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kidneys

kidneys The kidneys are situated on each side of the vertebral column, at the level of the last (twelfth) rib. Each kidney is about 12 cm long and weighs about 150 g — about the size of a fist. Despite their small size, the two kidneys receive an enormous blood flow — about 1.2 litres/min in an adult — which is a quarter of the total output of the heart (5 litres/min).

One of the main functions of the kidneys is the removal from the body (excretion) of waste products such as urea, uric acid, and creatinine. However, the kidneys' role is not merely excretion. They are also regulatory organs, controlling the volume and the composition of the body fluids and maintaining the correct osmolality, ion concentrations, and acid–base status of the body.

Each kidney is bean-shaped, with a slit opening — termed the hilus — through which pass the renal artery and vein, the renal nerves and lymphatics, and the ureter, which connects the kidney to the bladder (Fig. 1). A tough connective tissue capsule covers the outer layer of the kidney, the cortex. The deeper part of the kidney, the medulla, consists of a number (6–18) of conical pyramids, the tips of which (papillae) project into the funnel-shaped urine collectors — the renal calyxes (calices) — which merge to form the funnel-shaped upper end of the ureter — the renal pelvis. (Renal, pertaining to the kidney, from its Latin name, ren.)

The nephron is the functional unit of the kidney. (Nephros is the Greek for kidney.) Each kidney has about one million nephrons, and the total length of the nephrons in the body is about 100 miles!

The nephron begins as a Bowman's capsule — the blind end of the nephron — invaginated by a knot of capillaries, the glomerulus (glomerular capillaries). A Bowman's capsule and its glomerular capillaries are together termed a renal corpuscle. Sir William Bowman, British surgeon and histologist, described this in 1842.

The rest of the nephron consists of the proximal convoluted tubule, proximal straight tubule, loop of Henle, and distal convoluted tubule. The distal tubules join to form collecting tubules which in turn join to form collecting ducts, which open at the tip of the renal papilla (Fig. 2).

The Bowman's capsules, proximal tubules, and distal tubules are situated in the renal cortex, whereas the loops of Henle and the collecting ducts extend down through the medulla.

The function of the kidneys is to produce urine, a fluid of variable volume and composition (within limits), depending on the need of the body to excrete or conserve water or solutes. The first step in the production of urine is the filtration of plasma passing through the kidney. This filtration (sometimes called ultrafiltration as it occurs at the molecular level rather than gross particle level) occurs from the glomerular capillaries into the Bowman's capsule to form tubular fluid. The glomerular filter prevents plasma proteins from passing into the nephrons, but is permeable to all other plasma constituents (such as ions, glucose, amino acids, urea, etc). Thus filtration in the kidney is essentially non-selective — substances which the body needs to retain are filtered, as well as those substances which need to be excreted.

Filtration is the bulk flow of water through a semipermeable membrane (filter), carrying with it those solutes which can pass through the filter. As mentioned above, the glomerular filter only excludes plasma proteins. Water moves by bulk flow through the filter as a consequence of pressure gradients. Immediately upstream and downstream from the glomerular capillaries, there are blood vessels which have smooth muscle in their walls, so that they can constrict or dilate, and so alter the resistance to the flow of blood. These vessels are, respectively, the afferent and efferent arterioles. They permit precise regulation of the hydrostatic pressure of the blood in the glomerular capillaries, which is maintained at a higher level than in capillaries in other parts of the body. This force drives plasma from the glomerular capillaries into the nephrons. However, two forces work in opposition to this movement. One is the osmotic pressure exerted by the plasma proteins, which increases as filtration proceeds and the proteins, because they are not filtered, get more concentrated. The other force opposing filtration is the hydrostatic pressure within the Bowman's capsule. The resultant is a net filtration pressure which diminishes as blood flows through the glomerlus. The amount of filtration that actually occurs is known as the glomerular filtration rate, or GFR. It is about 120 ml/min (180 l/day). This seems an enormous volume — and it is an enormous volume — but it is important to realize that it is only a small fraction of the total plasma delivered to the kidneys in the blood. In this respect, the kidneys are rather different from our everyday experiences of filters. For example, when we make filter coffee, we pour water over coffee in the filter, and essentially all the water goes through the filter, leaving a ‘sludge’ of coffee grounds in the filter. If all of the plasma delivered to the kidneys passed through the glomerular filters into the nephrons, the filters would be clogged with a ‘sludge’ of red cells, white cells, and plasma proteins. This is prevented because only 20% of the plasma arriving at the filter actually passes through. The remaining 80% continues into the efferent arterioles.

