kidney

n., pl., -neys.

1. Anatomy. Either one of a pair of organs in the dorsal region of the vertebrate abdominal cavity, functioning to maintain proper water and electrolyte balance, regulate acid-base concentration, and filter the blood of metabolic wastes, which are then excreted as urine.
2. The kidney of certain animals, eaten as food.
3. An excretory organ of certain invertebrates.
4. Temperment; kind: a person of the same kidney.

[Middle English kidenei.]

An organ involved with the elimination of water and waste products from the body. In vertebrates the kidneys are paired organs located close to the spine dorsally in the body cavity. They consist of a number of smaller functional units called urinary tubules or nephrons. The nephrons open to large ducts, the collecting ducts, which open into a ureter. The two ureters run backward to open into the cloaca or into a urinary bladder. In mammals, the kidneys are bean-shaped and found between the thorax and the pelvis. The number, structure, and function of the nephrons vary with evolution and, in certain significant ways, with the adaptation of the animals to their various habitats.

In its most primitive form, found only in invertebrates, the nephron has a funnel opening into the coelomic cavity followed by a urinary tubule leading to an excretory pore. In amphibians, some of the tubules have this funnel, but most of the tubules have a Bowman capsule (see illustration). In all higher vertebrates, the nephron has the Bowman capsule, which surrounds a tuft of capillary loops, called the glomerulus, constituting the closed end of the nephron. The inner epithelial wall of the Bowman capsule is in intimate contact with the endothelial wall of the capillaries. The wall of the capillaries, together with the inner wall of the Bowman capsule, forms a membrane ideally suited for filtration of the blood.

Nephron from frog kidney, dissected to show glomerulus within Bowman capsule.

The blood pressure in the capillaries of the glomerulus causes filtering of blood by forcing fluid, small molecules, and ions through the membrane into the lumen of Bowman's capsule. This filtrate contains some of the proteins and all of the smaller molecules in the blood. As the filtrate passes down through the tubule, the walls of the tubule extract those substances not destined for excretion and return them to the blood in adjacent capillaries. Many substances which are toxic to the organism are moved in the opposite direction from the blood into the tubules. The urine thus produced by each nephron is conveyed by the collecting duct and ureter to the cloaca or bladder from which it can be eliminated.

In all classes of vertebrates the renal arteries deliver blood to the glomeruli and through a second capillary net to the tubules. The major blood supply to the kidney tubules comes, however, from the renal portal vein, which is found in all vertebrates except mammals and cyclostomes. Waste products from the venous blood can thus be secreted directly into the urinary tubules. See also Urinary system.

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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.

Fig. 1 Diagrammatic cross section of the kidney
(Click to enlarge)


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.

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).

One of the variety meats, the kidney is a glandular organ. The most popular kidneys for cooking are beef, veal, lamb and pork. They're easily distinguishable because beef and veal kidneys are multi-lobed while lamb and pork are single-lobed. In general, the texture is more tender and the flavor more delicate in younger animals. The kidneys from younger animals are pale while those from older animals become deep reddish-brown; they're also tougher and stronger-flavored. Look for kidneys that are firm, with a rich, even color. Avoid those with dry spots or a dull surface. Kidneys should be used the day they're purchased, or store loosely wrapped in the refrigerator for up to 1 day. Before cooking, remove skin and any excess fat. Soaking helps reduce the strong odor in kidneys from more mature animals. See a general cookbook for details pertaining to the particular type of kidney you wish to cook. Kidneys may be braised, broiled, simmered or cooked in casseroles, stews and dishes like the famous steak and kidney pie. All kidneys are a good source of protein, iron, phosphorus, vitamin A, thiamine and riboflavin.

n

One of a pair of bean-shaped urinary organs in the dorsal part of the abdomen, one on each side of the vertebral column. The kidneys produce and eliminate urine through a complex filtration network and reabsorption system comprising more than 2 million nephrons. More than 2500 pints of blood pass through the kidneys every day.

For more information on kidney, visit Britannica.com.

The major excretory and osmoregulatory organs in the body. A pair of kidneys lie dorsally (at the back), in the abdomen. The kidneys also act as endocrine organs releasing erythropoietin, a hormone that regulates red blood cell production.

A pair of organs, the principal parts of the excretory system, located above the waistline at the back of the abdominal cavity. The kidneys filter waste materials from the blood, excreting these wastes in the form of urine; they also regulate the amounts of water and other chemicals in body fluids.

Either of the two organs in the lumbar region that filter the blood, excreting the end-products of body metabolism in the form of urine, and regulating the concentrations of hydrogen, sodium, potassium, phosphate and other ions in the extracellular fluid. Bean-shaped in the dog, cat, sheep and laboratory animals, lobed in the ox and some fetal animals such as the horse; irregularly lobed in birds. See also renal.

Dog kidney. By permission from Sack W, Wensing CJG, Dyce KM, Textbook of Veterinary Anatomy, Saunders, 2002

• artificial k. — an extracorporeal device used as a substitute for nonfunctioning kidneys to remove endogenous metabolites from the blood, or as an emergency measure to remove exogenous poisons such as barbiturates. Called also hemodialyzer.
• balloon k. — meat hygiene term for cystic kidney.
• basal lamina k. — part of the filtration barrier of the kidney; is much thicker than most basal laminae.
• cake k. — a solid, irregularly lobed organ of bizarre shape, formed by fusion of the two renal anlagen. Called also lump kidney.
• cicatricial k. — a shriveled, irregular and scarred kidney due to suppurative pyelonephritis.
• contracted k. — an atrophic kidney that may be scarred and granular.
• duplicate k. — occurs in most species, without apparent increase in total renal mass.
• enlarged k. — may be due to polycystic kidney disease, hydronephrosis, pyelonephritis or congenital absence of one kidney resulting in hypertrophy of the other.
• fatty k. — one affected with fatty degeneration.
• floating k. — one that is freely movable, especially a human kidney (normally more firmly fixed than those in quadrupeds); called also hypermobile kidney. See also nephroptosis.

