a0acid<\/a>, produced in\u00a0nucleic acid<\/a>\u00a0catabolism.<\/p>\nComponents of a Nephron<\/h2>\n
Figure 16.4.5 shows in greater detail the components of a nephron<\/a>. Each nephron is composed of an initial filtering component that consists of a network of capillaries called the glomerulus<\/a><\/strong>\u00a0(plural, glomeruli), which is surrounded by a space within a structure\u00a0called\u00a0glomerular capsule<\/a> <\/strong>(also known as the Bowman’s capsule). Extending from glomerular capsule is the renal tubule<\/a><\/strong>. The proximal end (nearest glomerular capsule) of the renal tubule is called the proximal convoluted (coiled) tubule<\/a><\/strong>. From here, the renal tubule continues as a loop (known as the loop of Henle<\/strong><\/a>) (also known as the loop of the nephron), which in turn becomes the distal convoluted tubule<\/strong><\/a>. 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 peritubular capillary network<\/strong><\/a>.<\/p>\nFigure 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.<\/em><\/figcaption><\/figure>\n<\/div>\nFigure 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.<\/figcaption><\/figure>\nFunction of a Nephron<\/h2>\n
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\u2019t pass through the glomerulus \u2014 as well as blood after it passes through the glomerular capillaries \u2014 continues on through an arteriole called the efferent arteriole. The efferent arteriole follows the renal tubule of the nephron, where it continues\u00a0playing a role\u00a0in nephron functioning.<\/p>\n
<\/p>\n
Filtration<\/h3>\n
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 \u2014 called filtrate<\/a><\/strong> \u2014 pass into glomerular capsule, and from there into the proximal end of the renal tubule<\/a>.\u00a0 Anything too large to move through the pores in the glomerulus, such as blood cells, large proteins, etc., stay in the cardiovascular system.\u00a0 At this stage, filtrate (fluid in the nephron) includes water, salts, organic solids (such as nutrients), and waste products of metabolism (such as urea).<\/p>\nFigure 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. Reabsorption is the movement of substance back into the bloodstream and secretion is movement of substances from the blood into the nephron for excretion.<\/em><\/figcaption><\/figure>\nReabsorption and Secretion<\/h3>\n
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 peritubular capillary network<\/a>). 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 glucose<\/a> and amino acids<\/span><\/a><\/span>) are reabsorbed from the filtrate by the blood in the peritubular capillary network. Reabsorption<\/a> occurs mainly in the proximal convoluted tubule and the loop of Henle, as seen in Figure 16.4.7.<\/span><\/p>\nAt 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\u00a0secretion<\/a><\/strong>. 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 homeostasis<\/a> 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, parathyroid hormone<\/a> causes more calcium to be reabsorbed into the blood and more phosphorus to be secreted into the filtrate.<\/p>\nCollection of Urine and Excretion<\/h2>\nFigure 16.4.8 Fresh urine is typically yellow or amber in colour.<\/em><\/figcaption><\/figure>\nBy the time the filtrate has passed through the entire renal tubule, it has become the\u00a0liquid\u00a0waste known as urine<\/a>. Urine empties from the distal end of the renal tubule<\/a> into a\u00a0collecting duct<\/a><\/strong>. 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 antidiuretic\u00a0hormone\u00a0<\/a>from the\u00a0posterior\u00a0pituitary gland<\/a>. This\u00a0hormone\u00a0makes the collecting ducts permeable to water, allowing water molecules to pass through them into capillaries by\u00a0osmosis<\/a>, 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.<\/p>\nUrine finally exits the largest collecting ducts through the renal papillae. It empties into the renal calyces, and finally into the renal pelvis<\/a>. From there, it travels through the ureter<\/a> to the urinary bladder<\/a> 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<\/span>). The darker the colour, generally speaking, the more concentrated the urine is.<\/span><\/p>\n <\/p>\n
Other Functions of the Kidneys<\/span><\/div>\nBesides filtering blood and forming urine for excretion of soluble wastes, the kidneys have several vital functions in maintaining body-wide\u00a0homeostasis<\/a>. Most of these functions are related to the\u00a0composition\u00a0or volume of urine formed by the kidneys. The kidneys must maintain the proper balance of water and salts in the body, normal\u00a0blood pressure<\/a>, and the correct range of blood\u00a0pH<\/a>. 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.<\/p>\nBlood Pressure\u00a0Regulation<\/h2>\n
The kidneys do not control homeostasis all alone. As indicated above, endocrine hormones are also involved. Consider the regulation of\u00a0blood pressure\u00a0by 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\u00a0concentration\u00a0of sodium in the blood to control blood pressure.<\/p>\nFigure 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.<\/em><\/figcaption><\/figure>\nThe renin-angiotensin-aldosterone system is put into play when the\u00a0concentration\u00a0of sodium ions in the blood falls lower than normal. This causes the kidneys to secrete an\u00a0enzyme\u00a0called renin<\/a> into the blood. It also causes the liver to secrete a\u00a0protein\u00a0called angiotensinogen. Renin changes angiotensinogen into a proto-hormone\u00a0called\u00a0angiotensin I. This is converted to angiotensin II by an\u00a0enzyme\u00a0(angiotensin-converting enzyme) in lung capillaries.<\/p>\nAngiotensin 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\u00a0concentration\u00a0of sodium ions in the blood to normal. The increased water in the blood also increases blood volume and blood pressure.<\/p>\n
Other Kidney Hormones<\/h2>\n
Hormones other than renin are also produced and secreted by the kidneys. These include calcitriol and erythropoietin.<\/p>\n
\n- Calcitriol<\/a><\/strong>\u00a0is 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.<\/li>\n
- Erythropoietin<\/a><\/strong> is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of\u00a0 erythrocytes<\/a>\u00a0in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.<\/li>\n<\/ul>\n\n
Feature: Human Biology in the News<\/h1>\n<\/div>\n
Kidney failure is a complication of common disorders including diabetes mellitus<\/a> and hypertension<\/a>. It is estimated that approximately 12.5% of Canadians have some form of kidney disease.\u00a0 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.\u00a0 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.<\/p>\nFor 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\u2019s body in place of the failed kidney. According to Dr. Fissell, the artificial kidney is \u201c… a bio-hybrid device that can mimic a kidney to remove enough waste products, salt, and water to keep a patient off [hemo]dialysis.\u201d<\/p>\n
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\u00a0cells\u00a0that can mimic the natural functions of the kidney. The living cells form a membrane to filter the patient\u2019s blood as a biological kidney would, but with less risk of rejection by the patient\u2019s immune system, because they are embedded within the device. The new kidney doesn\u2019t 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.<\/p>\n
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.<\/p>\n
\n\n\n16.4 Summary<\/span><\/h1>\n<\/header>\n\n\n
Nephron Structure and Function<\/h1>\r\n<\/div>\r\nFigure 16.4.4 gives an indication of the complex structure of a nephron. The [pb_glossary id=\"4718\"]nephron[\/pb_glossary]<\/strong>\u00a0is 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 [pb_glossary id=\"5773\"]metabolism[\/pb_glossary]. At least half of the waste is [pb_glossary id=\"4708\"]urea[\/pb_glossary], a waste product produced by\u00a0[pb_glossary id=\"5813\"]protein[\/pb_glossary]\u00a0[pb_glossary id=\"5721\"]catabolism[\/pb_glossary]. Another important waste is [pb_glossary id=\"4709\"]uric\u00a0acid[\/pb_glossary], produced in\u00a0[pb_glossary id=\"4727\"]nucleic acid[\/pb_glossary]\u00a0catabolism.\r\nComponents of a Nephron<\/h2>\r\nFigure 16.4.5 shows in greater detail the components of a [pb_glossary id=\"4718\"]nephron[\/pb_glossary]. Each nephron is composed of an initial filtering component that consists of a network of capillaries called the [pb_glossary id=\"4747\"]glomerulus[\/pb_glossary]<\/strong>\u00a0(plural, glomeruli), which is surrounded by a space within a structure\u00a0called\u00a0[pb_glossary id=\"4748\"]glomerular capsule[\/pb_glossary] <\/strong>(also known as the Bowman's capsule). Extending from glomerular capsule is the [pb_glossary id=\"4749\"]renal tubule[\/pb_glossary]<\/strong>. The proximal end (nearest glomerular capsule) of the renal tubule is called the [pb_glossary id=\"4750\"]proximal convoluted (coiled) tubule[\/pb_glossary]<\/strong>. From here, the renal tubule continues as a loop (known as the [pb_glossary id=\"4751\"]loop of Henle<\/strong>[\/pb_glossary]) (also known as the loop of the nephron), which in turn becomes the [pb_glossary id=\"4752\"]distal convoluted tubule<\/strong>[\/pb_glossary]. 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 [pb_glossary id=\"4753\"]peritubular capillary network<\/strong>[\/pb_glossary].\r\n\r\n[caption id=\"attachment_4746\" align=\"aligncenter\" width=\"425\"] 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.<\/em>[\/caption]\r\n\r\n<\/div>\r\n\r\n[caption id=\"attachment_4754\" align=\"alignright\" width=\"257\"] 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.[\/caption]\r\nFunction of a Nephron<\/h2>\r\nThe 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\u2019t pass through the glomerulus \u2014 as well as blood after it passes through the glomerular capillaries \u2014 continues on through an arteriole called the efferent arteriole. The efferent arteriole follows the renal tubule of the nephron, where it continues\u00a0playing a role\u00a0in nephron functioning.\r\n\r\n \r\nFiltration<\/h3>\r\nAs 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 \u2014 called [pb_glossary id=\"5471\"]filtrate[\/pb_glossary]<\/strong> \u2014 pass into glomerular capsule, and from there into the proximal end of the [pb_glossary id=\"4749\"]renal tubule[\/pb_glossary].\u00a0 Anything too large to move through the pores in the glomerulus, such as blood cells, large proteins, etc., stay in the cardiovascular system.\u00a0 At this stage, filtrate (fluid in the nephron) includes water, salts, organic solids (such as nutrients), and waste products of metabolism (such as urea).\r\n\r\n[caption id=\"attachment_4756\" align=\"alignright\" width=\"403\"] 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. Reabsorption is the movement of substance back into the bloodstream and secretion is movement of substances from the blood into the nephron for excretion.<\/em>[\/caption]\r\nReabsorption and Secretion<\/h3>\r\nAs 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 [pb_glossary id=\"4753\"]peritubular capillary network[\/pb_glossary]). 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 [pb_glossary id=\"5451\"]glucose[\/pb_glossary] and [pb_glossary id=\"5707\"]amino acids<\/span>[\/pb_glossary]<\/span>) are reabsorbed from the filtrate by the blood in the peritubular capillary network. [pb_glossary id=\"4757\"]Reabsorption[\/pb_glossary] occurs mainly in the proximal convoluted tubule and the loop of Henle, as seen in Figure 16.4.7.<\/span>\r\n\r\nAt 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\u00a0[pb_glossary id=\"4758\"]secretion[\/pb_glossary]<\/strong>. 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 [pb_glossary id=\"5761\"]homeostasis[\/pb_glossary] 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, [pb_glossary id=\"3467\"]parathyroid hormone[\/pb_glossary] causes more calcium to be reabsorbed into the blood and more phosphorus to be secreted into the filtrate.\r\nCollection of Urine and Excretion<\/h2>\r\n[caption id=\"attachment_4760\" align=\"alignleft\" width=\"219\"] Figure 16.4.8 Fresh urine is typically yellow or amber in colour.<\/em>[\/caption]\r\n\r\nBy the time the filtrate has passed through the entire renal tubule, it has become the\u00a0liquid\u00a0waste known as [pb_glossary id=\"4717\"]urine[\/pb_glossary]. Urine empties from the distal end of the [pb_glossary id=\"4749\"]renal tubule[\/pb_glossary] into a\u00a0[pb_glossary id=\"4759\"]collecting duct[\/pb_glossary]<\/strong>. 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 [pb_glossary id=\"5533\"]antidiuretic\u00a0hormone\u00a0[\/pb_glossary]from the\u00a0posterior\u00a0[pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. This\u00a0hormone\u00a0makes the collecting ducts permeable to water, allowing water molecules to pass through them into capillaries by\u00a0[pb_glossary id=\"5663\"]osmosis[\/pb_glossary], 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.\r\n\r\nUrine finally exits the largest collecting ducts through the renal papillae. It empties into the renal calyces, and finally into the [pb_glossary id=\"4724\"]renal pelvis[\/pb_glossary]. From there, it travels through the [pb_glossary id=\"4725\"]ureter[\/pb_glossary] to the [pb_glossary id=\"4731\"]urinary bladder[\/pb_glossary] 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<\/span>). The darker the colour, generally speaking, the more concentrated the urine is.<\/span>\r\n\r\n \r\nOther Functions of the Kidneys<\/span><\/div>\r\nBesides filtering blood and forming urine for excretion of soluble wastes, the kidneys have several vital functions in maintaining body-wide\u00a0[pb_glossary id=\"5761\"]homeostasis[\/pb_glossary]. Most of these functions are related to the\u00a0composition\u00a0or volume of urine formed by the kidneys. The kidneys must maintain the proper balance of water and salts in the body, normal\u00a0[pb_glossary id=\"4480\"]blood pressure[\/pb_glossary], and the correct range of blood\u00a0[pb_glossary id=\"4330\"]pH[\/pb_glossary]. 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.\r\nBlood Pressure\u00a0Regulation<\/h2>\r\nThe kidneys do not control homeostasis all alone. As indicated above, endocrine hormones are also involved. Consider the regulation of\u00a0blood pressure\u00a0by 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\u00a0concentration\u00a0of sodium in the blood to control blood pressure.\r\n\r\n[caption id=\"attachment_4761\" align=\"aligncenter\" width=\"736\"] 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.<\/em>[\/caption]\r\n\r\nThe renin-angiotensin-aldosterone system is put into play when the\u00a0concentration\u00a0of sodium ions in the blood falls lower than normal. This causes the kidneys to secrete an\u00a0enzyme\u00a0called [pb_glossary id=\"4762\"]renin[\/pb_glossary] into the blood. It also causes the liver to secrete a\u00a0protein\u00a0called angiotensinogen. Renin changes angiotensinogen into a proto-hormone\u00a0called\u00a0angiotensin I. This is converted to angiotensin II by an\u00a0enzyme\u00a0(angiotensin-converting enzyme) in lung capillaries.\r\n\r\nAngiotensin 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\u00a0concentration\u00a0of sodium ions in the blood to normal. The increased water in the blood also increases blood volume and blood pressure.\r\nOther Kidney Hormones<\/h2>\r\nHormones other than renin are also produced and secreted by the kidneys. These include calcitriol and erythropoietin.\r\n\r\n \t- [pb_glossary id=\"4763\"]Calcitriol[\/pb_glossary]<\/strong>\u00a0is 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.<\/li>\r\n \t