The volume of plasma in the whole of the circulating blood is only about 3 litres, yet we filter 180 litres per day of it. This apparently paradoxical situation is possible because, after filtration, almost all (99%) of the plasma is reabsorbed along the nephron, so can be filtered again and again (60 times a day!). The selectivity of the kidney — how it is able to conserve some substances and excrete others — is due to the transport processes (reabsorption and secretion) which occur along the nephron, modifying the composition of the glomerular filtrate.

In the nephrons, the terms ‘reabsorption’ and ‘secretion’ indicate the direction of movement. Reabsorption is movement of a substance from the tubular fluid, through the tubular cells or between them and thence into the blood. Secretion is movement in the opposite direction.

If a transport process is directly linked to the consumption of metabolic energy, it is termed ‘active’. In the kidney, the quantitatively most important active transport process is the reabsorption of sodium ions (Na+). Up to 80% of the kidneys' oxygen consumption drives this process, and because the energy comes from the breakdown of adenosine triphosphate (ATP), Na+ active transporters are termed ATPases. There are many other transporter molecules in the nephron cells, many driven by gradients (e.g. for Na+) set up by active transport. Such transport is termed ‘secondary active’ for example, glucose reabsorption is via a transporter which also carries Na+ into the cell, with the driving force being the Na+ concentration gradient set up by the active transport of Na+ out of the cell. In addition to ATPases and transporter molecules, nephron cell membranes also contain proteins which constitute ‘channels’ for the passage of ions, neutral molecules or water.

The proximal tubule reabsorbs about 70% of the filtered Na+, 70% of the filtered water, and, normally, 100% of the filtered glucose and amino acids. Diabetes mellitus, the condition in which glucose is excreted in the urine, is caused by the failure to maintain the normal plasma level of glucose. In diabetes mellitus the plasma glucose concentration is increased, so the filtered load of glucose is increased; if the increase is big enough the nephrons are unable to reabsorb it all, and some appears in the urine.

The sodium which is reabsorbed in the ascending loop of Henle is not accompanied by water, since this part of the nephron is impermeable to water. Consequently, Na+ transport at this site lowers the solute concentration of the tubular fluid, and raises that of the fluid in the interstitial space of the medulla, which surrounds the tubules. This high medullary concentration is the osmotic driving force for water reabsorption in the collecting tubules under the influence of ADH (see below).

Just how efficient the kidneys are at controlling our body fluid volume is demonstrated by the constancy of the body weight from day to day. Even if you spend the evening in the pub and drink a couple of kilograms of beer, your body weight will be back to normal the next day!

The volume of urine excreted by the kidneys can vary between 400 ml/day, and about 25 L/day. The main determinants of urine volume are the osmotic concentration of the body fluids, and the effective circulating volume (the volume of blood circulating around the body in the vascular system). These regulate the urine volume primarily by affecting the release or production of hormones which control renal function.

If our fluid intake is less than the fluid loss, the body fluid osmotic concentration (osmolality) increases — the solutes of the body are in a smaller volume than normal, so their concentration is higher. This increased osmotic concentration is detected by ‘osmoreceptors’ in the brain, and these lead to the release, from the posterior pituitary gland, of the peptide antidiuretic hormone (ADH), also called vasopressin. This hormone circulates in the blood and binds to ‘V2’ receptors on the cells of the kidneys' collecting tubules. It causes them in effect to become more permeable to water, by incorporating water channels in their cell membranes. Because there is always an osmotic gradient tending to move water out of these tubules into the fluid around them and thence into the blood, more water is reabsorbed, the volume of urine is decreased and it becomes more concentrated. The raised osmolality of the body fluids is thus corrected. Because of this continual homeostatic mechanism, the urine volume, which can range from 400 ml/day to 25 litres/day is primarily determined by the level of circulating ADH. A typical volume is 1.5 litres/day.

Decreases in the effective circulating volume also increase ADH release, but in addition such decreases increase the release of renin from the juxtaglomerular apparatus of the kidney (a region of each nephron where the afferent arteriole and distal tubule are in contact). Renin is an enzyme, which acts on a plasma protein (a2 globulin) to release a 10-amino acid peptide, angiotensin I. This in turn is converted, by an enzyme present in blood vessel walls, (ACE — angiotensin converting enzyme), to an 8 amino acid peptide, angiotensin II.

Angiotensin II increases nephron Na+ reabsorption. Since water follows Na+, water reabsorption also increases, and urine volume falls. Angiotensin II acts directly on the nephrons, and also causes ADH release and the release of another Na+-retaining hormone, aldosterone.

Another important regulatory function of the kidney is the control of acid–base homeostasis. In general, the metabolism of the body produces excess H+, and this is secreted into the urine by the nephron cells. The pH of the blood and extracellular fluid is kept constant at 7.4, but to achieve this, the kidneys can vary the urine pH from 4.5–8.0.

