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either of the two bean-shaped 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.
Physiology. In an average adult each kidney is about 10 cm long, 5 cm wide, and 2.5 cm thick, and weighs 120 to 175 g. In this small area the kidney contains over a million microscopic filtering units, the nephrons. Blood arrives at the kidney by way of the renal artery, and is distributed through arterioles into many millions of capillaries which lead into the nephrons. Fluids and dissolved salts in the blood pass through the walls of the capillaries and are collected within the malpighian capsule, the central capsule of each nephron. Within the capsule is a tuft of capillaries called the glomerulus that acts as a semipermeable membrane permitting a protein-free ultrafiltrate of plasma to pass through. This filtrate is forced into the renal tubules, hairpin-shaped collecting channels in the nephrons. Capillaries in the walls of the tubules reabsorb the water and the salts required by the body and deliver them to a system of small kidney veins which, in turn, carry them into the renal vein and return them to the general circulation. Excess water and other waste materials remain in the tubules as urine. The urine contains, besides water, a quantity of urea, uric acid, yellow pigments, amino acids, and trace metals. The urine moves through a system of ducts into the funnel-shaped renal pelvis in each kidney, through which it is led into the two ureters.
Filtering Capacity. About 1500 ml of urine are excreted daily by the average adult. The efficiency of the normal kidney is one of the most remarkable aspects of the body. Ordinarily it draws off from the blood about 164 liters of fluid daily, and usually returns 98 to 99 per cent of the water plus the useful dissolved salts, according to the body's changing needs.
Maintaining Acid-Base Balance. The kidneys help control the body's acidity by reabsorbing filtered bicarbonate ions in exchange for chloride and by secreting hydrogen ions. When there is alkalosis, the kidney compensates by reabsorbing less bicarbonate ions and more hydrogen ions.
Regulation of Sodium-Water Balance. Normal osmolality and volume of body fluids are preserved by the normally functioning kidney. It does this by actively reabsorbing sodium and, by osmosis, reabsorbing more water, thus varying the urine concentration. The regulation of the sodium level in the blood is influenced by aldosterone, which increases sodium reabsorption; it is secreted by the adrenal gland in response to low serum sodium levels and the presence of angiotensin II. The reabsorption of water is affected not only by the reabsorption of sodium but also by antidiuretic hormone, which is secreted by the pituitary gland in response to high serum osmolality.
Endocrine Functions. In response to renal ischemia the kidneys regulate blood pressure by the renin-angiotensin-aldosterone system. Also, when kidney cells become hypoxic they release a hormone called erythropoietin, which stimulates the maturation of oxygen-bearing red blood cells in the bone marrow. The kidneys also are involved in the conversion of inactive vitamin D to the active form, which increases calcium absorption in the intestine and calcium uptake by the bones.
Disorders of the Kidneys. Disorders of the kidney include inflammation, infection, obstruction, structural defects, injuries, calculus formation, and tumors. Specific disorders include types of glomerulonephritis, nephritis, nephropathy, and pyelitis; kidney stones; polycystic kidney disease; and nephroptosis. See also renal failure.
Details of structure of the kidney.
amyloid kidney one with amyloidosis; called also waxy kidney.
artificial kidney popular name for an extracorporeal hemodialyzer, a device used as a substitute for nonfunctioning kidneys.
Ask-Upmark kidney a hypoplastic kidney with fewer lobules than usual and fissures on its surface; most affected persons have severe hypertension, sometimes with hypertensive encephalopathy and retinopathy. The condition may be either congenital or secondary to vesicoureteral reflux with pyelonephritis.
cake kidney a solid, irregularly lobed organ of bizarre shape, formed by fusion of the two renal anlagen; called also lump kidney.
cicatricial kidney a shriveled, irregular, and scarred kidney due to suppurative pyelonephritis.
fatty kidney one with fatty degeneration.
flea-bitten kidney one with small, randomly scattered petechiae on its surface.
floating kidney nephroptosis.
fused kidney a single anomalous organ developed as a result of fusion of the renal anlagen.
horseshoe kidney an anomaly in which the right and left kidneys are linked at one end by a band of tissue as a result of fusion of the poles of the renal anlagen.
hypermobile kidney nephroptosis.
lump kidney cake kidney.
medullary sponge kidney a usually asymptomatic congenital condition in which multiple small cystic dilatations of the collecting tubules of the medullary portion of the renal pyramids give the organ a spongy, porous feeling and appearance. Called also sponge kidney.
myeloma kidney renal changes seen in multiple myeloma, due to filtration of large amounts of Bence Jones protein; they include tubular atrophy with intraluminal casts and multinucleate giant cells in tubular walls and interstitium, resulting in renal failure.
kidney stone a calculus in the kidney, composed of crystals precipitated from the urine on a matrix of organic matter. Called also nephrolith and renal calculus.