- [pb_glossary id=\"4764\"]Erythropoietin[\/pb_glossary]<\/strong> is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of\u00a0 [pb_glossary id=\"4398\"]erythrocytes[\/pb_glossary]\u00a0in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.<\/li>\r\n<\/ul>\r\n\r\n
Feature: Human Biology in the News<\/h1>\r\n<\/div>\r\nKidney failure is a complication of common disorders including [pb_glossary id=\"2771\"]diabetes mellitus[\/pb_glossary] and [pb_glossary id=\"4231\"]hypertension[\/pb_glossary]. It is estimated that approximately 12.5% of Canadians have some form of kidney disease.\u00a0 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.\u00a0 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.\r\n\r\nFor 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\u2019s body in place of the failed kidney. According to Dr. Fissell, the artificial kidney is \u201c... a bio-hybrid device that can mimic a kidney to remove enough waste products, salt, and water to keep a patient off [hemo]dialysis.\u201d\r\n\r\nThe 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\u00a0cells\u00a0that can mimic the natural functions of the kidney. The living cells form a membrane to filter the patient\u2019s blood as a biological kidney would, but with less risk of rejection by the patient\u2019s immune system, because they are embedded within the device. The new kidney doesn\u2019t 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.\r\n\r\nBecause 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.\r\n\r\n\r\n16.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- 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\u00a0circulation.<\/li>\r\n \t
- 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 \u2014 which are the tiny functional units of the kidney \u2014 span the cortex and medulla. The entire kidney is surrounded by a fibrous capsule and protective fat layers.<\/li>\r\n \t
- 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.<\/li>\r\n \t
- In each nephron, the glomerulus and surrounding Bowman\u2019s capsule form the unit that filters blood. From Bowman\u2019s 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.<\/li>\r\n \t
- The kidneys control homeostasis with the help of endocrine hormones.\u00a0The 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\u00a0enzyme\u00a0renin secreted by the kidneys works with hormones from the liver and adrenal gland to stimulate nephrons to reabsorb more sodium and water from urine.<\/li>\r\n \t
- The kidneys also secrete endocrine hormones, including calcitriol \u2014 which helps control the level of calcium in the blood \u2014 and erythropoietin, which stimulates bone marrow to produce red blood\u00a0cells.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n16.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"632\"]<\/li>\r\n \t
- Contrast the renal artery and renal vein.<\/li>\r\n \t
- Identify the functions of a nephron.\u00a0Describe in detail what happens to fluids (blood, filtrate, and urine) as they pass through the parts of a nephron.<\/li>\r\n \t
- Identify two endocrine hormones secreted by the kidneys, along with the functions they control.<\/li>\r\n \t
- Name two regions in the kidney where water is reabsorbed.<\/li>\r\n \t
- Is the blood in the glomerular capillaries more or less filtered than the blood in the peritubular capillaries? Explain your answer.<\/li>\r\n \t
- What do you think would happen if\u00a0blood flow to the kidneys is blocked?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n16.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/FN3MFhYPWWo\r\nHow do your kidneys work? - Emma Bryce, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/es-t8lO1KpA\r\n
Urine Formation, Hamada Abass, 2013.<\/p>\r\nhttps:\/\/youtu.be\/bX3C201O4MA\r\n
Printing a human kidney - Anthony Atala, TED-Ed, 2013.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 16.4.1<\/strong>\r\n\r\nSteak and Kidney Pie<\/a> by Charles Haynes<\/a> on Flickr<\/a> is used under a CC BY-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/2.0\/) license.\r\n\r\nFigure 16.4.2<\/strong>\r\n\r\nGray Kidneys<\/a> by\u00a0Henry Vandyke Carter<\/a>\u00a0(1831-1897) on Wikimedia Commons is in the \u00a0public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain). (Bartleby.com<\/a>:\u00a0Gray\u2019s Anatomy<\/a>, Plate 1120<\/a>).\r\n\r\nFigure 16.4.3<\/strong>\r\n\r\nBlausen_0592_KidneyAnatomy_01<\/a> by BruceBlaus<\/a>\u00a0on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.4<\/strong>\r\n\r\nDiagram_showing_how_the_kidneys_work_CRUK_138.svg<\/a> by Cancer Research UK<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 16.4.5<\/strong>\r\n\r\nBlood_Flow_in_the_Nephron<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.6<\/strong>\r\n\r\n1024px-Physiology_of_Nephron<\/a> by Madhero88<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.7<\/strong>\r\n\r\nNephron_Secretion_Reabsorption<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.8<\/strong>\r\n\r\nUrine<\/a> by User:Markhamilton<\/a> at English Wikipedia<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 16.4.9<\/strong>\r\n\r\nRenin_Angiotensin_System-01<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\nReferences<\/h2>\r\n
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\u00a0<\/span><\/span>25.10<\/span><\/span>\u00a0<\/span>Blood flow in the nephron [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 25.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/25-3-gross-anatomy-of-the-kidney<\/span><\/span><\/p>\r\nBetts, 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\u00a0<\/span><\/span>25.17<\/span><\/span>\u00a0<\/span>Locations of secretion and reabsorption in the nephron [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 25.6). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/25-6-tubular-reabsorption<\/span><\/span><\/span><\/span><\/p>\r\nBetts, 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\u00a0<\/span><\/span>26.14<\/span><\/span>\u00a0<\/span>The renin-angiotensin system [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 26.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/26-3-electrolyte-balance<\/span><\/span><\/span><\/span><\/span><\/p>\r\nBlausen.com Staff. (2014). Medical gallery of Blausen Medical 2014.\u00a0WikiJournal of Medicine<\/i>\u00a01<\/b>\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436<\/p>\r\n
Hamada Abass. (2013). Urine formation. YouTube. https:\/\/www.youtube.com\/watch?v=es-t8lO1KpA&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, February 9). How do your kidneys work? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=FN3MFhYPWWo&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, March 15). Printing a human kidney - Anthony Atala. YouTube. https:\/\/www.youtube.com\/watch?v=bX3C201O4MA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\nFigure 16.4.1 Steak and kidney pie!<\/em><\/figcaption><\/figure>\n\nKidneys on the Menu<\/h1>\n<\/div>\n
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.<\/p>\n
\nLocation\u00a0of the Kidneys<\/h1>\n<\/div>\n
The two bean-shaped\u00a0kidneys<\/a><\/strong>\u00a0are located high in the back of the abdominal cavity<\/a>, one on each side of the spine. Both kidneys sit just below the diaphragm<\/a>, the large\u00a0breathing\u00a0muscle that separates the abdominal and thoracic cavities. As you can see in the\u00a0following\u00a0figure, the right kidney is slightly smaller and lower than the left kidney. The right kidney is behind the liver<\/a>, and the left kidney is behind the spleen<\/a>. The\u00a0location\u00a0of the liver explains why the right kidney is smaller and lower than the left.<\/p>\nFigure 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\u2019s main artery (aorta) and vein (inferior vena cava). The ureter exiting each kidney is also shown in the diagram.<\/em><\/figcaption><\/figure>\nKidney Anatomy<\/span><\/p>\nThe 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 hilum<\/a><\/strong>. The entire kidney is surrounded by tough fibrous tissue \u2014 called the\u00a0renal capsule<\/a><\/strong>\u00a0\u2014 which, in turn, is surrounded by two layers of protective, cushioning fat.<\/p>\nFigure 16.4.3 This diagram shows the location and relative size of the two kidneys, as well as the internal structure of each kidney.<\/em><\/figcaption><\/figure>\nInternally, each kidney is divided into two major layers: the outer renal cortex<\/strong><\/a> and the inner renal medulla<\/strong><\/a> (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 renal pyramid<\/strong><\/a>. Within the renal pyramids are the structural and functional units of the kidneys, the tiny nephrons<\/span><\/a><\/span>. Between the renal pyramids are projections of cortex called renal columns<\/strong><\/a>. 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 renal pelvis<\/a>, which becomes the ureter as it leaves the kidney.<\/span><\/p>\n\nRenal\u00a0Circulation<\/h3>\n<\/div>\n
The renal circulation is an important part of the kidney\u2019s primary function of filtering waste products from the blood. Blood<\/a> 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.<\/p>\nAs 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 arteries<\/a> that extend through the renal columns<\/a> between the renal pyramids<\/a>. These arteries, in turn, branch into arterioles that penetrate the renal pyramids. Blood in the arterioles passes through nephrons<\/a>, 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 veins<\/a>. Small veins merge into increasingly larger ones, and ultimately into the renal vein, which carries clean blood away from the kidney to the inferior vena cava<\/a>.<\/p>\nFigure 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).<\/em><\/figcaption><\/figure>\n\nNephron Structure and Function<\/h1>\n<\/div>\n
Figure 16.4.4 gives an indication of the complex structure of a nephron. The nephron<\/a><\/strong>\u00a0is 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 metabolism<\/a>. At least half of the waste is urea<\/a>, a waste product produced by\u00a0protein<\/a>\u00a0catabolism<\/a>. Another important waste is uric\u00a0acid<\/a>, produced in\u00a0nucleic acid<\/a>\u00a0catabolism.<\/p>\nComponents of a Nephron<\/h2>\n
Figure 16.4.5 shows in greater detail the components of a nephron<\/a>. Each nephron is composed of an initial filtering component that consists of a network of capillaries called the glomerulus<\/a><\/strong>\u00a0(plural, glomeruli), which is surrounded by a space within a structure\u00a0called\u00a0glomerular capsule<\/a> <\/strong>(also known as the Bowman’s capsule). Extending from glomerular capsule is the renal tubule<\/a><\/strong>. The proximal end (nearest glomerular capsule) of the renal tubule is called the proximal convoluted (coiled) tubule<\/a><\/strong>. From here, the renal tubule continues as a loop (known as the loop of Henle<\/strong><\/a>) (also known as the loop of the nephron), which in turn becomes the distal convoluted tubule<\/strong><\/a>. 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 peritubular capillary network<\/strong><\/a>.<\/p>\nFigure 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.<\/em><\/figcaption><\/figure>\n<\/div>\nFigure 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.<\/figcaption><\/figure>\nFunction of a Nephron<\/h2>\n
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\u2019t pass through the glomerulus \u2014 as well as blood after it passes through the glomerular capillaries \u2014 continues on through an arteriole called the efferent arteriole. The efferent arteriole follows the renal tubule of the nephron, where it continues\u00a0playing a role\u00a0in nephron functioning.<\/p>\n
<\/p>\n
Filtration<\/h3>\n
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 \u2014 called filtrate<\/a><\/strong> \u2014 pass into glomerular capsule, and from there into the proximal end of the renal tubule<\/a>.\u00a0 Anything too large to move through the pores in the glomerulus, such as blood cells, large proteins, etc., stay in the cardiovascular system.\u00a0 At this stage, filtrate (fluid in the nephron) includes water, salts, organic solids (such as nutrients), and waste products of metabolism (such as urea).<\/p>\nFigure 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. Reabsorption is the movement of substance back into the bloodstream and secretion is movement of substances from the blood into the nephron for excretion.<\/em><\/figcaption><\/figure>\nReabsorption and Secretion<\/h3>\n
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 peritubular capillary network<\/a>). 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 glucose<\/a> and amino acids<\/span><\/a><\/span>) are reabsorbed from the filtrate by the blood in the peritubular capillary network. Reabsorption<\/a> occurs mainly in the proximal convoluted tubule and the loop of Henle, as seen in Figure 16.4.7.<\/span><\/p>\nAt 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\u00a0secretion<\/a><\/strong>. 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 homeostasis<\/a> 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, parathyroid hormone<\/a> causes more calcium to be reabsorbed into the blood and more phosphorus to be secreted into the filtrate.<\/p>\nCollection of Urine and Excretion<\/h2>\nFigure 16.4.8 Fresh urine is typically yellow or amber in colour.<\/em><\/figcaption><\/figure>\nBy the time the filtrate has passed through the entire renal tubule, it has become the\u00a0liquid\u00a0waste known as urine<\/a>. Urine empties from the distal end of the renal tubule<\/a> into a\u00a0collecting duct<\/a><\/strong>. 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 antidiuretic\u00a0hormone\u00a0<\/a>from the\u00a0posterior\u00a0pituitary gland<\/a>. This\u00a0hormone\u00a0makes the collecting ducts permeable to water, allowing water molecules to pass through them into capillaries by\u00a0osmosis<\/a>, 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.<\/p>\nUrine finally exits the largest collecting ducts through the renal papillae. It empties into the renal calyces, and finally into the renal pelvis<\/a>. From there, it travels through the ureter<\/a> to the urinary bladder<\/a> 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<\/span>). The darker the colour, generally speaking, the more concentrated the urine is.<\/span><\/p>\n <\/p>\n