Kidney function may become impaired, leading to renal failure. There are many potential causes of renal failure, including reduction of the renal blood supply (e.g. as a result of major haemorrhage), toxins and disease organisms, and blockages of the urinary tract. If the kidneys fail, one of the first signs is the accumulation of urea and other nitrogenous waste in the blood —uraemia. This may require treatment by dialysis or by organ transplantation. However, other problems associated with failing kidneys relate to the fact that the kidneys are themselves important endocrine glands. They produce the hormone erythropoietin, which stimulates bone marrow to produce red blood cells, and also convert the precursor form of vitamin D to the active form. Both of these functions can be disrupted in renal failure, leading to anaemia and to disturbance of calcium supply to the bones.

If just one kidney fails, or is surgically removed, then changes take place in the remaining one to enable it to maintain homeostasis. Although the number of nephrons in the surviving kidney does not increase, the glomerular filtration rate of each individual nephron increases, so that the overall glomerular filtration rate increases to approach that which was previously achieved with two kidneys.

Chris Lote

Bibliography

Lote, C. J. (2000). Principles of renal physiology, (4th edn). Kluwer, Amsterdam.


See also acid–base homeostasis; dialysis; urine; water balance.See urogenital system.

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Kidney

Kidney

The kidneys of vertebrates have the vital function of removing metabolic wastes from the blood and otherwise maintaining its normal composition. The two kidneys of a normal human adult produce 1 to 2 liters (about 30 to 70 fluid ounces) of urine each day that contain wastes, excess water, and other unneeded molecules. Production of less than 0.4 liter (13.5 fluid ounces) of urine per day is insufficient to eliminate wastes and regulate the composition of blood. Such a condition is always fatal within a few weeks unless the underlying cause is corrected, a new kidney is transplanted, or the blood is artificially cleared by dialysis .

The human kidney belongs to one of three kinds of kidneys that occur among different vertebrates at various developmental stages. The first type, called the pronephros, lies toward the front of some fishes and the embryos of many vertebrates. The mesonephros lies more posteriorly and occurs in most adult fishes and amphibians and in the embryo of humans and other mammals. The metanephros occurs still farther posteriorly and is the type of kidney in adult reptiles, birds, and mammals, including humans.

Each human kidney is about the size of a fist, shaped like a kidney bean, and located on one side of the lower abdomen toward the back. At any given time about one-fifth of the body's blood is flowing through the kidneys. The blood enters each kidney from the body's major artery, the aorta, by means of the renal artery. (The word "renal" refers to kidney.) Blood leaving the kidney enters the major vein, the vena cava, via the renal vein. Also connecting to the kidney is a third tube, the ureter, which conducts urine to the urinary bladder for temporary storage.


Alcohol suppresses antidiuretic hormone (ADH), which normally reabsorbs water in the kidneys, and so increases urine volume.


From this "plumbing diagram" one can get an overview of renal function: blood enters the kidney, wastes and excess molecules are removed with the urine, and the blood is returned to the circulatory system. To appreciate how the kidneys function, however, one must take a microscopic view of one of the million or so structures called nephrons within each kidney. Each nephron begins its work by producing a filtrate of blood. Filtration occurs in a tuft of capillaries called the glomerulus. The lining of the glomerulus is leaky enough to allow blood pressure to force water, ions , and small molecules out while retaining cells and very large molecules in the blood. The filtrate, which is very much like the fluid portion of blood (plasma), enters Bowman's capsule, which encloses the glomerulus like a helmet. Bowman's capsule conducts the filtrate into the first part of the nephron tubule, called the proximal convoluted tubule. In humans approximately 180 liters of filtrate (almost enough to fill a 50-gallon drum) make it this far each day. Fortunately, not all of it goes into urine. In the proximal tubule, many of the inorganic ions and almost all of the glucose and amino acids get pumped out of the filtrate and go back into the blood. Most of the water in the filtrate is also drawn back into the blood.

The tubular fluid next passes through a hairpin turn called the loop of Henle, which helps the nephron return more water to the bloodstream rather than allowing it to be lost in the urine. How this works will be explained later. Tubular fluid then enters the distal convoluted tubule of the nephron. Here further transport of particular ions may occur, depending on whether the concentration of that ion in the blood is too high or too low. For example, if the pH of the blood is too low, hydrogen ions (H+) are transported out of the blood and into the tubular fluid. If the pH is too high, H+ ions are transported from the fluid into the blood.

By the time the fluid has completed its journey through the distal convoluted tubule, it is essentially dilute urine, called preurine. Preurine from several nephrons enters a tube called the collecting duct. As preurine passes through the collecting duct, more water can be removed and returned to the blood.