About 80 per cent of kidney stones are composed of calcium salts, which precipitate out of their normally soluble form in urine, usually because the patient has an inherited tendency to excrete excessive amounts of calcium (idiopathic hypercalcemia). A very small percentage of kidney stones are associated with a parathyroid tumor that increases production of parathyroid hormone and thus raises the serum calcium level. Persons with intestinal absorption problems, including those who have had intestinal bypass surgery for obesity, sometimes develop calcium stones because of excessive absorption of dietary oxalate, which is eventually excreted by the kidneys. Since vitamin C is converted by the body into oxalate, large doses of the vitamin can predispose one to stone formation.

The most common type of stones is the oxalate calculi, hard ones consisting of calcium oxalate; some have sharp spines that can abrade the renal pelvic epithelium, and others are smooth. Another common type is the phosphate calculi, which contain calcium phosphate in a mineral form such as brushite or whitlockite; they may be hard, soft, or friable and range from small to so large that they fill the renal pelvis. Struvite stones are composed of the salt magnesium ammonium phosphate and form in alkaline urine such as that produced in urinary tract infections. Uric acid stones form when there is an increased excretion of uric acid, as in gout or certain malignancies. An acid urine favors their formation. Cystine stones are associated with cystinuria, a hereditary kidney disorder in which there is excessive excretion of cystine. “Staghorn stones” are ones that have extended from the renal pelvis into the calyces, giving them sharp protrusions like the antlers of a stag.
Prevention. No matter what the type of kidney stone, an essential preventive measure is high fluid intake to prevent urinary stasis. In order to dilute the urine sufficiently, an adult must put out almost 4000 ml of urine every 24 hours. A continuous flow of adequate amounts of urine has both a mechanical and a chemical effect. The fluids flush the urinary tract and remove substances essential to stone formation. Also, the urine itself contains substances that bind with potential precipitates, making them more soluble and less able to form a mass.

Additional preventive measures include avoidance or prompt treatment of urinary tract infections, changing the urinary pH in cases in which acidity or alkalinity predisposes to stone formation, treatment of underlying pathologies such as parathyroid tumor, and careful long-term follow-up of patients who have had intestinal bypass surgery or a history of intestinal malabsorption.

Uric acid stones can be prevented by administering the drug allopurinol, which inhibits the formation of uric acid, and by keeping the urine relatively alkaline. An alkaline urine and high intake are effective means of preventing cystine stones. If these measures fail, however, the drug penicillamine may be prescribed.

A specific strategy for prevention of stone formation in an individual patient requires chemical analysis of the stones, urine, and blood to determine the type of stone being formed.
Symptoms. Kidney stones do not always produce symptoms. However, they can lead to infections and inflammations that do produce symptoms. A definitive diagnosis is established by examination of the urine for hematuria, an abdominal x-ray (which can detect stones of calcium salts), or an intravenous or retrograde pyelogram using a radiopaque dye. The pyelogram will not show the stone itself but there will be a gap in the stream of dye as it courses down the ureter.