Other Functions of the Kidneys<\/span><\/div>\nBesides filtering blood and forming urine for excretion of soluble wastes, the kidneys have several vital functions in maintaining body-wide\u00a0homeostasis<\/a>. Most of these functions are related to the\u00a0composition\u00a0or volume of urine formed by the kidneys. The kidneys must maintain the proper balance of water and salts in the body, normal\u00a0blood pressure<\/a>, and the correct range of blood\u00a0pH<\/a>. 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.<\/p>\nBlood Pressure\u00a0Regulation<\/h2>\n
The kidneys do not control homeostasis all alone. As indicated above, endocrine hormones are also involved. Consider the regulation of\u00a0blood pressure\u00a0by 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\u00a0concentration\u00a0of sodium in the blood to control blood pressure.<\/p>\nFigure 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.<\/em><\/figcaption><\/figure>\nThe renin-angiotensin-aldosterone system is put into play when the\u00a0concentration\u00a0of sodium ions in the blood falls lower than normal. This causes the kidneys to secrete an\u00a0enzyme\u00a0called renin<\/a> into the blood. It also causes the liver to secrete a\u00a0protein\u00a0called angiotensinogen. Renin changes angiotensinogen into a proto-hormone\u00a0called\u00a0angiotensin I. This is converted to angiotensin II by an\u00a0enzyme\u00a0(angiotensin-converting enzyme) in lung capillaries.<\/p>\nAngiotensin 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\u00a0concentration\u00a0of sodium ions in the blood to normal. The increased water in the blood also increases blood volume and blood pressure.<\/p>\n
Other Kidney Hormones<\/h2>\n
Hormones other than renin are also produced and secreted by the kidneys. These include calcitriol and erythropoietin.<\/p>\n
\n- Calcitriol<\/a><\/strong>\u00a0is 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.<\/li>\n
- Erythropoietin<\/a><\/strong> is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of\u00a0 erythrocytes<\/a>\u00a0in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.<\/li>\n<\/ul>\n\n
Feature: Human Biology in the News<\/h1>\n<\/div>\n
Kidney failure is a complication of common disorders including diabetes mellitus<\/a> and hypertension<\/a>. It is estimated that approximately 12.5% of Canadians have some form of kidney disease.\u00a0 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.\u00a0 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.<\/p>\nFor 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\u2019s body in place of the failed kidney. According to Dr. Fissell, the artificial kidney is \u201c… a bio-hybrid device that can mimic a kidney to remove enough waste products, salt, and water to keep a patient off [hemo]dialysis.\u201d<\/p>\n
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\u00a0cells\u00a0that can mimic the natural functions of the kidney. The living cells form a membrane to filter the patient\u2019s blood as a biological kidney would, but with less risk of rejection by the patient\u2019s immune system, because they are embedded within the device. The new kidney doesn\u2019t 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.<\/p>\n
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.<\/p>\n
\n\n\n16.4 Summary<\/span><\/h1>\n<\/header>\n\n\n
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.<\/em>[\/caption]\r\n\r\n 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.[\/caption]\r\nFunction of a Nephron<\/h2>\r\nThe 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\u2019t pass through the glomerulus \u2014 as well as blood after it passes through the glomerular capillaries \u2014 continues on through an arteriole called the efferent arteriole. The efferent arteriole follows the renal tubule of the nephron, where it continues\u00a0playing a role\u00a0in nephron functioning.\r\n\r\n \r\nFiltration<\/h3>\r\nAs 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 \u2014 called [pb_glossary id=\"5471\"]filtrate[\/pb_glossary]<\/strong> \u2014 pass into glomerular capsule, and from there into the proximal end of the [pb_glossary id=\"4749\"]renal tubule[\/pb_glossary].\u00a0 Anything too large to move through the pores in the glomerulus, such as blood cells, large proteins, etc., stay in the cardiovascular system.\u00a0 At this stage, filtrate (fluid in the nephron) includes water, salts, organic solids (such as nutrients), and waste products of metabolism (such as urea).\r\n\r\n[caption id=\"attachment_4756\" align=\"alignright\" width=\"403\"] 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. Reabsorption is the movement of substance back into the bloodstream and secretion is movement of substances from the blood into the nephron for excretion.<\/em>[\/caption]\r\nReabsorption and Secretion<\/h3>\r\nAs 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 [pb_glossary id=\"4753\"]peritubular capillary network[\/pb_glossary]). 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 [pb_glossary id=\"5451\"]glucose[\/pb_glossary] and [pb_glossary id=\"5707\"]amino acids<\/span>[\/pb_glossary]<\/span>) are reabsorbed from the filtrate by the blood in the peritubular capillary network. [pb_glossary id=\"4757\"]Reabsorption[\/pb_glossary] occurs mainly in the proximal convoluted tubule and the loop of Henle, as seen in Figure 16.4.7.<\/span>\r\n\r\nAt 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\u00a0[pb_glossary id=\"4758\"]secretion[\/pb_glossary]<\/strong>. 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 [pb_glossary id=\"5761\"]homeostasis[\/pb_glossary] 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, [pb_glossary id=\"3467\"]parathyroid hormone[\/pb_glossary] causes more calcium to be reabsorbed into the blood and more phosphorus to be secreted into the filtrate.\r\nCollection of Urine and Excretion<\/h2>\r\n[caption id=\"attachment_4760\" align=\"alignleft\" width=\"219\"] Figure 16.4.8 Fresh urine is typically yellow or amber in colour.<\/em>[\/caption]\r\n\r\nBy the time the filtrate has passed through the entire renal tubule, it has become the\u00a0liquid\u00a0waste known as [pb_glossary id=\"4717\"]urine[\/pb_glossary]. Urine empties from the distal end of the [pb_glossary id=\"4749\"]renal tubule[\/pb_glossary] into a\u00a0[pb_glossary id=\"4759\"]collecting duct[\/pb_glossary]<\/strong>. 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 [pb_glossary id=\"5533\"]antidiuretic\u00a0hormone\u00a0[\/pb_glossary]from the\u00a0posterior\u00a0[pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. This\u00a0hormone\u00a0makes the collecting ducts permeable to water, allowing water molecules to pass through them into capillaries by\u00a0[pb_glossary id=\"5663\"]osmosis[\/pb_glossary], 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.\r\n\r\nUrine finally exits the largest collecting ducts through the renal papillae. It empties into the renal calyces, and finally into the [pb_glossary id=\"4724\"]renal pelvis[\/pb_glossary]. From there, it travels through the [pb_glossary id=\"4725\"]ureter[\/pb_glossary] to the [pb_glossary id=\"4731\"]urinary bladder[\/pb_glossary] 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<\/span>). The darker the colour, generally speaking, the more concentrated the urine is.<\/span>\r\n\r\n \r\nOther Functions of the Kidneys<\/span><\/div>\r\nBesides filtering blood and forming urine for excretion of soluble wastes, the kidneys have several vital functions in maintaining body-wide\u00a0[pb_glossary id=\"5761\"]homeostasis[\/pb_glossary]. Most of these functions are related to the\u00a0composition\u00a0or volume of urine formed by the kidneys. The kidneys must maintain the proper balance of water and salts in the body, normal\u00a0[pb_glossary id=\"4480\"]blood pressure[\/pb_glossary], and the correct range of blood\u00a0[pb_glossary id=\"4330\"]pH[\/pb_glossary]. 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.\r\nBlood Pressure\u00a0Regulation<\/h2>\r\nThe kidneys do not control homeostasis all alone. As indicated above, endocrine hormones are also involved. Consider the regulation of\u00a0blood pressure\u00a0by 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\u00a0concentration\u00a0of sodium in the blood to control blood pressure.\r\n\r\n[caption id=\"attachment_4761\" align=\"aligncenter\" width=\"736\"] 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.<\/em>[\/caption]\r\n\r\nThe renin-angiotensin-aldosterone system is put into play when the\u00a0concentration\u00a0of sodium ions in the blood falls lower than normal. This causes the kidneys to secrete an\u00a0enzyme\u00a0called [pb_glossary id=\"4762\"]renin[\/pb_glossary] into the blood. It also causes the liver to secrete a\u00a0protein\u00a0called angiotensinogen. Renin changes angiotensinogen into a proto-hormone\u00a0called\u00a0angiotensin I. This is converted to angiotensin II by an\u00a0enzyme\u00a0(angiotensin-converting enzyme) in lung capillaries.\r\n\r\nAngiotensin 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\u00a0concentration\u00a0of sodium ions in the blood to normal. The increased water in the blood also increases blood volume and blood pressure.\r\nOther Kidney Hormones<\/h2>\r\nHormones other than renin are also produced and secreted by the kidneys. These include calcitriol and erythropoietin.\r\n\r\n \t- [pb_glossary id=\"4763\"]Calcitriol[\/pb_glossary]<\/strong>\u00a0is 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.<\/li>\r\n \t
- [pb_glossary id=\"4764\"]Erythropoietin[\/pb_glossary]<\/strong> is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of\u00a0 [pb_glossary id=\"4398\"]erythrocytes[\/pb_glossary]\u00a0in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.<\/li>\r\n<\/ul>\r\n\r\n
Feature: Human Biology in the News<\/h1>\r\n<\/div>\r\nKidney failure is a complication of common disorders including [pb_glossary id=\"2771\"]diabetes mellitus[\/pb_glossary] and [pb_glossary id=\"4231\"]hypertension[\/pb_glossary]. It is estimated that approximately 12.5% of Canadians have some form of kidney disease.\u00a0 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.\u00a0 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.\r\n\r\nFor 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\u2019s body in place of the failed kidney. According to Dr. Fissell, the artificial kidney is \u201c... a bio-hybrid device that can mimic a kidney to remove enough waste products, salt, and water to keep a patient off [hemo]dialysis.\u201d\r\n\r\nThe 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\u00a0cells\u00a0that can mimic the natural functions of the kidney. The living cells form a membrane to filter the patient\u2019s blood as a biological kidney would, but with less risk of rejection by the patient\u2019s immune system, because they are embedded within the device. The new kidney doesn\u2019t 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.\r\n\r\nBecause 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.\r\n\r\n\r\n16.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- 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\u00a0circulation.<\/li>\r\n \t
- 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 \u2014 which are the tiny functional units of the kidney \u2014 span the cortex and medulla. The entire kidney is surrounded by a fibrous capsule and protective fat layers.<\/li>\r\n \t
- 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.<\/li>\r\n \t
- In each nephron, the glomerulus and surrounding Bowman\u2019s capsule form the unit that filters blood. From Bowman\u2019s 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.<\/li>\r\n \t
- The kidneys control homeostasis with the help of endocrine hormones.\u00a0The 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\u00a0enzyme\u00a0renin secreted by the kidneys works with hormones from the liver and adrenal gland to stimulate nephrons to reabsorb more sodium and water from urine.<\/li>\r\n \t
- The kidneys also secrete endocrine hormones, including calcitriol \u2014 which helps control the level of calcium in the blood \u2014 and erythropoietin, which stimulates bone marrow to produce red blood\u00a0cells.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n16.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"632\"]<\/li>\r\n \t
- Contrast the renal artery and renal vein.<\/li>\r\n \t
- Identify the functions of a nephron.\u00a0Describe in detail what happens to fluids (blood, filtrate, and urine) as they pass through the parts of a nephron.<\/li>\r\n \t
- Identify two endocrine hormones secreted by the kidneys, along with the functions they control.<\/li>\r\n \t
- Name two regions in the kidney where water is reabsorbed.<\/li>\r\n \t
- Is the blood in the glomerular capillaries more or less filtered than the blood in the peritubular capillaries? Explain your answer.<\/li>\r\n \t
- What do you think would happen if\u00a0blood flow to the kidneys is blocked?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n16.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/FN3MFhYPWWo\r\nHow do your kidneys work? - Emma Bryce, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/es-t8lO1KpA\r\n
Urine Formation, Hamada Abass, 2013.<\/p>\r\nhttps:\/\/youtu.be\/bX3C201O4MA\r\n
Printing a human kidney - Anthony Atala, TED-Ed, 2013.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 16.4.1<\/strong>\r\n\r\nSteak and Kidney Pie<\/a> by Charles Haynes<\/a> on Flickr<\/a> is used under a CC BY-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/2.0\/) license.\r\n\r\nFigure 16.4.2<\/strong>\r\n\r\nGray Kidneys<\/a> by\u00a0Henry Vandyke Carter<\/a>\u00a0(1831-1897) on Wikimedia Commons is in the \u00a0public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain). (Bartleby.com<\/a>:\u00a0Gray\u2019s Anatomy<\/a>, Plate 1120<\/a>).\r\n\r\nFigure 16.4.3<\/strong>\r\n\r\nBlausen_0592_KidneyAnatomy_01<\/a> by BruceBlaus<\/a>\u00a0on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.4<\/strong>\r\n\r\nDiagram_showing_how_the_kidneys_work_CRUK_138.svg<\/a> by Cancer Research UK<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 16.4.5<\/strong>\r\n\r\nBlood_Flow_in_the_Nephron<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.6<\/strong>\r\n\r\n1024px-Physiology_of_Nephron<\/a> by Madhero88<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.7<\/strong>\r\n\r\nNephron_Secretion_Reabsorption<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.8<\/strong>\r\n\r\nUrine<\/a> by User:Markhamilton<\/a> at English Wikipedia<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 16.4.9<\/strong>\r\n\r\nRenin_Angiotensin_System-01<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\nReferences<\/h2>\r\n