Water is drawn out of the collecting duct by osmosis due to an increasing concentration of ions surrounding the collecting duct. The loops of Henle produce this concentration gradient by a combination of transport and diffusion of ions and urea. Urea is a molecule that temporarily stores the nitrogen produced by the metabolism of proteins . After helping to create the concentration gradient, urea is eventually eliminated with the urine.

see also Blood; Drug Testing; Excretory Systems; Heart and Circulation; Osmoregulation; Pituitary Gland

C. Leon Harris

Bibliography

The Kidney. <http://www.ultranet.com/~jkimball/biologyPages/K/Kidney.html>.

Saladin, Kenneth S. "The Urinary System." In Anatomy and Physiology, 2nd ed. Dubuque, IA: McGraw-Hill Higher Education, 2001.

Supplemental Image Database, The Kidney. <http://www.kumc.edu/instruction/medicine/pathology/ed/ch_16/mainframe.html>.

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kidney

kidney In vertebrates, one of a pair of bean-shaped organs responsible for regulating blood composition and the excretion of waste products. The kidneys are at the back of the abdomen, one on each side of the backbone. Each is about 10cm (4in) long and 6.5cm (2.5in) wide. The human kidney consists of an outer cortex and an inner medulla with about one million tubules (nephrons). Nephrons contain numerous capillaries, which filter the blood entering from the renal artery. Some substances, including water, are reabsorbed into the blood. Urine remains, which is passed to the ureter and on to the bladder. One healthy kidney is needed to maintain life. A serious kidney disease may require a kidney transplant or dialysis using a machine. See also capillary; homeostasis

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kidney

kidney organ that secretes urine XIV; transf. temperament, nature XVI. of obscure orig. ME. sg. kidnei and pl. kidneiren suggest that the word was a comp. of ei, pl. eiren (OE. ǣġ, pl. ǣġru) EGG1, the pl. kidneires being partly analogical, partly due to assoc. with ME. and dial. nere(s) kidney(s) :- OE. *nēore, corr. to OHG. nioro m. (G. niere fem.), ON. nýra, rel. to Gr. nephrós kidney, scrotum, L. nefrōnes loins. On the other hand, if the first el. is (dial.) kid pod (:- OE. *cydda :- *kuddja; see COD1), the word may have been OE. *cyd(e)nēora, the ME. repr. of which was assoc. with ei, eiren, the shape of the kidney assisting the comparison.

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kidney

kidney The main organ of excretion and osmoregulation in vertebrates, through which nitrogenous waste material (usually in the form of urine) and surplus water, ions, etc., are eliminated from the body. In mammals there is a pair of kidneys situated in the abdomen (see illustration). Each has an outer cortex and an inner medulla and is made up of tubular units called nephrons, through which nitrogenous waste is filtered from the blood, with the formation of urine. The nephrons drain into a basin-like cavity in the kidney (the renal pelvis), which leads to the ureter and bladder.

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kidney

kidney (kid-ni) n. either of the pair of organs responsible for the excretion of nitrogenous wastes, principally urea, from the blood (see illustration). The active units of the kidney are the nephrons, which filter the blood under pressure and then reabsorb water and selected substances back into the blood. The urine thus formed is conducted via the renal tubules to the ureter, which leads to the bladder. See also haemodialysis, horseshoe kidney, renal (function tests).

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kidney

kidney Usually from lamb, ox, or pig; a 150‐g portion is a rich source of protein, niacin, iron, zinc, copper, selenium, vitamins A, B1, B2, B12, and folate; a good source of vitamin B6 and, unusually for a meat product, vitamin C; a source of iodine; contains about 9 g of fat, of which one‐third is saturated; supplies 150 kcal (630 kJ).

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kidney

kid·ney / ˈkidnē/ • n. (pl. -neys) each of a pair of organs in the abdominal cavity of mammals, birds, and reptiles, excreting urine. ∎  the kidney of a sheep, ox, or pig as food. ∎  temperament, nature, or kind: I hoped that he would not prove of similar kidney.

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kidney

kidney An organ that may have arisen in freshwater animals for the purpose of voiding excess water and that is concerned in modern forms with excretion and/or the retention of water.

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kidney

kidney temperament, nature, as in of the same kidney. The kidneys were anciently thought to control disposition and temperament.

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kidneys

kidneys: see urinary system.