  Bovine kidney. By permission from Sack W, Wensing CJG, Dyce KM, Textbook of Veterinary Anatomy, Saunders, 2002

• fused k. — a single anomalous organ developed as a result of fusion of the renal anlagen.
• giant k. worm — dioctophyme renale.
• Goldblatt k. — one with obstruction of its blood flow, resulting in renal hypertension. Produced experimentally in dogs.
• horseshoe k. — an anomalous organ resulting from fusion of the corresponding poles of the renal anlagen.
• hypermobile k. — one that is freely movable; called also floating kidney. See also nephroptosis.
• lump k. — cake kidney.
• k. meridian points — acupuncture points on the kidney meridian.
• pelvic k. — a kidney which has failed to ascend from its primordial site to the roof of the abdomen.
• polycystic k. disease — the most common congenital renal defect but most cases are sporadic and do not cause clinical illness because there is still sufficient renal mass to avoid uremia. In some cases the enlarged kidney is detected incidentally during a clinical examination. Rarely both kidneys are badly involved and the animal is dead at birth or dies soon afterwards. In some cases, there are signs of progressive renal failure, perhaps not until later in life. The defect is inherited in Persian cats, Cairn terriers and pigs. In Cairn terriers, cysts may also occur in the liver. See also feline perirenal cysts.
• pulpy k. disease — see Clostridium perfringens enterotoxemia.
• k. scan — radioimaging of a kidney by the use of a rectilinear scanner after the intravenous administration of a radiopaque material.
• k. stones — see urolithiasis.
• supernumerary k. — additional kidneys which develop as a consequence of two ureteric buds arising from one mesonephric duct so that two kidneys develop on the one side.
• k. transplant — commonly and successfully performed in experimental dogs. Increasingly used as a therapeutic procedure in clinical veterinary medicine for renal failure in cats and dogs.
• turkey egg k. — a speckled pattern caused by hemorrhagic glomeruli in diseases such as porcine erysipelas.
• wandering k. — floating or hypermobile kidney. See also nephroptosis.
• waxy k. — amyloid kidney.
• white-spotted k. — focal nonsuppurative interstitial nephritis, seen most commonly in calves.

IN BRIEF: An organ that filters the blood.

The sister donated her kidney to her twin.

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Dansk (Danish)
n. - nyre

idioms:

• kidney bean    havebønne, snittebønne, krybbønne, pralbønne
• kidney machine    dialysemaskine
• kidney stone    nyresten

Nederlands (Dutch)
nier

Français (French)
n. - (Culin) rognon, (Anat) rein, (fig) acabit

idioms:

• kidney bean    haricot rouge
• kidney machine    (Méd) rein artificiel, dialyseur, (être) en hémodialyse, (être) en épuration extrarénale, (être) sous rein artificiel
• kidney stone    calcul rénal

Deutsch (German)
n. - Niere

idioms:

• kidney bean    Weiße Bohne, Feuerbohne
• kidney machine    künstliche Niere, Dialysegerät
• kidney stone    Nierenstein

Ελληνική (Greek)
n. - (ανατ.) νεφρό(ς)

idioms:

• kidney bean    κοινό φασόλι
• kidney machine    μηχάνημα τεχνητού νεφρού
• kidney stone    πέτρα του νεφρού

Italiano (Italian)
rene

idioms:

• kidney bean    fagiolo marrone-purpureo, fagiolo della regina
• kidney machine    rene artificiale
• kidney stone    calcolo renale

Português (Portuguese)
n. - rim (m) (Anat.), índole (f) (coloq.), tipo (m) (coloq.)

idioms:

• kidney bean    feijão (m)
• kidney machine    máquina (f) de hemodiálise (Med.)
• kidney stone    pedra (f) no rim (Med.), cálculo (m) renal (Med.)

Русский (Russian)
почка, темперамент, склад характера

idioms:

• kidney bean    фасоль обыкновенная
• kidney machine    искусственная почка
• kidney stone    галька, нефрит

Español (Spanish)
n. - riñón, temperamento, especie

idioms:

• kidney bean    frijol, poroto, judía, alubia, alubia pinta
• kidney machine    riñón artificial
• kidney stone    cálculo renal

Svenska (Swedish)
n. - njure, slag, sort

中文（简体）(Chinese (Simplified))
肾, 性格, 个性

idioms:

• kidney bean    菜豆, 四季豆, 云豆
• kidney machine    人工肾血液透析器
• kidney stone    肾结石

中文（繁體）(Chinese (Traditional))
n. - 腎, 性格, 個性

idioms:

• kidney bean    菜豆, 四季豆, 雲豆
• kidney machine    人工腎血液透析器
• kidney stone    腎結石

한국어 (Korean)
n. - 신장, 기질, 종류

日本語 (Japanese)
n. - 腎臓, 気質, 性質

idioms:

• kidney bean    いんげん豆, インゲンマメ
• kidney machine    人工腎臓
• kidney stone    腎臓形の小石, 軟玉, 腎石

العربيه (Arabic)
‏(الاسم) كليه الإنسان‏

עברית (Hebrew)
n. - ‮כליה, סוג, טבע, מזג‬

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Kidney Beans	Bmw Oem Front Kidney Grill