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\u00a0<\/span><\/span>25.10<\/span><\/span>\u00a0<\/span>Blood flow in the nephron [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 25.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/25-3-gross-anatomy-of-the-kidney<\/span><\/span><\/p>\r\nBetts, 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\u00a0<\/span><\/span>25.17<\/span><\/span>\u00a0<\/span>Locations of secretion and reabsorption in the nephron [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 25.6). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/25-6-tubular-reabsorption<\/span><\/span><\/span><\/span><\/p>\r\nBetts, 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\u00a0<\/span><\/span>26.14<\/span><\/span>\u00a0<\/span>The renin-angiotensin system [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 26.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/26-3-electrolyte-balance<\/span><\/span><\/span><\/span><\/span><\/p>\r\nBlausen.com Staff. (2014). Medical gallery of Blausen Medical 2014.\u00a0WikiJournal of Medicine<\/i>\u00a01<\/b>\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436<\/p>\r\n
Hamada Abass. (2013). Urine formation. YouTube. https:\/\/www.youtube.com\/watch?v=es-t8lO1KpA&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, February 9). How do your kidneys work? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=FN3MFhYPWWo&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, March 15). Printing a human kidney - Anthony Atala. YouTube. https:\/\/www.youtube.com\/watch?v=bX3C201O4MA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\nFigure 16.4.1 Steak and kidney pie!<\/em><\/figcaption><\/figure>\n\nKidneys on the Menu<\/h1>\n<\/div>\n
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.<\/p>\n
\nLocation\u00a0of the Kidneys<\/h1>\n<\/div>\n
The two bean-shaped\u00a0kidneys<\/a><\/strong>\u00a0are located high in the back of the abdominal cavity<\/a>, one on each side of the spine. Both kidneys sit just below the diaphragm<\/a>, the large\u00a0breathing\u00a0muscle that separates the abdominal and thoracic cavities. As you can see in the\u00a0following\u00a0figure, the right kidney is slightly smaller and lower than the left kidney. The right kidney is behind the liver<\/a>, and the left kidney is behind the spleen<\/a>. The\u00a0location\u00a0of the liver explains why the right kidney is smaller and lower than the left.<\/p>\nFigure 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\u2019s main artery (aorta) and vein (inferior vena cava). The ureter exiting each kidney is also shown in the diagram.<\/em><\/figcaption><\/figure>\nKidney Anatomy<\/span><\/p>\nThe 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 hilum<\/a><\/strong>. The entire kidney is surrounded by tough fibrous tissue \u2014 called the\u00a0renal capsule<\/a><\/strong>\u00a0\u2014 which, in turn, is surrounded by two layers of protective, cushioning fat.<\/p>\nFigure 16.4.3 This diagram shows the location and relative size of the two kidneys, as well as the internal structure of each kidney.<\/em><\/figcaption><\/figure>\nInternally, each kidney is divided into two major layers: the outer renal cortex<\/strong><\/a> and the inner renal medulla<\/strong><\/a> (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 renal pyramid<\/strong><\/a>. Within the renal pyramids are the structural and functional units of the kidneys, the tiny nephrons<\/span><\/a><\/span>. Between the renal pyramids are projections of cortex called renal columns<\/strong><\/a>. 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 renal pelvis<\/a>, which becomes the ureter as it leaves the kidney.<\/span><\/p>\n\nRenal\u00a0Circulation<\/h3>\n<\/div>\n
The renal circulation is an important part of the kidney\u2019s primary function of filtering waste products from the blood. Blood<\/a> 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.<\/p>\nAs 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 arteries<\/a> that extend through the renal columns<\/a> between the renal pyramids<\/a>. These arteries, in turn, branch into arterioles that penetrate the renal pyramids. Blood in the arterioles passes through nephrons<\/a>, 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 veins<\/a>. Small veins merge into increasingly larger ones, and ultimately into the renal vein, which carries clean blood away from the kidney to the inferior vena cava<\/a>.<\/p>\nFigure 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).<\/em><\/figcaption><\/figure>\n\nNephron Structure and Function<\/h1>\n<\/div>\n
Figure 16.4.4 gives an indication of the complex structure of a nephron. The nephron<\/a><\/strong>\u00a0is 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 metabolism<\/a>. At least half of the waste is urea<\/a>, a waste product produced by\u00a0protein<\/a>\u00a0catabolism<\/a>. Another important waste is uric\u00a0acid<\/a>, produced in\u00a0nucleic acid<\/a>\u00a0catabolism.<\/p>\nComponents of a Nephron<\/h2>\n
Figure 16.4.5 shows in greater detail the components of a nephron<\/a>. Each nephron is composed of an initial filtering component that consists of a network of capillaries called the glomerulus<\/a><\/strong>\u00a0(plural, glomeruli), which is surrounded by a space within a structure\u00a0called\u00a0glomerular capsule<\/a> <\/strong>(also known as the Bowman’s capsule). Extending from glomerular capsule is the renal tubule<\/a><\/strong>. The proximal end (nearest glomerular capsule) of the renal tubule is called the proximal convoluted (coiled) tubule<\/a><\/strong>. From here, the renal tubule continues as a loop (known as the loop of Henle<\/strong><\/a>) (also known as the loop of the nephron), which in turn becomes the distal convoluted tubule<\/strong><\/a>. 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 peritubular capillary network<\/strong><\/a>.<\/p>\nFigure 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.<\/em><\/figcaption><\/figure>\n<\/div>\nFigure 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.<\/figcaption><\/figure>\nFunction of a Nephron<\/h2>\n
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\u2019t pass through the glomerulus \u2014 as well as blood after it passes through the glomerular capillaries \u2014 continues on through an arteriole called the efferent arteriole. The efferent arteriole follows the renal tubule of the nephron, where it continues\u00a0playing a role\u00a0in nephron functioning.<\/p>\n
<\/p>\n
Filtration<\/h3>\n
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 \u2014 called filtrate<\/a><\/strong> \u2014 pass into glomerular capsule, and from there into the proximal end of the renal tubule<\/a>.\u00a0 Anything too large to move through the pores in the glomerulus, such as blood cells, large proteins, etc., stay in the cardiovascular system.\u00a0 At this stage, filtrate (fluid in the nephron) includes water, salts, organic solids (such as nutrients), and waste products of metabolism (such as urea).<\/p>\nFigure 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. Reabsorption is the movement of substance back into the bloodstream and secretion is movement of substances from the blood into the nephron for excretion.<\/em><\/figcaption><\/figure>\nReabsorption and Secretion<\/h3>\n
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 peritubular capillary network<\/a>). 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 glucose<\/a> and amino acids<\/span><\/a><\/span>) are reabsorbed from the filtrate by the blood in the peritubular capillary network. Reabsorption<\/a> occurs mainly in the proximal convoluted tubule and the loop of Henle, as seen in Figure 16.4.7.<\/span><\/p>\nAt 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\u00a0secretion<\/a><\/strong>. 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 homeostasis<\/a> 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, parathyroid hormone<\/a> causes more calcium to be reabsorbed into the blood and more phosphorus to be secreted into the filtrate.<\/p>\nCollection of Urine and Excretion<\/h2>\nFigure 16.4.8 Fresh urine is typically yellow or amber in colour.<\/em><\/figcaption><\/figure>\nBy the time the filtrate has passed through the entire renal tubule, it has become the\u00a0liquid\u00a0waste known as urine<\/a>. Urine empties from the distal end of the renal tubule<\/a> into a\u00a0collecting duct<\/a><\/strong>. 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 antidiuretic\u00a0hormone\u00a0<\/a>from the\u00a0posterior\u00a0pituitary gland<\/a>. This\u00a0hormone\u00a0makes the collecting ducts permeable to water, allowing water molecules to pass through them into capillaries by\u00a0osmosis<\/a>, 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.<\/p>\nUrine finally exits the largest collecting ducts through the renal papillae. It empties into the renal calyces, and finally into the renal pelvis<\/a>. From there, it travels through the ureter<\/a> to the urinary bladder<\/a> 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<\/span>). The darker the colour, generally speaking, the more concentrated the urine is.<\/span><\/p>\n <\/p>\n
Other Functions of the Kidneys<\/span><\/div>\nBesides filtering blood and forming urine for excretion of soluble wastes, the kidneys have several vital functions in maintaining body-wide\u00a0homeostasis<\/a>. Most of these functions are related to the\u00a0composition\u00a0or volume of urine formed by the kidneys. The kidneys must maintain the proper balance of water and salts in the body, normal\u00a0blood pressure<\/a>, and the correct range of blood\u00a0pH<\/a>. 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.<\/p>\nBlood Pressure\u00a0Regulation<\/h2>\n
The kidneys do not control homeostasis all alone. As indicated above, endocrine hormones are also involved. Consider the regulation of\u00a0blood pressure\u00a0by 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\u00a0concentration\u00a0of sodium in the blood to control blood pressure.<\/p>\nFigure 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.<\/em><\/figcaption><\/figure>\nThe renin-angiotensin-aldosterone system is put into play when the\u00a0concentration\u00a0of sodium ions in the blood falls lower than normal. This causes the kidneys to secrete an\u00a0enzyme\u00a0called renin<\/a> into the blood. It also causes the liver to secrete a\u00a0protein\u00a0called angiotensinogen. Renin changes angiotensinogen into a proto-hormone\u00a0called\u00a0angiotensin I. This is converted to angiotensin II by an\u00a0enzyme\u00a0(angiotensin-converting enzyme) in lung capillaries.<\/p>\nAngiotensin 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\u00a0concentration\u00a0of sodium ions in the blood to normal. The increased water in the blood also increases blood volume and blood pressure.<\/p>\n
Other Kidney Hormones<\/h2>\n
Hormones other than renin are also produced and secreted by the kidneys. These include calcitriol and erythropoietin.<\/p>\n
\n- Calcitriol<\/a><\/strong>\u00a0is 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.<\/li>\n
- Erythropoietin<\/a><\/strong> is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of\u00a0 erythrocytes<\/a>\u00a0in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.<\/li>\n<\/ul>\n\n
Feature: Human Biology in the News<\/h1>\n<\/div>\n
Kidney failure is a complication of common disorders including diabetes mellitus<\/a> and hypertension<\/a>. It is estimated that approximately 12.5% of Canadians have some form of kidney disease.\u00a0 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.\u00a0 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.<\/p>\nFor 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\u2019s body in place of the failed kidney. According to Dr. Fissell, the artificial kidney is \u201c… a bio-hybrid device that can mimic a kidney to remove enough waste products, salt, and water to keep a patient off [hemo]dialysis.\u201d<\/p>\n
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\u00a0cells\u00a0that can mimic the natural functions of the kidney. The living cells form a membrane to filter the patient\u2019s blood as a biological kidney would, but with less risk of rejection by the patient\u2019s immune system, because they are embedded within the device. The new kidney doesn\u2019t 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.<\/p>\n
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.<\/p>\n
\n\n\n16.4 Summary<\/span><\/h1>\n<\/header>\n\n\n
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. Reabsorption is the movement of substance back into the bloodstream and secretion is movement of substances from the blood into the nephron for excretion.<\/em>[\/caption]\r\nReabsorption and Secretion<\/h3>\r\nAs 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 [pb_glossary id=\"4753\"]peritubular capillary network[\/pb_glossary]). 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 [pb_glossary id=\"5451\"]glucose[\/pb_glossary] and [pb_glossary id=\"5707\"]amino acids<\/span>[\/pb_glossary]<\/span>) are reabsorbed from the filtrate by the blood in the peritubular capillary network. [pb_glossary id=\"4757\"]Reabsorption[\/pb_glossary] occurs mainly in the proximal convoluted tubule and the loop of Henle, as seen in Figure 16.4.7.<\/span>\r\n\r\nAt 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\u00a0[pb_glossary id=\"4758\"]secretion[\/pb_glossary]<\/strong>. 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 [pb_glossary id=\"5761\"]homeostasis[\/pb_glossary] 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, [pb_glossary id=\"3467\"]parathyroid hormone[\/pb_glossary] causes more calcium to be reabsorbed into the blood and more phosphorus to be secreted into the filtrate.\r\nCollection of Urine and Excretion<\/h2>\r\n[caption id=\"attachment_4760\" align=\"alignleft\" width=\"219\"] Figure 16.4.8 Fresh urine is typically yellow or amber in colour.<\/em>[\/caption]\r\n\r\nBy the time the filtrate has passed through the entire renal tubule, it has become the\u00a0liquid\u00a0waste known as [pb_glossary id=\"4717\"]urine[\/pb_glossary]. Urine empties from the distal end of the [pb_glossary id=\"4749\"]renal tubule[\/pb_glossary] into a\u00a0[pb_glossary id=\"4759\"]collecting duct[\/pb_glossary]<\/strong>. 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 [pb_glossary id=\"5533\"]antidiuretic\u00a0hormone\u00a0[\/pb_glossary]from the\u00a0posterior\u00a0[pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. This\u00a0hormone\u00a0makes the collecting ducts permeable to water, allowing water molecules to pass through them into capillaries by\u00a0[pb_glossary id=\"5663\"]osmosis[\/pb_glossary], 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.