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kidney

kidneyblini, cine, Finney, finny, Ginny, guinea, hinny, mini, Minnie, ninny, pinny, Pliny, shinny, skinny, spinney, tinny, whinny •kidney, Sidney, Sydney •chimney •jitney, Whitney •Disney •aborigine, polygeny, polygyny •androgyny, homogeny, misogyny, progeny •Gemininiminy-piminy, Rimini •dominie, hominy, Melpomene •ignominy • Panini • larceny • telecine •satiny • destiny • mountainy •mutiny, scrutiny •briny, Heine, liny, piny, shiny, spiny, tiny, whiny •sunshiny •Bonnie, bonny, Connie, johnny, Lonnie, Ronnie, Suwannee •Rodney •Cockney, Procne •Romney • Novotný • Grozny •brawny, corny, horny, lawny, mulligatawny, scrawny, tawny, thorny •Orkney • Courtney •brownie, browny, downy, townie

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Kidneys

Kidneys

Definition

The kidneys maintain body fluid volumes and blood pressure, filter blood, and contribute to waste removal by producing urine.

Description

The kidneys are two bean-shaped organs that sit just below the rib cage on either side of the spinal cord. Each is about the size of a bar of soap. At any one time 20-25% of the body's blood flows through them, even though they comprise only 0.5% of the body's total weight. At this rate, the kidneys filter the entire blood supply 60 times per day.

Blood flows into the kidney through the renal artery and exits through the renal vein. Within the kidney are many small capillaries that perfuse it with blood, giving the organ its reddish-brown color.

The gross anatomy of the kidney can be divided into four parts:

  • Capsule: A thin but tough outer membrane that protects the kidney against infection and trauma.
  • Cortex: The outer layer of the kidney's interior, about 1 cm (0.4 inch) thick.
  • Medulla: The inner layer of the kidney's interior, which contains triangular structures called renal pyramids. Between pyramids are sections of cortex called renal columns.
  • Renal pelvis: A large funnel for collecting urine from all parts of the kidney, connected to the bladder by the ureter.

Each cortex and medulla together contain about a million nephrons, microscopic filtering systems that are the basic unit of each kidney. Each nephron has two main components. The first is a vascular system that includes 1) the glomerulus, 2) afferent and efferent arterioles, and 3) peritubular capillaries. The second, tubular component contains five main parts: Bowman's capsule, proximal tubule, loop of Henle, distal tubule, and the collecting duct.

Bowman's capsule forms one end of each nephron. It contains a bundle of tiny capillaries called the glomerulus, which receives its bloodflow from the afferent arteriole. The glomerulus filters minerals, nutrients, wastes, and water from the blood that flows through it, and passes them down into the proximal tubule. The glomerulus also returns large plasma proteins and red blood cells to the blood supply through the efferent arteriole. The efferent arteriole is connected to a second capillary bed called the peritubular capillaries. These two successive capillary beds create a pressure difference that forces fluid through the nephron.

Once filtrate enters the proximal tubule, specialized cells reabsorb sodium and other ions, water, glucose, and amino acids back into the blood. The fluid then goes into the loop of Henle, which helps concentrate the waste products to be excreted in urine. After the loop of Henle, fluid then flows into the highly coiled distal tubule, where potassium is secreted and more water and sodium are reabsorbed back into the blood. The fluid then flows into the last part of the nephron, the collecting duct, where final adjustments are made to the urine concentration. The collecting ducts respond to the antidiuretic hormone (ADH), which regulates the amount of water reabsorbed by the blood. The urine then flows through the renal pelvis to the ureter, which delivers urine to the bladder for excretion.

Function

By filtering the blood, the kidneys play a very important role in the body. They adjust the water volume, remove wastes such as urea, ammonia, and drugs, establish acid-base balance, determine the composition of blood, help maintain blood pressure, stimulate the production of red blood cells, and determine calcium levels.

The maintenance of water volume is a particularly important function. When the body sweats on a hot day or during exercise, it needs a way to sense water loss to avoid dehydration. Water volume is monitored by specialized osmoreceptors in the hypothalamus that measure sodium concentration in the blood. A high sodium concentration means there is insufficient water; this signals the hypothalamus to increase ADH secretion, which in turn prevents the kidneys from reabsorbing water from the blood in the collecting ducts. If the sodium concentration is low, there is too much water in the blood, so the hypothalamus reduces ADH secretion, which tells the kidneys to increase the water concentration of the urine.

The kidneys are also crucial in removing waste products such as urea, ammonia, and any chemical compounds such as medications from the blood. For this reason patients with damaged kidneys must be monitored closely when they take medications that are excreted in the urine. If the kidneys are not working properly, drug concentrations in the blood could rise to fatal levels.

In addition, the kidneys play a pivotal role in the body's acid-base balance. The blood's pH is maintained by a fixed ratio of hydrogen-to-bicarbonate ions in the blood. If the number of hydrogen ions increase, then the blood becomes acidic, a condition known as acidosis. Likewise, if the number of sodium bicarbonate ions rise, the blood becomes basic, a condition known as alkalosis. The kidneys help sustain this hydrogen-to-bicarbonate ratio by adjusting the amount of bicarbonate in the blood. If the blood is too basic the proximal and distal tubules of the kidney will decrease bicarbonate reabsorption and more bicarbonate will be excreted into the urine. If the blood is acidic, then the proximal tubule will allow reabsorption of bicarbonate back into the blood and excrete more hydrogen into the urine.

