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[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.
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.
— Chris Lote
Bibliography
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.
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.
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The sister donated her kidney to her twin.
| kidney | |
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| Human kidneys viewed from behind with spine removed | |
| Latin | ren |
| Gray's | subject #253 1215 |
| Artery | renal artery |
| Vein | renal vein |
| Nerve | renal plexus |
| MeSH | Kidney |
| Dorlands/Elsevier | k_03/12470097 |
The kidneys are organs that filter wastes (such as urea) from the blood and excrete them, along with water, as urine. The medical field that studies the kidneys and diseases of the kidney is called nephrology[1]. The prefix nephro- meaning kidney is from the Ancient Greek word nephros (νεφρός); the adjective renal meaning related to the kidney is from Latin rēnēs, meaning kidneys.
In humans, the kidneys are located in the posterior part of the abdomen. There is one on each side of the spine; the right kidney sits just below the liver, the left below the diaphragm and adjacent to the spleen. Above each kidney is an adrenal gland (also called the suprarenal gland). The asymmetry within the abdominal cavity caused by the liver results in the right kidney being slightly lower than the left one while the left kidney is located slightly more medial.
The kidneys are retroperitoneal. They are approximately at the vertebral level T12 to L3. The upper parts of the kidneys are partially protected by the eleventh and twelfth ribs, and each whole kidney is surrounded by two layers of fat (the perirenal and pararenal fat) which help to cushion it. Congenital absence of one or both kidneys, known as unilateral or bilateral renal agenesis, can occur.
In a normal human adult, each kidney is about 10 cm long, 5.5 cm in width and about 3 cm thick, weighing 150 grams.[2] Together, kidneys weigh about 0.5% of a person's total body weight.[citation needed] The kidneys are bean-shaped organs, and have a concave side facing inwards (medially). On this medial aspect of each kidney is an opening, called the hilum, which admits the renal artery, the renal vein, nerves, and the ureter.
The outer portion of the kidney is called the renal cortex, which sits directly beneath the kidney's loose connective tissue/fibrous capsule. Deep to the cortex lies the renal medulla, which is divided into 10-20 renal pyramids in humans. Each pyramid together with the associated overlying cortex forms a renal lobe. The tip of each pyramid (called a papilla) empties into a minor calyx, several minor calyces empty into a major calyx, and the major calyces empty into the renal pelvis. The pelvis transmits urine to the urinary bladder via the ureter.
Each kidney receives its blood supply from the renal artery, two of which branch from the abdominal aorta. Upon entering the hilum of the kidney, the renal artery divides into smaller interlobar arteries situated between the renal papillae. At the outer medulla, the interlobar arteries branch into arcuate arteries, which course along the border between the renal medulla and cortex, giving off still smaller branches, the cortical radial arteries (sometimes called interlobular arteries). Branching off these cortical arteries are the afferent arterioles supplying the glomerular capillaries, which drain into efferent arterioles. Efferent arterioles divide into peritubular capillaries that provide an extensive blood supply to the cortex. Blood from these capillaries collects in renal venules and leaves the kidney via the renal vein. Efferent arterioles of glomeruli closest to the medulla (those that belong to juxtamedullary nephrons) send branches into the medulla, forming the vasa recta. Blood supply is intimately linked to blood pressure.
The basic functional unit of the kidney is the nephron, of which there are more than a million within the cortex and medulla of each normal adult human kidney. Nephrons regulate water and solute within the cortex and medulla of each normal adult human kidney. Nephrons regulate water and soluble matter (especially electrolytes) in the body by first filtering the blood under pressure, and then reabsorbing some necessary fluid and molecules back into the blood while secreting other, unneeded molecules. Reabsorption and secretion are accomplished with both cotransport and countertransport mechanisms established in the nephrons and associated collecting ducts.