\r\n\r\nUrine finally exits the largest collecting ducts through the renal papillae. It empties into the renal calyces, and finally into the [pb_glossary id=\"4724\"]renal pelvis[\/pb_glossary]. From there, it travels through the [pb_glossary id=\"4725\"]ureter[\/pb_glossary] to the [pb_glossary id=\"4731\"]urinary bladder[\/pb_glossary] 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<\/span>). The darker the colour, generally speaking, the more concentrated the urine is.<\/span>\r\n\r\n \r\nOther Functions of the Kidneys<\/span><\/div>\r\nBesides filtering blood and forming urine for excretion of soluble wastes, the kidneys have several vital functions in maintaining body-wide\u00a0[pb_glossary id=\"5761\"]homeostasis[\/pb_glossary]. Most of these functions are related to the\u00a0composition\u00a0or volume of urine formed by the kidneys. The kidneys must maintain the proper balance of water and salts in the body, normal\u00a0[pb_glossary id=\"4480\"]blood pressure[\/pb_glossary], and the correct range of blood\u00a0[pb_glossary id=\"4330\"]pH[\/pb_glossary]. 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.\r\nBlood Pressure\u00a0Regulation<\/h2>\r\nThe kidneys do not control homeostasis all alone. As indicated above, endocrine hormones are also involved. Consider the regulation of\u00a0blood pressure\u00a0by 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\u00a0concentration\u00a0of sodium in the blood to control blood pressure.\r\n\r\n[caption id=\"attachment_4761\" align=\"aligncenter\" width=\"736\"] 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.<\/em>[\/caption]\r\n\r\nThe renin-angiotensin-aldosterone system is put into play when the\u00a0concentration\u00a0of sodium ions in the blood falls lower than normal. This causes the kidneys to secrete an\u00a0enzyme\u00a0called [pb_glossary id=\"4762\"]renin[\/pb_glossary] into the blood. It also causes the liver to secrete a\u00a0protein\u00a0called angiotensinogen. Renin changes angiotensinogen into a proto-hormone\u00a0called\u00a0angiotensin I. This is converted to angiotensin II by an\u00a0enzyme\u00a0(angiotensin-converting enzyme) in lung capillaries.\r\n\r\nAngiotensin 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\u00a0concentration\u00a0of sodium ions in the blood to normal. The increased water in the blood also increases blood volume and blood pressure.\r\nOther Kidney Hormones<\/h2>\r\nHormones other than renin are also produced and secreted by the kidneys. These include calcitriol and erythropoietin.\r\n\r\n \t- [pb_glossary id=\"4763\"]Calcitriol[\/pb_glossary]<\/strong>\u00a0is 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.<\/li>\r\n \t
- [pb_glossary id=\"4764\"]Erythropoietin[\/pb_glossary]<\/strong> is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of\u00a0 [pb_glossary id=\"4398\"]erythrocytes[\/pb_glossary]\u00a0in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.<\/li>\r\n<\/ul>\r\n\r\n
Feature: Human Biology in the News<\/h1>\r\n<\/div>\r\nKidney failure is a complication of common disorders including [pb_glossary id=\"2771\"]diabetes mellitus[\/pb_glossary] and [pb_glossary id=\"4231\"]hypertension[\/pb_glossary]. It is estimated that approximately 12.5% of Canadians have some form of kidney disease.\u00a0 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.\u00a0 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.\r\n\r\nFor 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\u2019s body in place of the failed kidney. According to Dr. Fissell, the artificial kidney is \u201c... a bio-hybrid device that can mimic a kidney to remove enough waste products, salt, and water to keep a patient off [hemo]dialysis.\u201d\r\n\r\nThe 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\u00a0cells\u00a0that can mimic the natural functions of the kidney. The living cells form a membrane to filter the patient\u2019s blood as a biological kidney would, but with less risk of rejection by the patient\u2019s immune system, because they are embedded within the device. The new kidney doesn\u2019t 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.\r\n\r\nBecause 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.\r\n\r\n\r\n16.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- 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\u00a0circulation.<\/li>\r\n \t
- 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 \u2014 which are the tiny functional units of the kidney \u2014 span the cortex and medulla. The entire kidney is surrounded by a fibrous capsule and protective fat layers.<\/li>\r\n \t
- 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.<\/li>\r\n \t
- In each nephron, the glomerulus and surrounding Bowman\u2019s capsule form the unit that filters blood. From Bowman\u2019s 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.<\/li>\r\n \t
- The kidneys control homeostasis with the help of endocrine hormones.\u00a0The 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\u00a0enzyme\u00a0renin secreted by the kidneys works with hormones from the liver and adrenal gland to stimulate nephrons to reabsorb more sodium and water from urine.<\/li>\r\n \t
- The kidneys also secrete endocrine hormones, including calcitriol \u2014 which helps control the level of calcium in the blood \u2014 and erythropoietin, which stimulates bone marrow to produce red blood\u00a0cells.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n16.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"632\"]<\/li>\r\n \t
- Contrast the renal artery and renal vein.<\/li>\r\n \t
- Identify the functions of a nephron.\u00a0Describe in detail what happens to fluids (blood, filtrate, and urine) as they pass through the parts of a nephron.<\/li>\r\n \t
- Identify two endocrine hormones secreted by the kidneys, along with the functions they control.<\/li>\r\n \t
- Name two regions in the kidney where water is reabsorbed.<\/li>\r\n \t
- Is the blood in the glomerular capillaries more or less filtered than the blood in the peritubular capillaries? Explain your answer.<\/li>\r\n \t
- What do you think would happen if\u00a0blood flow to the kidneys is blocked?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n16.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/FN3MFhYPWWo\r\nHow do your kidneys work? - Emma Bryce, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/es-t8lO1KpA\r\n
Urine Formation, Hamada Abass, 2013.<\/p>\r\nhttps:\/\/youtu.be\/bX3C201O4MA\r\n
Printing a human kidney - Anthony Atala, TED-Ed, 2013.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 16.4.1<\/strong>\r\n\r\nSteak and Kidney Pie<\/a> by Charles Haynes<\/a> on Flickr<\/a> is used under a CC BY-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/2.0\/) license.\r\n\r\nFigure 16.4.2<\/strong>\r\n\r\nGray Kidneys<\/a> by\u00a0Henry Vandyke Carter<\/a>\u00a0(1831-1897) on Wikimedia Commons is in the \u00a0public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain). (Bartleby.com<\/a>:\u00a0Gray\u2019s Anatomy<\/a>, Plate 1120<\/a>).\r\n\r\nFigure 16.4.3<\/strong>\r\n\r\nBlausen_0592_KidneyAnatomy_01<\/a> by BruceBlaus<\/a>\u00a0on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.4<\/strong>\r\n\r\nDiagram_showing_how_the_kidneys_work_CRUK_138.svg<\/a> by Cancer Research UK<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 16.4.5<\/strong>\r\n\r\nBlood_Flow_in_the_Nephron<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.6<\/strong>\r\n\r\n1024px-Physiology_of_Nephron<\/a> by Madhero88<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.7<\/strong>\r\n\r\nNephron_Secretion_Reabsorption<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.8<\/strong>\r\n\r\nUrine<\/a> by User:Markhamilton<\/a> at English Wikipedia<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 16.4.9<\/strong>\r\n\r\nRenin_Angiotensin_System-01<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\nReferences<\/h2>\r\n
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\u00a0<\/span><\/span>25.10<\/span><\/span>\u00a0<\/span>Blood flow in the nephron [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 25.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/25-3-gross-anatomy-of-the-kidney<\/span><\/span><\/p>\r\nBetts, 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\u00a0<\/span><\/span>25.17<\/span><\/span>\u00a0<\/span>Locations of secretion and reabsorption in the nephron [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 25.6). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/25-6-tubular-reabsorption<\/span><\/span><\/span><\/span><\/p>\r\nBetts, 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\u00a0<\/span><\/span>26.14<\/span><\/span>\u00a0<\/span>The renin-angiotensin system [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 26.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/26-3-electrolyte-balance<\/span><\/span><\/span><\/span><\/span><\/p>\r\nBlausen.com Staff. (2014). Medical gallery of Blausen Medical 2014.\u00a0WikiJournal of Medicine<\/i>\u00a01<\/b>\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436<\/p>\r\n
Hamada Abass. (2013). Urine formation. YouTube. https:\/\/www.youtube.com\/watch?v=es-t8lO1KpA&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, February 9). How do your kidneys work? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=FN3MFhYPWWo&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, March 15). Printing a human kidney - Anthony Atala. YouTube. https:\/\/www.youtube.com\/watch?v=bX3C201O4MA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\nFigure 16.4.1 Steak and kidney pie!<\/em><\/figcaption><\/figure>\n\nKidneys on the Menu<\/h1>\n<\/div>\n
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.<\/p>\n
\nLocation\u00a0of the Kidneys<\/h1>\n<\/div>\n
The two bean-shaped\u00a0kidneys<\/a><\/strong>\u00a0are located high in the back of the abdominal cavity<\/a>, one on each side of the spine. Both kidneys sit just below the diaphragm<\/a>, the large\u00a0breathing\u00a0muscle that separates the abdominal and thoracic cavities. As you can see in the\u00a0following\u00a0figure, the right kidney is slightly smaller and lower than the left kidney. The right kidney is behind the liver<\/a>, and the left kidney is behind the spleen<\/a>. The\u00a0location\u00a0of the liver explains why the right kidney is smaller and lower than the left.<\/p>\nFigure 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\u2019s main artery (aorta) and vein (inferior vena cava). The ureter exiting each kidney is also shown in the diagram.<\/em><\/figcaption><\/figure>\nKidney Anatomy<\/span><\/p>\nThe 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 hilum<\/a><\/strong>. The entire kidney is surrounded by tough fibrous tissue \u2014 called the\u00a0renal capsule<\/a><\/strong>\u00a0\u2014 which, in turn, is surrounded by two layers of protective, cushioning fat.<\/p>\nFigure 16.4.3 This diagram shows the location and relative size of the two kidneys, as well as the internal structure of each kidney.<\/em><\/figcaption><\/figure>\nInternally, each kidney is divided into two major layers: the outer renal cortex<\/strong><\/a> and the inner renal medulla<\/strong><\/a> (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 renal pyramid<\/strong><\/a>. Within the renal pyramids are the structural and functional units of the kidneys, the tiny nephrons<\/span><\/a><\/span>. Between the renal pyramids are projections of cortex called renal columns<\/strong><\/a>. 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 renal pelvis<\/a>, which becomes the ureter as it leaves the kidney.<\/span><\/p>\n\nRenal\u00a0Circulation<\/h3>\n<\/div>\n
The renal circulation is an important part of the kidney\u2019s primary function of filtering waste products from the blood. Blood<\/a> 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.<\/p>\nAs 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 arteries<\/a> that extend through the renal columns<\/a> between the renal pyramids<\/a>. These arteries, in turn, branch into arterioles that penetrate the renal pyramids. Blood in the arterioles passes through nephrons<\/a>, 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 veins<\/a>. Small veins merge into increasingly larger ones, and ultimately into the renal vein, which carries clean blood away from the kidney to the inferior vena cava<\/a>.<\/p>\nFigure 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).<\/em><\/figcaption><\/figure>\n\nNephron Structure and Function<\/h1>\n<\/div>\n
Figure 16.4.4 gives an indication of the complex structure of a nephron. The nephron<\/a><\/strong>\u00a0is 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 metabolism<\/a>. At least half of the waste is urea<\/a>, a waste product produced by\u00a0protein<\/a>\u00a0catabolism<\/a>. Another important waste is uric\u00a0acid<\/a>, produced in\u00a0nucleic acid<\/a>\u00a0catabolism.<\/p>\nComponents of a Nephron<\/h2>\n
Figure 16.4.5 shows in greater detail the components of a nephron<\/a>. Each nephron is composed of an initial filtering component that consists of a network of capillaries called the glomerulus<\/a><\/strong>\u00a0(plural, glomeruli), which is surrounded by a space within a structure\u00a0called\u00a0glomerular capsule<\/a> <\/strong>(also known as the Bowman’s capsule). Extending from glomerular capsule is the renal tubule<\/a><\/strong>. The proximal end (nearest glomerular capsule) of the renal tubule is called the proximal convoluted (coiled) tubule<\/a><\/strong>. From here, the renal tubule continues as a loop (known as the loop of Henle<\/strong><\/a>) (also known as the loop of the nephron), which in turn becomes the distal convoluted tubule<\/strong><\/a>. 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 peritubular capillary network<\/strong><\/a>.<\/p>\nFigure 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.<\/em><\/figcaption><\/figure>\n<\/div>\nFigure 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.<\/figcaption><\/figure>\nFunction of a Nephron<\/h2>\n
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\u2019t pass through the glomerulus \u2014 as well as blood after it passes through the glomerular capillaries \u2014 continues on through an arteriole called the efferent arteriole. The efferent arteriole follows the renal tubule of the nephron, where it continues\u00a0playing a role\u00a0in nephron functioning.<\/p>\n
<\/p>\n
Filtration<\/h3>\n
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 \u2014 called filtrate<\/a><\/strong> \u2014 pass into glomerular capsule, and from there into the proximal end of the renal tubule<\/a>.\u00a0 Anything too large to move through the pores in the glomerulus, such as blood cells, large proteins, etc., stay in the cardiovascular system.\u00a0 At this stage, filtrate (fluid in the nephron) includes water, salts, organic solids (such as nutrients), and waste products of metabolism (such as urea).<\/p>\nFigure 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. Reabsorption is the movement of substance back into the bloodstream and secretion is movement of substances from the blood into the nephron for excretion.<\/em><\/figcaption><\/figure>\nReabsorption and Secretion<\/h3>\n