Another major task the kidneys perform is to help maintain blood pressure. Kidney cells can recognize when a drop in blood pressure occurs, because when this happens, blood flow to the kidney decreases. This means less sodium is present in the kidney cells, a condition that causes the kidney cells to release an enzyme called renin. Renin converts angiotensin I into angiotensin II, which in turn constricts blood vessels and causes sodium retention by the kidneys, thereby raising blood pressure. This is known as the renin-angiotensin system. Angiotensin II also causes the adrenal glands to release the hormone aldosterone, which tells the kidneys to allow more sodium and water to be reabsorbed back into the blood. This increase in water volume in the blood increases blood pressure. Many medications for high blood pressure act by working on the kidneys to decrease blood volume and therefore blood pressure. These blood pressure medications are collectively known as diuretics.

Role in human health

The kidneys play a crucial role in human health because they perform many vital functions. The kidneys work constantly, simultaneously, and influence each other. Individuals are born with two kidneys but can function with one. However, a person with kidney function at 10-15% of capacity will require dialysis or a kidney transplant to sustain life. Individuals with high blood pressure and diabetes have a significant risk of kidney disease.

Common diseases and disorders

Diabetic nephropathy

Diabetic patients cannot process blood glucose properly, and if their disease is untreated or poorly controlled, it can lead to high blood sugar levels. This can damage the nephrons, leading to diabetic neuropathy. This usually means that soft kidney tissue hardens and thickens, a process called sclerosis; this is especially true for the glomerulus. The American Diabetes Association estimates that 35-45% of type 1 diabetics and 20-30% of type 2 diabetics have damaged kidneys. Because the symptoms of nephropathy may not appear until 80% of kidney function is gone, periodic tests of kidney function and strict compliance with diet and treatment regimens are important for patients with diabetes.

High blood pressure

The kidneys use small blood vessels called capillaries to filter blood and to help create a pressure gradient to move fluid through the nephron. Continuous high blood pressure can damage the fragile walls of these vessels. When this happens, blood may not filter properly, allowing waste products and/or drug levels to build up, sometimes to dangerous or fatal levels.

Kidney stones

Kidneys stones occur when crystals form in the lumen of the tubules or in the ureters. The stones are most commonly made of calcium and oxalate or phosphate. The basis of stone formation is not clear but certain foods in certain people can cause them to accrete. Kidney stones can be extremely painful, and can also cause hydronephrosis. Patients with kidney stones are encouraged to drink plenty of water in effort to have the stone excreted in the urine. In some cases, kidney stones must be surgically removed.

Polycystic kidney disease

Polycystic kidney disease (PKD) is an inherited disease in which cysts form in the kidney. These fluidfilled cysts can take over a significant amount of space in the kidney, eventually reducing kidney function and causing kidney failure. Most cases of PKD show no symptoms until the patient is well into adulthood. PKD that appears in children is often more virulent, frequently leading to kidney failure and death. Nutrition and dietary modification play a major role in controlling the progression of PKD.

Wilms' tumor

Wilms' tumor, or nephroblastoma, is a cancer of the kidney that appears during childhood. Both sporadic cases and a few rare inherited cases have been linked to mutations in the Wilm's tumor gene (WT1) on chromsome 11. Many cases of Wilms' tumor are curable if caught early enough.

KEY TERMS

Adrenal gland— Small gland on top of each kidney that produces and releases several different hormones that are involved in maintaining internal fluid and salt levels and also mediates stress responses.

Angiotensin I— Inactive form of angiotensin that circulates in the blood; it is a precursor of angiotensin II.

Angiotensin II— Active form of angiotensin that constricts blood vessels, thus raising blood pressure.

Capillary— Small blood vessel that is the point of connection for blood and veins and where exchanges occur between the blood and tissue.

Hydronephrosis— Distention of the renal pelvis that occurs when urine is trapped in the kidney and blocked from flowing into the bladder.

Ureter— Carries urine from the kidney to the bladder.

Resources

BOOKS

Cohen, Barbara, and Dena Wood. Structure and Function of the Human Body, 7th ed. Philadelphia, PA: Lippincott Williams and Wilkins, 2000.

Johnson, R.J., and J. Feehally. Comprehensive Clinical Nephrology. London: Harcourt Publishers, 2000.

ORGANIZATIONS

National Kidney Foundation. 30 East 33rd St. Suite 1100, New York, NY 10016. (800) 622-9010. 〈http://www.kidney.org〉.