The fluid flows from the nephron into the collecting duct system. This segment of the nephron is crucial to the process of water conservation by the organism. In the presence of antidiuretic hormone (ADH; also called vasopressin), these ducts become permeable to water and facilitate its reabsorption, thus concentrating the urine and reducing its volume. When the organism must eliminate excess water, such as after excess fluid drinking, the production of ADH is decreased and the collecting tubule becomes less permeable to water, rendering urine dilute and abundant. Failure of the organism to decrease ADH production appropriately, a condition known as syndrome of inappropriate ADH (SIADH), may lead to water retention and dangerous dilution of body fluids, which in turn may cause severe neurological damage. Failure to produce ADH (or inability of the collecting ducts to respond to it) may cause excessive urination, called diabetes insipidus (DI). Alcohol inhibits the action of ADH, causing excess urination.
A second major function of the collecting duct system is the maintenance of acid-base homeostasis.
After being processed along the collecting tubules and ducts, the fluid, now called urine, is drained into the bladder via the ureter, to be finally excluded from the organism.
The kidneys excrete a variety of waste products produced by
The kidney is one of the major organs involved in whole-body homeostasis. Among its homeostatic functions are acid-base balance, regulation of electrolyte concentrations, control of blood volume, and regulation of blood pressure. The kidneys accomplish these homeostatic functions independently and through coordination with other organs, particularly those of the endocrine system. The kidney communicates with these organs through hormones secreted into the bloodstream.
Together with the lungs, the kidneys are major players in the regulation of acid-base homeostasis. Because the pH of the blood is determined in large part by the levels of carbon dioxide and bicarbonate in the blood, control of acid-base balance is achieved by regulating the blood levels of these compounds. The lung contributes to carbon dioxide balance, while the kidney regulates bicarbonate levels. Whereas carbon dioxide levels can be rapidly adjusted by changing respiratory rate, adjusting bicarbonate concentration is more time-intensive. Thus the lung contributes to short-term regulation, while the kidneys contribute to long-term regulation of acid-base balance.
The kidneys regulate bicarbonate concentration through the secretion and reabsorption of bicarbonate and hydrogen ions. The secretion of hydrogen ions and reabsorption of bicarbonate ions are two mechanisms by which the kidney protects against and recovers from acidosis. Conversely, the kidney responds to alkalosis by secreting bicarbonate and reabsorbing hydrogen ions. Under normal circumstances, whether the kidney mostly secretes or reabsorbs bicarbonate and hydrogen ions is largely dependent on diet. For example, a typical Western diet tends to acidify the blood, causing the kidney to favor the secretion of hydrogen ions and reabsorption of bicarbonate.
Changes in bicarbonate concentration alter pH. When the plasma bicarbonate concentration increases to such an extent that the pH exceeds the maximum limit of normal (approximately 7.45), an individual is said to be alkalemic (having alkaline blood). The underlying condition that caused the increase in plasma bicarbonate concentration is called a metabolic alkalosis. Similarly, when plasma bicarbonate concentration decreases and causes pH to drop below a normal minimum limit (approximately 7.35), an individual is considered acidemic (having acidic blood). The underlying condition causing the decrease in bicarbonate concentration is called a metabolic acidosis.
Sodium ions are controlled in a homeostatic process involving aldosterone which increases sodium ion reabsorption in the distal convoluted tubules.
When blood pressure becomes low, a proteolytic enzyme called Renin is secreted by cells of the juxtaglomerular apparatus (part of the distal convoluted tubule) which are sensitive to pressure. Renin acts on a blood protein, angiotensinogen, converting it to angiotensin I (10 amino acids). Angiotensin I is then converted by the Angiotensin-converting enzyme (ACE) in the lung capillaries to Angiotensin II (8 amino acids), which stimulates the secretion of Aldosterone by the adrenal cortex, which then affects the renal tubules.
Aldosterone stimulates an increase in the reabsorption of sodium ions from the kidney tubules which causes an increase in the volume of water that is reabsorbed from the tubule. This increase in water reabsorption increases the volume of blood which ultimately raises the blood pressure.
Any significant rise or drop in plasma osmolality is detected by the hypothalamus, which communicates directly with the posterior pituitary gland. A rise in osmolality causes the gland to secrete antidiuretic hormone, resulting in water reabsorption by the kidney and an increase in urine concentration. The two factors work together to return the plasma osmolality to its normal levels.
The kidneys secrete a variety of hormones, including erythropoietin, urodilatin, renin and vitamin D.