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 peritubular capillary network<\/a>). 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 glucose<\/a> and amino acids<\/span><\/a><\/span>) are reabsorbed from the filtrate by the blood in the peritubular capillary network. Reabsorption<\/a> occurs mainly in the proximal convoluted tubule and the loop of Henle, as seen in Figure 16.4.7.<\/span><\/p>\nAt 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\u00a0secretion<\/a><\/strong>. 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 homeostasis<\/a> 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, parathyroid hormone<\/a> causes more calcium to be reabsorbed into the blood and more phosphorus to be secreted into the filtrate.<\/p>\nCollection of Urine and Excretion<\/h2>\nFigure 16.4.8 Fresh urine is typically yellow or amber in colour.<\/em><\/figcaption><\/figure>\nBy the time the filtrate has passed through the entire renal tubule, it has become the\u00a0liquid\u00a0waste known as urine<\/a>. Urine empties from the distal end of the renal tubule<\/a> into a\u00a0collecting duct<\/a><\/strong>. 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 antidiuretic\u00a0hormone\u00a0<\/a>from the\u00a0posterior\u00a0pituitary gland<\/a>. This\u00a0hormone\u00a0makes the collecting ducts permeable to water, allowing water molecules to pass through them into capillaries by\u00a0osmosis<\/a>, 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.<\/p>\nUrine finally exits the largest collecting ducts through the renal papillae. It empties into the renal calyces, and finally into the renal pelvis<\/a>. From there, it travels through the ureter<\/a> to the urinary bladder<\/a> 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<\/span>). The darker the colour, generally speaking, the more concentrated the urine is.<\/span><\/p>\n <\/p>\n
Other Functions of the Kidneys<\/span><\/div>\nBesides filtering blood and forming urine for excretion of soluble wastes, the kidneys have several vital functions in maintaining body-wide\u00a0homeostasis<\/a>. Most of these functions are related to the\u00a0composition\u00a0or volume of urine formed by the kidneys. The kidneys must maintain the proper balance of water and salts in the body, normal\u00a0blood pressure<\/a>, and the correct range of blood\u00a0pH<\/a>. 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.<\/p>\nBlood Pressure\u00a0Regulation<\/h2>\n
The kidneys do not control homeostasis all alone. As indicated above, endocrine hormones are also involved. Consider the regulation of\u00a0blood pressure\u00a0by 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\u00a0concentration\u00a0of sodium in the blood to control blood pressure.<\/p>\nFigure 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.<\/em><\/figcaption><\/figure>\nThe renin-angiotensin-aldosterone system is put into play when the\u00a0concentration\u00a0of sodium ions in the blood falls lower than normal. This causes the kidneys to secrete an\u00a0enzyme\u00a0called renin<\/a> into the blood. It also causes the liver to secrete a\u00a0protein\u00a0called angiotensinogen. Renin changes angiotensinogen into a proto-hormone\u00a0called\u00a0angiotensin I. This is converted to angiotensin II by an\u00a0enzyme\u00a0(angiotensin-converting enzyme) in lung capillaries.<\/p>\nAngiotensin 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\u00a0concentration\u00a0of sodium ions in the blood to normal. The increased water in the blood also increases blood volume and blood pressure.<\/p>\n
Other Kidney Hormones<\/h2>\n
Hormones other than renin are also produced and secreted by the kidneys. These include calcitriol and erythropoietin.<\/p>\n
\n- Calcitriol<\/a><\/strong>\u00a0is 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.<\/li>\n
- Erythropoietin<\/a><\/strong> is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of\u00a0 erythrocytes<\/a>\u00a0in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.<\/li>\n<\/ul>\n\n
Feature: Human Biology in the News<\/h1>\n<\/div>\n
Kidney failure is a complication of common disorders including diabetes mellitus<\/a> and hypertension<\/a>. It is estimated that approximately 12.5% of Canadians have some form of kidney disease.\u00a0 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.\u00a0 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.<\/p>\nFor 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\u2019s body in place of the failed kidney. According to Dr. Fissell, the artificial kidney is \u201c… a bio-hybrid device that can mimic a kidney to remove enough waste products, salt, and water to keep a patient off [hemo]dialysis.\u201d<\/p>\n
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\u00a0cells\u00a0that can mimic the natural functions of the kidney. The living cells form a membrane to filter the patient\u2019s blood as a biological kidney would, but with less risk of rejection by the patient\u2019s immune system, because they are embedded within the device. The new kidney doesn\u2019t 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.<\/p>\n
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.<\/p>\n
\n\n\n16.4 Summary<\/span><\/h1>\n<\/header>\n\n\n
Figure 16.4.8 Fresh urine is typically yellow or amber in colour.<\/em>[\/caption]\r\n\r\nBy the time the filtrate has passed through the entire renal tubule, it has become the\u00a0liquid\u00a0waste known as [pb_glossary id=\"4717\"]urine[\/pb_glossary]. Urine empties from the distal end of the [pb_glossary id=\"4749\"]renal tubule[\/pb_glossary] into a\u00a0[pb_glossary id=\"4759\"]collecting duct[\/pb_glossary]<\/strong>. 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 [pb_glossary id=\"5533\"]antidiuretic\u00a0hormone\u00a0[\/pb_glossary]from the\u00a0posterior\u00a0[pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. This\u00a0hormone\u00a0makes the collecting ducts permeable to water, allowing water molecules to pass through them into capillaries by\u00a0[pb_glossary id=\"5663\"]osmosis[\/pb_glossary], 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.\r\n\r\nUrine finally exits the largest collecting ducts through the renal papillae. It empties into the renal calyces, and finally into the [pb_glossary id=\"4724\"]renal pelvis[\/pb_glossary]. From there, it travels through the [pb_glossary id=\"4725\"]ureter[\/pb_glossary] to the [pb_glossary id=\"4731\"]urinary bladder[\/pb_glossary] 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<\/span>). The darker the colour, generally speaking, the more concentrated the urine is.<\/span>\r\n\r\n \r\nBlood Pressure\u00a0Regulation<\/h2>\r\nThe kidneys do not control homeostasis all alone. As indicated above, endocrine hormones are also involved. Consider the regulation of\u00a0blood pressure\u00a0by 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\u00a0concentration\u00a0of sodium in the blood to control blood pressure.\r\n\r\n[caption id=\"attachment_4761\" align=\"aligncenter\" width=\"736\"] 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.<\/em>[\/caption]\r\n\r\nThe renin-angiotensin-aldosterone system is put into play when the\u00a0concentration\u00a0of sodium ions in the blood falls lower than normal. This causes the kidneys to secrete an\u00a0enzyme\u00a0called [pb_glossary id=\"4762\"]renin[\/pb_glossary] into the blood. It also causes the liver to secrete a\u00a0protein\u00a0called angiotensinogen. Renin changes angiotensinogen into a proto-hormone\u00a0called\u00a0angiotensin I. This is converted to angiotensin II by an\u00a0enzyme\u00a0(angiotensin-converting enzyme) in lung capillaries.\r\n\r\nAngiotensin 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\u00a0concentration\u00a0of sodium ions in the blood to normal. The increased water in the blood also increases blood volume and blood pressure.\r\nOther Kidney Hormones<\/h2>\r\nHormones other than renin are also produced and secreted by the kidneys. These include calcitriol and erythropoietin.\r\n\r\n \t- [pb_glossary id=\"4763\"]Calcitriol[\/pb_glossary]<\/strong>\u00a0is 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.<\/li>\r\n \t
- [pb_glossary id=\"4764\"]Erythropoietin[\/pb_glossary]<\/strong> is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of\u00a0 [pb_glossary id=\"4398\"]erythrocytes[\/pb_glossary]\u00a0in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.<\/li>\r\n<\/ul>\r\n\r\n
Feature: Human Biology in the News<\/h1>\r\n<\/div>\r\nKidney failure is a complication of common disorders including [pb_glossary id=\"2771\"]diabetes mellitus[\/pb_glossary] and [pb_glossary id=\"4231\"]hypertension[\/pb_glossary]. It is estimated that approximately 12.5% of Canadians have some form of kidney disease.\u00a0 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.\u00a0 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.\r\n\r\nFor 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\u2019s body in place of the failed kidney. According to Dr. Fissell, the artificial kidney is \u201c... a bio-hybrid device that can mimic a kidney to remove enough waste products, salt, and water to keep a patient off [hemo]dialysis.\u201d\r\n\r\nThe 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\u00a0cells\u00a0that can mimic the natural functions of the kidney. The living cells form a membrane to filter the patient\u2019s blood as a biological kidney would, but with less risk of rejection by the patient\u2019s immune system, because they are embedded within the device. The new kidney doesn\u2019t 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.\r\n\r\nBecause 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.\r\n\r\n\r\n16.4 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- 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\u00a0circulation.<\/li>\r\n \t
- 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 \u2014 which are the tiny functional units of the kidney \u2014 span the cortex and medulla. The entire kidney is surrounded by a fibrous capsule and protective fat layers.<\/li>\r\n \t
- 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.<\/li>\r\n \t
- In each nephron, the glomerulus and surrounding Bowman\u2019s capsule form the unit that filters blood. From Bowman\u2019s 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.<\/li>\r\n \t
- The kidneys control homeostasis with the help of endocrine hormones.\u00a0The 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\u00a0enzyme\u00a0renin secreted by the kidneys works with hormones from the liver and adrenal gland to stimulate nephrons to reabsorb more sodium and water from urine.<\/li>\r\n \t
- The kidneys also secrete endocrine hormones, including calcitriol \u2014 which helps control the level of calcium in the blood \u2014 and erythropoietin, which stimulates bone marrow to produce red blood\u00a0cells.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n16.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"632\"]<\/li>\r\n \t
- Contrast the renal artery and renal vein.<\/li>\r\n \t
- Identify the functions of a nephron.\u00a0Describe in detail what happens to fluids (blood, filtrate, and urine) as they pass through the parts of a nephron.<\/li>\r\n \t
- Identify two endocrine hormones secreted by the kidneys, along with the functions they control.<\/li>\r\n \t
- Name two regions in the kidney where water is reabsorbed.<\/li>\r\n \t
- Is the blood in the glomerular capillaries more or less filtered than the blood in the peritubular capillaries? Explain your answer.<\/li>\r\n \t
- What do you think would happen if\u00a0blood flow to the kidneys is blocked?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n16.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/FN3MFhYPWWo\r\nHow do your kidneys work? - Emma Bryce, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/es-t8lO1KpA\r\n
Urine Formation, Hamada Abass, 2013.<\/p>\r\nhttps:\/\/youtu.be\/bX3C201O4MA\r\n
Printing a human kidney - Anthony Atala, TED-Ed, 2013.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 16.4.1<\/strong>\r\n\r\nSteak and Kidney Pie<\/a> by Charles Haynes<\/a> on Flickr<\/a> is used under a CC BY-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/2.0\/) license.\r\n\r\nFigure 16.4.2<\/strong>\r\n\r\nGray Kidneys<\/a> by\u00a0Henry Vandyke Carter<\/a>\u00a0(1831-1897) on Wikimedia Commons is in the \u00a0public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain). (Bartleby.com<\/a>:\u00a0Gray\u2019s Anatomy<\/a>, Plate 1120<\/a>).\r\n\r\nFigure 16.4.3<\/strong>\r\n\r\nBlausen_0592_KidneyAnatomy_01<\/a> by BruceBlaus<\/a>\u00a0on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.4<\/strong>\r\n\r\nDiagram_showing_how_the_kidneys_work_CRUK_138.svg<\/a> by Cancer Research UK<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 16.4.5<\/strong>\r\n\r\nBlood_Flow_in_the_Nephron<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.6<\/strong>\r\n\r\n1024px-Physiology_of_Nephron<\/a> by Madhero88<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.7<\/strong>\r\n\r\nNephron_Secretion_Reabsorption<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.8<\/strong>\r\n\r\nUrine<\/a> by User:Markhamilton<\/a> at English Wikipedia<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 16.4.9<\/strong>\r\n\r\nRenin_Angiotensin_System-01<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\nReferences<\/h2>\r\n
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\u00a0<\/span><\/span>25.10<\/span><\/span>\u00a0<\/span>Blood flow in the nephron [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 25.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/25-3-gross-anatomy-of-the-kidney<\/span><\/span><\/p>\r\nBetts, 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\u00a0<\/span><\/span>25.17<\/span><\/span>\u00a0<\/span>Locations of secretion and reabsorption in the nephron [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 25.6). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/25-6-tubular-reabsorption<\/span><\/span><\/span><\/span><\/p>\r\nBetts, 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\u00a0<\/span><\/span>26.14<\/span><\/span>\u00a0<\/span>The renin-angiotensin system [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 26.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/26-3-electrolyte-balance<\/span><\/span><\/span><\/span><\/span><\/p>\r\nBlausen.com Staff. (2014). Medical gallery of Blausen Medical 2014.\u00a0WikiJournal of Medicine<\/i>\u00a01<\/b>\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436<\/p>\r\n