Polycystic Kidney Disease Foundation. 9221 Ward Pkwy., Suite 400, Kansas City, MO 64114. (800) PKD-CURE. 〈http://www.pkdcure.org〉.

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Kidneys

Kidneys

Definition

The kidneys maintain body fluid volumes and blood pressure , filter blood , and contribute to waste removal by producing urine.

Description

The kidneys are two bean-shaped organs that sit just below the rib cage on either side of the spinal cord . Each is about the size of a bar of soap. At any one time 20–25% of the body's blood flows through them, even though they comprise only 0.5% of the body's total weight. At this rate, the kidneys filter the entire blood supply 60 times per day.

Blood flows into the kidney through the renal artery and exits through the renal vein. Within the kidney are many small capillaries that perfuse it with blood, giving the organ its reddish-brown color.

The gross anatomy of the kidney can be divided into four parts:

  • Capsule: A thin but tough outer membrane that protects the kidney against infection and trauma.
  • Cortex: The outer layer of the kidney's interior, about 1 cm (0.4 inch) thick.
  • Medulla: The inner layer of the kidney's interior, which contains triangular structures called renal pyramids. Between pyramids are sections of cortex called renal columns.
  • Renal pelvis: A large funnel for collecting urine from all parts of the kidney, connected to the bladder by the ureter.

Each cortex and medulla together contain about a million nephrons, microscopic filtering systems that are the basic unit of each kidney. Each nephron has two main components. The first is a vascular system that includes1) the glomerulus, 2) afferent and efferent arterioles, and3) peritubular capillaries. The second, tubular component contains five main parts: Bowman's capsule, proximal tubule, loop of Henle, distal tubule, and the collecting duct.

Bowman's capsule forms one end of each nephron. It contains a bundle of tiny capillaries called the glomerulus, which receives its bloodflow from the afferent arteriole. The glomerulus filters minerals , nutrients, wastes, and water from the blood that flows through it, and passes them down into the proximal tubule. The glomerulus also returns large plasma proteins and red blood cells to the blood supply through the efferent arteriole. The efferent arteriole is connected to a second capillary bed called the peritubular capillaries. These two successive capillary beds create a pressure difference that forces fluid through the nephron.

Once filtrate enters the proximal tubule, specialized cells reabsorb sodium and other ions, water, glucose, and amino acids back into the blood. The fluid then goes into the loop of Henle, which helps concentrate the waste products to be excreted in urine. After the loop of Henle, fluid then flows into the highly coiled distal tubule, where potassium is secreted and more water and sodium are reabsorbed back into the blood. The fluid then flows into the last part of the nephron, the collecting duct, where final adjustments are made to the urine concentration. The collecting ducts respond to the antidiuretic hormone (ADH), which regulates the amount of water reabsorbed by the blood. The urine then flows through the renal pelvis to the ureter, which delivers urine to the bladder for excretion.

Function

By filtering the blood, the kidneys play a very important role in the body. They adjust the water volume, remove wastes such as urea, ammonia, and drugs, establish acid-base balance , determine the composition of blood, help maintain blood pressure, stimulate the production of red blood cells, and determine calcium levels.

The maintenance of water volume is a particularly important function. When the body sweats on a hot day or during exercise , it needs a way to sense water loss to avoid dehydration . Water volume is monitored by specialized osmoreceptors in the hypothalamus that measure sodium concentration in the blood. A high sodium concentration means there is insufficient water; this signals the hypothalamus to increase ADH secretion, which in turn prevents the kidneys from reabsorbing water from the blood in the collecting ducts. If the sodium concentration is low, there is too much water in the blood, so the hypothalamus reduces ADH secretion, which tells the kidneys to increase the water concentration of the urine.

The kidneys are also crucial in removing waste products such as urea, ammonia, and any chemical compounds such as medications from the blood. For this reason patients with damaged kidneys must be monitored closely when they take medications that are excreted in the urine. If the kidneys are not working properly, drug concentrations in the blood could rise to fatal levels.

In addition, the kidneys play a pivotal role in the body's acid-base balance. The blood's pH is maintained by a fixed ratio of hydrogen-to-bicarbonate ions in the blood. If the number of hydrogen ions increase, then the blood becomes acidic, a condition known as acidosis. Likewise, if the number of sodium bicarbonate ions rise, the blood becomes basic, a condition known as alkalosis. The kidneys help sustain this hydrogen-to-bicarbonate ratio by adjusting the amount of bicarbonate in the blood. If the blood is too basic the proximal and distal tubules of the kidney will decrease bicarbonate reabsorption and more bicarbonate will be excreted into the urine. If the blood is acidic, then the proximal tubule will allow reab-sorption of bicarbonate back into the blood and excrete more hydrogen into the urine.