In the case of starvation, in the kidneys, an amino group is removed from protein and glucose is formed in the process of gluconeogenesis.
The mammalian kidney develops from intermediate mesoderm. Kidney development, also called nephrogenesis, proceeds through a series of three successive phases, each marked by the development of a more advanced pair of kidneys: the pronephros, mesonephros, and metanephros.[3] (The plural forms of these terms end in -oi.)
During approximately day 22 of human gestation, the paired pronephroi appear towards the cranial end of the intermediate mesoderm. In this region, epithelial cells arrange themselves in a series of tubules called nephrotomes and join laterally with the pronephric duct, which does not reach the outside of the embryo. Thus the pronephros is considered nonfunctional in mammals because it cannot excrete waste from the embryo.
Each pronephric duct grows towards the tail of the embryo, and in doing so induces intermediate mesoderm in the thoracolumbar area to become epithelial tubules called mesonephric tubules. Each mesonephric tubule receives a blood supply from a branch of the aorta, ending in a capillary tuft analogous to the glomerulus of the definitive nephron. The mesonephric tubule forms a capsule around the capillary tuft, allowing for filtration of blood. This filtrate flows through the mesonephric tubule and is drained into the continuation of the pronephric duct, now called the mesonephric duct or Wolffian duct. The nephrotomes of the pronephros degenerate while the mesonephric duct extends towards the most caudal end of the embryo, ultimately attaching to the cloaca. The mammalian mesonephros is similar to the kidneys of aquatic amphibians and fishes.
During the fifth week of gestation, the mesonephric duct develops an outpouching, the ureteric bud, near its attachment to the cloaca. This bud, also called the metanephrogenic diverticulum, grows posteriorly and towards the head of the embryo. The elongated stalk of the ureteric bud, the metanephric duct, later forms the ureter. As the cranial end of the bud extends into the intermediate mesoderm, it undergoes a series of branchings to form the collecting duct system of the kidney. It also forms the major and minor calyces and the renal pelvis.
The portion of undifferentiated intermediate mesoderm in contact with the tips of the branching ureteric bud is known as the metanephrogenic blastema. Signals released from the ureteric bud induce the differentiation of the metanephrogenic blastema into the renal tubules. As the renal tubules grow, they come into contact and join with connecting tubules of the collecting duct system, forming a continuous passage for flow from the renal tubule to the collecting duct. Simultaneously, precursors of vascular endothelial cells begin to take their position at the tips of the renal tubules. These cells differentiate into the cells of the definitive glomerulus.
Generally, humans can live normally with just one kidney, as one has more functioning renal tissue than is needed to survive, possibly due to the nature of the prehistoric human diet. Only when the amount of functioning kidney tissue is greatly diminished will chronic renal failure develop. If the glomerular filtration rate (a measure of renal function) has fallen very low (end-stage renal failure), or if the renal dysfunction leads to severe symptoms, then renal replacement therapy is indicated, either dialysis or renal transplantation.
The kidneys of animals can be cooked and eaten by humans (along with other offal). If prepared properly, they can be nutritious and pleasant tasting (if somewhat bland). Veal kidneys and lamb kidneys are particularly prized for their tenderness and flavour. Kidneys can be grilled or sautéed, though they become tough and unpleasant if overcooked.
Chinese cuisine includes sauteed pork kidneys, which are first soaked in water and then cleaned with scissors to remove nephrons and excess urine.
Pork kidneys, along with pork tongue and beef tongue, are some of the most cholesterol intense sources. A serving of pork kidney or beef tongue can yield more than 200% of the allotted daily intake of cholesterol.
World Kidney Day is observed on the second Thursday of March every year. [4] It was held for the first time in 2006, to increase awareness of kidney disease and educate persons at risk regarding the importance of prevention and early detection. [5] It is a joint initiative by the International Society of Nephrology (ISF) and International Federation of Kidney Foundations (IFKF). The next World Kidney Day will be held on 13 March 2008. In 2007, it was held on 8th March.
| Anatomy: urinary system |
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| Kidneys • Ureters • Urinary bladder (Uvula) • Urethral sphincters • Urethra |
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