Hamada Abass. (2013). Urine formation. YouTube. https:\/\/www.youtube.com\/watch?v=es-t8lO1KpA&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, February 9). How do your kidneys work? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=FN3MFhYPWWo&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, March 15). Printing a human kidney - Anthony Atala. YouTube. https:\/\/www.youtube.com\/watch?v=bX3C201O4MA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\nFigure 16.4.1 Steak and kidney pie!<\/em><\/figcaption><\/figure>\n\nKidneys on the Menu<\/h1>\n<\/div>\n
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.<\/p>\n
\nLocation\u00a0of the Kidneys<\/h1>\n<\/div>\n
The two bean-shaped\u00a0kidneys<\/a><\/strong>\u00a0are located high in the back of the abdominal cavity<\/a>, one on each side of the spine. Both kidneys sit just below the diaphragm<\/a>, the large\u00a0breathing\u00a0muscle that separates the abdominal and thoracic cavities. As you can see in the\u00a0following\u00a0figure, the right kidney is slightly smaller and lower than the left kidney. The right kidney is behind the liver<\/a>, and the left kidney is behind the spleen<\/a>. The\u00a0location\u00a0of the liver explains why the right kidney is smaller and lower than the left.<\/p>\nFigure 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\u2019s main artery (aorta) and vein (inferior vena cava). The ureter exiting each kidney is also shown in the diagram.<\/em><\/figcaption><\/figure>\nKidney Anatomy<\/span><\/p>\nThe 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 hilum<\/a><\/strong>. The entire kidney is surrounded by tough fibrous tissue \u2014 called the\u00a0renal capsule<\/a><\/strong>\u00a0\u2014 which, in turn, is surrounded by two layers of protective, cushioning fat.<\/p>\nFigure 16.4.3 This diagram shows the location and relative size of the two kidneys, as well as the internal structure of each kidney.<\/em><\/figcaption><\/figure>\nInternally, each kidney is divided into two major layers: the outer renal cortex<\/strong><\/a> and the inner renal medulla<\/strong><\/a> (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 renal pyramid<\/strong><\/a>. Within the renal pyramids are the structural and functional units of the kidneys, the tiny nephrons<\/span><\/a><\/span>. Between the renal pyramids are projections of cortex called renal columns<\/strong><\/a>. 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 renal pelvis<\/a>, which becomes the ureter as it leaves the kidney.<\/span><\/p>\n\nRenal\u00a0Circulation<\/h3>\n<\/div>\n
The renal circulation is an important part of the kidney\u2019s primary function of filtering waste products from the blood. Blood<\/a> 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.<\/p>\nAs 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 arteries<\/a> that extend through the renal columns<\/a> between the renal pyramids<\/a>. These arteries, in turn, branch into arterioles that penetrate the renal pyramids. Blood in the arterioles passes through nephrons<\/a>, 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 veins<\/a>. Small veins merge into increasingly larger ones, and ultimately into the renal vein, which carries clean blood away from the kidney to the inferior vena cava<\/a>.<\/p>\nFigure 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).<\/em><\/figcaption><\/figure>\n\nNephron Structure and Function<\/h1>\n<\/div>\n
Figure 16.4.4 gives an indication of the complex structure of a nephron. The nephron<\/a><\/strong>\u00a0is 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 metabolism<\/a>. At least half of the waste is urea<\/a>, a waste product produced by\u00a0protein<\/a>\u00a0catabolism<\/a>. Another important waste is uric\u00a0acid<\/a>, produced in\u00a0nucleic acid<\/a>\u00a0catabolism.<\/p>\nComponents of a Nephron<\/h2>\n
Figure 16.4.5 shows in greater detail the components of a nephron<\/a>. Each nephron is composed of an initial filtering component that consists of a network of capillaries called the glomerulus<\/a><\/strong>\u00a0(plural, glomeruli), which is surrounded by a space within a structure\u00a0called\u00a0glomerular capsule<\/a> <\/strong>(also known as the Bowman’s capsule). Extending from glomerular capsule is the renal tubule<\/a><\/strong>. The proximal end (nearest glomerular capsule) of the renal tubule is called the proximal convoluted (coiled) tubule<\/a><\/strong>. From here, the renal tubule continues as a loop (known as the loop of Henle<\/strong><\/a>) (also known as the loop of the nephron), which in turn becomes the distal convoluted tubule<\/strong><\/a>. 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 peritubular capillary network<\/strong><\/a>.<\/p>\nFigure 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.<\/em><\/figcaption><\/figure>\n<\/div>\nFigure 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.<\/figcaption><\/figure>\nFunction of a Nephron<\/h2>\n
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\u2019t pass through the glomerulus \u2014 as well as blood after it passes through the glomerular capillaries \u2014 continues on through an arteriole called the efferent arteriole. The efferent arteriole follows the renal tubule of the nephron, where it continues\u00a0playing a role\u00a0in nephron functioning.<\/p>\n
<\/p>\n
Filtration<\/h3>\n
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 \u2014 called filtrate<\/a><\/strong> \u2014 pass into glomerular capsule, and from there into the proximal end of the renal tubule<\/a>.\u00a0 Anything too large to move through the pores in the glomerulus, such as blood cells, large proteins, etc., stay in the cardiovascular system.\u00a0 At this stage, filtrate (fluid in the nephron) includes water, salts, organic solids (such as nutrients), and waste products of metabolism (such as urea).<\/p>\nFigure 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. Reabsorption is the movement of substance back into the bloodstream and secretion is movement of substances from the blood into the nephron for excretion.<\/em><\/figcaption><\/figure>\nReabsorption and Secretion<\/h3>\n
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 peritubular capillary network<\/a>). 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 glucose<\/a> and amino acids<\/span><\/a><\/span>) are reabsorbed from the filtrate by the blood in the peritubular capillary network. Reabsorption<\/a> occurs mainly in the proximal convoluted tubule and the loop of Henle, as seen in Figure 16.4.7.<\/span><\/p>\nAt 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\u00a0secretion<\/a><\/strong>. 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 homeostasis<\/a> 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, parathyroid hormone<\/a> causes more calcium to be reabsorbed into the blood and more phosphorus to be secreted into the filtrate.<\/p>\nCollection of Urine and Excretion<\/h2>\nFigure 16.4.8 Fresh urine is typically yellow or amber in colour.<\/em><\/figcaption><\/figure>\nBy the time the filtrate has passed through the entire renal tubule, it has become the\u00a0liquid\u00a0waste known as urine<\/a>. Urine empties from the distal end of the renal tubule<\/a> into a\u00a0collecting duct<\/a><\/strong>. 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 antidiuretic\u00a0hormone\u00a0<\/a>from the\u00a0posterior\u00a0pituitary gland<\/a>. This\u00a0hormone\u00a0makes the collecting ducts permeable to water, allowing water molecules to pass through them into capillaries by\u00a0osmosis<\/a>, 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.<\/p>\nUrine finally exits the largest collecting ducts through the renal papillae. It empties into the renal calyces, and finally into the renal pelvis<\/a>. From there, it travels through the ureter<\/a> to the urinary bladder<\/a> 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<\/span>). The darker the colour, generally speaking, the more concentrated the urine is.<\/span><\/p>\n <\/p>\n
Other Functions of the Kidneys<\/span><\/div>\nBesides filtering blood and forming urine for excretion of soluble wastes, the kidneys have several vital functions in maintaining body-wide\u00a0homeostasis<\/a>. Most of these functions are related to the\u00a0composition\u00a0or volume of urine formed by the kidneys. The kidneys must maintain the proper balance of water and salts in the body, normal\u00a0blood pressure<\/a>, and the correct range of blood\u00a0pH<\/a>. 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.<\/p>\nBlood Pressure\u00a0Regulation<\/h2>\n
The kidneys do not control homeostasis all alone. As indicated above, endocrine hormones are also involved. Consider the regulation of\u00a0blood pressure\u00a0by 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\u00a0concentration\u00a0of sodium in the blood to control blood pressure.<\/p>\nFigure 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.<\/em><\/figcaption><\/figure>\nThe renin-angiotensin-aldosterone system is put into play when the\u00a0concentration\u00a0of sodium ions in the blood falls lower than normal. This causes the kidneys to secrete an\u00a0enzyme\u00a0called renin<\/a> into the blood. It also causes the liver to secrete a\u00a0protein\u00a0called angiotensinogen. Renin changes angiotensinogen into a proto-hormone\u00a0called\u00a0angiotensin I. This is converted to angiotensin II by an\u00a0enzyme\u00a0(angiotensin-converting enzyme) in lung capillaries.<\/p>\nAngiotensin 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\u00a0concentration\u00a0of sodium ions in the blood to normal. The increased water in the blood also increases blood volume and blood pressure.<\/p>\n
Other Kidney Hormones<\/h2>\n
Hormones other than renin are also produced and secreted by the kidneys. These include calcitriol and erythropoietin.<\/p>\n
\n- Calcitriol<\/a><\/strong>\u00a0is 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.<\/li>\n
- Erythropoietin<\/a><\/strong> is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of\u00a0 erythrocytes<\/a>\u00a0in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.<\/li>\n<\/ul>\n\n
Feature: Human Biology in the News<\/h1>\n<\/div>\n
Kidney failure is a complication of common disorders including diabetes mellitus<\/a> and hypertension<\/a>. It is estimated that approximately 12.5% of Canadians have some form of kidney disease.\u00a0 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.\u00a0 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.<\/p>\nFor 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\u2019s body in place of the failed kidney. According to Dr. Fissell, the artificial kidney is \u201c… a bio-hybrid device that can mimic a kidney to remove enough waste products, salt, and water to keep a patient off [hemo]dialysis.\u201d<\/p>\n
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\u00a0cells\u00a0that can mimic the natural functions of the kidney. The living cells form a membrane to filter the patient\u2019s blood as a biological kidney would, but with less risk of rejection by the patient\u2019s immune system, because they are embedded within the device. The new kidney doesn\u2019t 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.<\/p>\n
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.<\/p>\n
\n\n\n16.4 Summary<\/span><\/h1>\n<\/header>\n\n\n
- \r\n \t
- [pb_glossary id=\"4763\"]Calcitriol[\/pb_glossary]<\/strong>\u00a0is 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.<\/li>\r\n \t
- [pb_glossary id=\"4764\"]Erythropoietin[\/pb_glossary]<\/strong> is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of\u00a0 [pb_glossary id=\"4398\"]erythrocytes[\/pb_glossary]\u00a0in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.<\/li>\r\n<\/ul>\r\n
\r\nFeature: Human Biology in the News<\/h1>\r\n<\/div>\r\nKidney failure is a complication of common disorders including [pb_glossary id=\"2771\"]diabetes mellitus[\/pb_glossary] and [pb_glossary id=\"4231\"]hypertension[\/pb_glossary]. It is estimated that approximately 12.5% of Canadians have some form of kidney disease.\u00a0 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.\u00a0 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.\r\n\r\nFor 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\u2019s body in place of the failed kidney. According to Dr. Fissell, the artificial kidney is \u201c... a bio-hybrid device that can mimic a kidney to remove enough waste products, salt, and water to keep a patient off [hemo]dialysis.\u201d\r\n\r\nThe 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\u00a0cells\u00a0that can mimic the natural functions of the kidney. The living cells form a membrane to filter the patient\u2019s blood as a biological kidney would, but with less risk of rejection by the patient\u2019s immune system, because they are embedded within the device. The new kidney doesn\u2019t 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.\r\n\r\nBecause 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.\r\n
\r\n\r\n 16.4 Summary<\/span><\/h1>\r\n<\/header>\r\n
\r\n- \r\n \t
- 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\u00a0circulation.<\/li>\r\n \t
- 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 \u2014 which are the tiny functional units of the kidney \u2014 span the cortex and medulla. The entire kidney is surrounded by a fibrous capsule and protective fat layers.<\/li>\r\n \t
- 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.<\/li>\r\n \t
- In each nephron, the glomerulus and surrounding Bowman\u2019s capsule form the unit that filters blood. From Bowman\u2019s 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.<\/li>\r\n \t
- The kidneys control homeostasis with the help of endocrine hormones.\u00a0The 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\u00a0enzyme\u00a0renin secreted by the kidneys works with hormones from the liver and adrenal gland to stimulate nephrons to reabsorb more sodium and water from urine.<\/li>\r\n \t
- The kidneys also secrete endocrine hormones, including calcitriol \u2014 which helps control the level of calcium in the blood \u2014 and erythropoietin, which stimulates bone marrow to produce red blood\u00a0cells.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n 16.4 Review Questions<\/span><\/h1>\r\n<\/header>\r\n
\r\n- \r\n \t
- [h5p id=\"632\"]<\/li>\r\n \t
- Contrast the renal artery and renal vein.<\/li>\r\n \t
- Identify the functions of a nephron.\u00a0Describe in detail what happens to fluids (blood, filtrate, and urine) as they pass through the parts of a nephron.<\/li>\r\n \t
- Identify two endocrine hormones secreted by the kidneys, along with the functions they control.<\/li>\r\n \t
- Name two regions in the kidney where water is reabsorbed.<\/li>\r\n \t
- Is the blood in the glomerular capillaries more or less filtered than the blood in the peritubular capillaries? Explain your answer.<\/li>\r\n \t
- What do you think would happen if\u00a0blood flow to the kidneys is blocked?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n 16.4 Explore More<\/span><\/h1>\r\n<\/header>\r\n