Another major task the kidneys perform is to help maintain blood pressure. Kidney cells can recognize when a drop in blood pressure occurs, because when this happens, blood flow to the kidney decreases. This means less sodium is present in the kidney cells, a condition that causes the kidney cells to release an enzyme called renin. Renin converts angiotensin I into angiotensin II, which in turn constricts blood vessels and causes sodium retention by the kidneys, thereby raising blood pressure. This is known as the renin-angiotensin system. Angiotensin II also causes the adrenal glands to release the hormone aldosterone, which tells the kidneys to allow more sodium and water to be reabsorbed back into the blood. This


KEY TERMS


Adrenal gland —Small gland on top of each kidney that produces and releases several different hormones that are involved in maintaining internal fluid and salt levels and also mediates stress responses.

Angiotensin I —Inactive form of angiotensin that circulates in the blood; it is a precursor of angiotensin II.

Angiotensin II —Active form of angiotensin that constricts blood vessels, thus raising blood pressure.

Capillary —Small blood vessel that is the point of connection for blood and veins and where exchanges occur between the blood and tissue.

Hydronephrosis —Distention of the renal pelvis that occurs when urine is trapped in the kidney and blocked from flowing into the bladder.

Ureter —Carries urine from the kidney to the bladder.


increase in water volume in the blood increases blood pressure. Many medications for high blood pressure act by working on the kidneys to decrease blood volume and therefore blood pressure. These blood pressure medications are collectively known as diuretics.

Role in human health

The kidneys play a crucial role in human health because they perform many vital functions. The kidneys work constantly, simultaneously, and influence each other. Individuals are born with two kidneys but can function with one. However, a person with kidney function at 10–15% of capacity will require dialysis or a kidney transplant to sustain life. Individuals with high blood pressure and diabetes have a significant risk of kidney disease.

Common diseases and disorders

Diabetic nephropathy

Diabetic patients cannot process blood glucose properly, and if their disease is untreated or poorly controlled, it can lead to high blood sugar levels. This can damage the nephrons, leading to diabetic neuropathy. This usually means that soft kidney tissue hardens and thickens, a process called sclerosis; this is especially true for the glomerulus. The American Diabetes Association estimates that 35–45% of type 1 diabetics and 20–30% of type 2 diabetics have damaged kidneys. Because the symptoms of nephropathy may not appear until 80% of kidney function is gone, periodic tests of kidney function and strict compliance with diet and treatment regimens are important for patients with diabetes.

High blood pressure

The kidneys use small blood vessels called capillaries to filter blood and to help create a pressure gradient to move fluid through the nephron. Continuous high blood pressure can damage the fragile walls of these vessels. When this happens, blood may not filter properly, allowing waste products and/or drug levels to build up, some times to dangerous or fatal levels. Kidney stones Kidneys stones occur when crystals form in the lumen of the tubules or in the ureters. The stones are most commonly made of calcium and oxalate or phosphate. The basis of stone formation is not clear but certain foods in certain people can cause them to accrete. Kidney stones can be extremely painful, and can also cause hydronephrosis. Patients with kidney stones are encour aged to drink plenty of water in effort to have the stone excreted in the urine. In some cases, kidney stones must be surgically removed.

Polycystic kidney disease

Polycystic kidney disease (PKD) is an inherited disease in which cysts form in the kidney. These fluid-filled cysts can take over a significant amount of space in the kidney, eventually reducing kidney function and causing kidney failure. Most cases of PKD show no symptoms until the patient is well into adulthood. PKD that appears in children is often more virulent, frequently leading to kidney failure and death. Nutrition and dietary modification play a major role in controlling the progression of PKD.

Wilms' tumor

Wilms' tumor, or nephroblastoma, is a cancer of the kidney that appears during childhood. Both sporadic cases and a few rare inherited cases have been linked to mutations in the Wilm's tumor gene (WT1) on chrom-some 11. Many cases of Wilms' tumor are curable if caught early enough.

Resources

BOOKS

Cohen, Barbara, and Dena Wood. Structure and Function of the Human Body, 7th ed. Philadelphia, PA: Lippincott Williams and Wilkins, 2000.

Johnson, R.J., and J. Feehally. Comprehensive Clinical Nephrology. London: Harcourt Publishers, 2000.

ORGANIZATIONS

National Kidney Foundation. 30 East 33rd St. Suite 1100, New York, NY 10016. (800) 622-9010. <http://www.kidney.org>.

Polycystic Kidney Disease Foundation. 4901 Main St. Suite 200, Kansas City, MO 64112. (800) PKD-CURE. <http://www.pkdcure.org>.

Susan M. Mockus, Ph.D.

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