\r\n\r\nhttps:\/\/youtu.be\/FN3MFhYPWWo\r\nHow do your kidneys work? - Emma Bryce, TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/es-t8lO1KpA\r\n
Urine Formation, Hamada Abass, 2013.<\/p>\r\nhttps:\/\/youtu.be\/bX3C201O4MA\r\n
Printing a human kidney - Anthony Atala, TED-Ed, 2013.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 16.4.1<\/strong>\r\n\r\nSteak and Kidney Pie<\/a> by Charles Haynes<\/a> on Flickr<\/a> is used under a CC BY-SA 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/2.0\/) license.\r\n\r\nFigure 16.4.2<\/strong>\r\n\r\nGray Kidneys<\/a> by\u00a0Henry Vandyke Carter<\/a>\u00a0(1831-1897) on Wikimedia Commons is in the \u00a0public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain). (Bartleby.com<\/a>:\u00a0Gray\u2019s Anatomy<\/a>, Plate 1120<\/a>).\r\n\r\nFigure 16.4.3<\/strong>\r\n\r\nBlausen_0592_KidneyAnatomy_01<\/a> by BruceBlaus<\/a>\u00a0on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.4<\/strong>\r\n\r\nDiagram_showing_how_the_kidneys_work_CRUK_138.svg<\/a> by Cancer Research UK<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 16.4.5<\/strong>\r\n\r\nBlood_Flow_in_the_Nephron<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.6<\/strong>\r\n\r\n1024px-Physiology_of_Nephron<\/a> by Madhero88<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.7<\/strong>\r\n\r\nNephron_Secretion_Reabsorption<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 16.4.8<\/strong>\r\n\r\nUrine<\/a> by User:Markhamilton<\/a> at English Wikipedia<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/Public_domain).\r\n\r\nFigure 16.4.9<\/strong>\r\n\r\nRenin_Angiotensin_System-01<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n
References<\/h2>\r\n
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\u00a0<\/span><\/span>25.10<\/span><\/span>\u00a0<\/span>Blood flow in the nephron [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 25.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/25-3-gross-anatomy-of-the-kidney<\/span><\/span><\/p>\r\n
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\u00a0<\/span><\/span>25.17<\/span><\/span>\u00a0<\/span>Locations of secretion and reabsorption in the nephron [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 25.6). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/25-6-tubular-reabsorption<\/span><\/span><\/span><\/span><\/p>\r\n
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\u00a0<\/span><\/span>26.14<\/span><\/span>\u00a0<\/span>The renin-angiotensin system [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 26.3). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/26-3-electrolyte-balance<\/span><\/span><\/span><\/span><\/span><\/p>\r\n
Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014.\u00a0WikiJournal of Medicine<\/i>\u00a01<\/b>\u00a0(2).\u00a0DOI:10.15347\/wjm\/2014.010.\u00a0ISSN\u00a02002-4436<\/p>\r\n
Hamada Abass. (2013). Urine formation. YouTube. https:\/\/www.youtube.com\/watch?v=es-t8lO1KpA&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, February 9). How do your kidneys work? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=FN3MFhYPWWo&feature=youtu.be<\/p>\r\n
TED-Ed. (2013, March 15). Printing a human kidney - Anthony Atala. YouTube. https:\/\/www.youtube.com\/watch?v=bX3C201O4MA&feature=youtu.be<\/p>\r\n ","rendered":"
<\/p>\n
Figure 16.4.1 Steak and kidney pie!<\/em><\/figcaption><\/figure>\n \nKidneys on the Menu<\/h1>\n<\/div>\n
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.<\/p>\n
\nLocation\u00a0of the Kidneys<\/h1>\n<\/div>\n
The two bean-shaped\u00a0kidneys<\/a><\/strong>\u00a0are located high in the back of the abdominal cavity<\/a>, one on each side of the spine. Both kidneys sit just below the diaphragm<\/a>, the large\u00a0breathing\u00a0muscle that separates the abdominal and thoracic cavities. As you can see in the\u00a0following\u00a0figure, the right kidney is slightly smaller and lower than the left kidney. The right kidney is behind the liver<\/a>, and the left kidney is behind the spleen<\/a>. The\u00a0location\u00a0of the liver explains why the right kidney is smaller and lower than the left.<\/p>\n
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\u2019s main artery (aorta) and vein (inferior vena cava). The ureter exiting each kidney is also shown in the diagram.<\/em><\/figcaption><\/figure>\n Kidney Anatomy<\/span><\/p>\n
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 hilum<\/a><\/strong>. The entire kidney is surrounded by tough fibrous tissue \u2014 called the\u00a0renal capsule<\/a><\/strong>\u00a0\u2014 which, in turn, is surrounded by two layers of protective, cushioning fat.<\/p>\n
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.<\/em><\/figcaption><\/figure>\n Internally, each kidney is divided into two major layers: the outer renal cortex<\/strong><\/a> and the inner renal medulla<\/strong><\/a> (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 renal pyramid<\/strong><\/a>. Within the renal pyramids are the structural and functional units of the kidneys, the tiny nephrons<\/span><\/a><\/span>. Between the renal pyramids are projections of cortex called renal columns<\/strong><\/a>. 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 renal pelvis<\/a>, which becomes the ureter as it leaves the kidney.<\/span><\/p>\n
\nRenal\u00a0Circulation<\/h3>\n<\/div>\n
The renal circulation is an important part of the kidney\u2019s primary function of filtering waste products from the blood. Blood<\/a> 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.<\/p>\n
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 arteries<\/a> that extend through the renal columns<\/a> between the renal pyramids<\/a>. These arteries, in turn, branch into arterioles that penetrate the renal pyramids. Blood in the arterioles passes through nephrons<\/a>, 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 veins<\/a>. Small veins merge into increasingly larger ones, and ultimately into the renal vein, which carries clean blood away from the kidney to the inferior vena cava<\/a>.<\/p>\n
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).<\/em><\/figcaption><\/figure>\n \nNephron Structure and Function<\/h1>\n<\/div>\n
Figure 16.4.4 gives an indication of the complex structure of a nephron. The nephron<\/a><\/strong>\u00a0is 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 metabolism<\/a>. At least half of the waste is urea<\/a>, a waste product produced by\u00a0protein<\/a>\u00a0catabolism<\/a>. Another important waste is uric\u00a0acid<\/a>, produced in\u00a0nucleic acid<\/a>\u00a0catabolism.<\/p>\n
Components of a Nephron<\/h2>\n
Figure 16.4.5 shows in greater detail the components of a nephron<\/a>. Each nephron is composed of an initial filtering component that consists of a network of capillaries called the glomerulus<\/a><\/strong>\u00a0(plural, glomeruli), which is surrounded by a space within a structure\u00a0called\u00a0glomerular capsule<\/a> <\/strong>(also known as the Bowman’s capsule). Extending from glomerular capsule is the renal tubule<\/a><\/strong>. The proximal end (nearest glomerular capsule) of the renal tubule is called the proximal convoluted (coiled) tubule<\/a><\/strong>. From here, the renal tubule continues as a loop (known as the loop of Henle<\/strong><\/a>) (also known as the loop of the nephron), which in turn becomes the distal convoluted tubule<\/strong><\/a>. 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 peritubular capillary network<\/strong><\/a>.<\/p>\n
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.<\/em><\/figcaption><\/figure>\n <\/div>\nFigure 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.<\/figcaption><\/figure>\n Function of a Nephron<\/h2>\n
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\u2019t pass through the glomerulus \u2014 as well as blood after it passes through the glomerular capillaries \u2014 continues on through an arteriole called the efferent arteriole. The efferent arteriole follows the renal tubule of the nephron, where it continues\u00a0playing a role\u00a0in nephron functioning.<\/p>\n
<\/p>\n
Filtration<\/h3>\n
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 \u2014 called filtrate<\/a><\/strong> \u2014 pass into glomerular capsule, and from there into the proximal end of the renal tubule<\/a>.\u00a0 Anything too large to move through the pores in the glomerulus, such as blood cells, large proteins, etc., stay in the cardiovascular system.\u00a0 At this stage, filtrate (fluid in the nephron) includes water, salts, organic solids (such as nutrients), and waste products of metabolism (such as urea).<\/p>\n
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. Reabsorption is the movement of substance back into the bloodstream and secretion is movement of substances from the blood into the nephron for excretion.<\/em><\/figcaption><\/figure>\n Reabsorption and Secretion<\/h3>\n
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 peritubular capillary network<\/a>). 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 glucose<\/a> and amino acids<\/span><\/a><\/span>) are reabsorbed from the filtrate by the blood in the peritubular capillary network. Reabsorption<\/a> occurs mainly in the proximal convoluted tubule and the loop of Henle, as seen in Figure 16.4.7.<\/span><\/p>\n
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\u00a0secretion<\/a><\/strong>. 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 homeostasis<\/a> 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, parathyroid hormone<\/a> causes more calcium to be reabsorbed into the blood and more phosphorus to be secreted into the filtrate.<\/p>\n
Collection of Urine and Excretion<\/h2>\n
Figure 16.4.8 Fresh urine is typically yellow or amber in colour.<\/em><\/figcaption><\/figure>\n By the time the filtrate has passed through the entire renal tubule, it has become the\u00a0liquid\u00a0waste known as urine<\/a>. Urine empties from the distal end of the renal tubule<\/a> into a\u00a0collecting duct<\/a><\/strong>. 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 antidiuretic\u00a0hormone\u00a0<\/a>from the\u00a0posterior\u00a0pituitary gland<\/a>. This\u00a0hormone\u00a0makes the collecting ducts permeable to water, allowing water molecules to pass through them into capillaries by\u00a0osmosis<\/a>, 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.<\/p>\n
Urine finally exits the largest collecting ducts through the renal papillae. It empties into the renal calyces, and finally into the renal pelvis<\/a>. From there, it travels through the ureter<\/a> to the urinary bladder<\/a> 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<\/span>). The darker the colour, generally speaking, the more concentrated the urine is.<\/span><\/p>\n
<\/p>\n
Other Functions of the Kidneys<\/span><\/div>\nBesides filtering blood and forming urine for excretion of soluble wastes, the kidneys have several vital functions in maintaining body-wide\u00a0homeostasis<\/a>. Most of these functions are related to the\u00a0composition\u00a0or volume of urine formed by the kidneys. The kidneys must maintain the proper balance of water and salts in the body, normal\u00a0blood pressure<\/a>, and the correct range of blood\u00a0pH<\/a>. 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.<\/p>\n
Blood Pressure\u00a0Regulation<\/h2>\n
The kidneys do not control homeostasis all alone. As indicated above, endocrine hormones are also involved. Consider the regulation of\u00a0blood pressure\u00a0by 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\u00a0concentration\u00a0of sodium in the blood to control blood pressure.<\/p>\n
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.<\/em><\/figcaption><\/figure>\n The renin-angiotensin-aldosterone system is put into play when the\u00a0concentration\u00a0of sodium ions in the blood falls lower than normal. This causes the kidneys to secrete an\u00a0enzyme\u00a0called renin<\/a> into the blood. It also causes the liver to secrete a\u00a0protein\u00a0called angiotensinogen. Renin changes angiotensinogen into a proto-hormone\u00a0called\u00a0angiotensin I. This is converted to angiotensin II by an\u00a0enzyme\u00a0(angiotensin-converting enzyme) in lung capillaries.<\/p>\n
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\u00a0concentration\u00a0of sodium ions in the blood to normal. The increased water in the blood also increases blood volume and blood pressure.<\/p>\n
Other Kidney Hormones<\/h2>\n
Hormones other than renin are also produced and secreted by the kidneys. These include calcitriol and erythropoietin.<\/p>\n
- \n
- Calcitriol<\/a><\/strong>\u00a0is 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.<\/li>\n
- Erythropoietin<\/a><\/strong> is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of\u00a0 erythrocytes<\/a>\u00a0in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.<\/li>\n<\/ul>\n
\nFeature: Human Biology in the News<\/h1>\n<\/div>\n
Kidney failure is a complication of common disorders including diabetes mellitus<\/a> and hypertension<\/a>. It is estimated that approximately 12.5% of Canadians have some form of kidney disease.\u00a0 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.\u00a0 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.<\/p>\n
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\u2019s body in place of the failed kidney. According to Dr. Fissell, the artificial kidney is \u201c… a bio-hybrid device that can mimic a kidney to remove enough waste products, salt, and water to keep a patient off [hemo]dialysis.\u201d<\/p>\n
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\u00a0cells\u00a0that can mimic the natural functions of the kidney. The living cells form a membrane to filter the patient\u2019s blood as a biological kidney would, but with less risk of rejection by the patient\u2019s immune system, because they are embedded within the device. The new kidney doesn\u2019t 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.<\/p>\n
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.<\/p>\n
\n\n\n 16.4 Summary<\/span><\/h1>\n<\/header>\n
\n- \n
- Erythropoietin<\/a><\/strong> is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of\u00a0 erythrocytes<\/a>\u00a0in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.<\/li>\n<\/ul>\n
- Calcitriol<\/a><\/strong>\u00a0is 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.<\/li>\n
- [pb_glossary id=\"4764\"]Erythropoietin[\/pb_glossary]<\/strong> is secreted by the kidneys in response to low levels of oxygen in the blood. This hormone stimulates erythropoiesis, which is the production of\u00a0 [pb_glossary id=\"4398\"]erythrocytes[\/pb_glossary]\u00a0in bone marrow. Extra red blood cells increase the level of oxygen carried in the blood.<\/li>\r\n<\/ul>\r\n
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