The classic symptoms of renal colic occur when a small calculus is dislodged from the renal pelvis and begins to travel down the ureter. Many stones have sharp spicules or spikes on their surfaces; as they roll along the ureter they can scrape the lining, causing excruciating pain and bleeding. The pain is typically felt in the flank over the affected kidney and ureter and radiates downward toward the genitalia and inner thigh. Nausea and vomiting can occur as a result of the severe pain. If an infection is present the patient experiences fever and chills.
Treatment. Stones that are less than 5 mm can usually be eliminated with the normal passage of urine; this is the most desirable method of treatment. Adequate medication is given to relieve pain and relax the muscular walls of the ureter, thus easing passage of the stone. Fluids are given orally or intravenously to aid mechanical flushing. During this period the urine is strained in order to determine whether the stone is passed and, if it is, to collect it for laboratory analysis.

If the stone is not passed, the traditional treatment has been surgical intervention to remove it via ureteroscopy. A newer noninvasive technique is lithotripsy, which involves crushing the stone into fragments small enough to be passed in the urine; this is done using any of a variety of techniques, the most common being ultrasound.
Patient Care. Prevention of kidney stones requires a knowledge of patients most at risk. Males are much more at risk than females for development of calcium stones; also at high risk are those of either sex who have a family history of stone formation. Other persons at risk are those who are immobilized for any reason, have a urinary tract infection, or have a history of intestinal bypass or malabsorption.

Analgesics should be administered promptly to provide relief of pain and facilitate passage of the stone. Fluid intake and output are measured; the intake is encouraged to be 4000 ml every 24 hours. Characteristics of the urine are noted, and all urine is strained until the stone is either passed or removed surgically. Dietary restrictions and recommendations to alter urinary pH and the reason for increased fluid intake are explained to the patient and family members as appropriate. The patient also is taught to take prescribed medications faithfully and to report symptoms of urinary tract infection promptly.
wandering kidney nephroptosis.
waxy kidney amyloid kidney.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.


(kid'nē), [TA]
One of the paired organs that excrete urine, remove nitrogenous wastes of metabolism, reclaim important electrolytes and water, contribute to blood pressure control(renin-angiotensin system) and erythropoiesis (via erythropoietin production). The kidneys are bean-shaped organs about 11-cm long, 5-cm wide, and 3-cm thick, lying on either side of the vertebral column, posterior to the peritoneum, opposite the 12th thoracic and 1st-3rd lumbar vertebrae. In animals, the kidney has variable size and location.
Synonym(s): ren [TA], nephros
[A.S. cwith, womb, belly, + neere, kidney (L. ren, G. nephros)]
Farlex Partner Medical Dictionary © Farlex 2012


n. pl. kid·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. An excretory organ of certain invertebrates.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.


See Cake kidney, Goldblatt kidney, Horseshoe kidney, Large white kidney, Polycystic kidney, Rat-bitten kidney.
McGraw-Hill Concise Dictionary of Modern Medicine. © 2002 by The McGraw-Hill Companies, Inc.


(kid'nē) [TA]
One of the two organs that excrete urine. The kidneys are bean-shaped organs (about 11 cm long, 5 cm wide, and 3 cm thick) lying on either side of the vertebral column, posterior to the peritoneum, opposite the 12th thoracic and first three lumbar vertebrae.
Synonym(s): ren [TA] .
Medical Dictionary for the Health Professions and Nursing © Farlex 2012


Enlarge picture
KIDNEY: A. Urinary Structures; B. Cross section of the kidney
Enlarge picture
One of a pair of purple-brown organs situated at the back (retroperitoneal area) of the abdominal cavity; each is lateral to the spinal column. The kidneys form urine from blood plasma. They are the major regulators of the water, electrolyte, and acid-base content of the blood and, indirectly, all body fluids.


The top of each kidney is opposite the 12th thoracic vertebra; the bottom is opposite the third lumbar vertebra. The right kidney is slightly lower than the left one. Each kidney weighs 113 to 170 g (4 to 6 oz), and each is about 11.4 cm (4 1 2 in) long, 5 to 7.5 cm (2 to 3 in) broad, and 2.5 cm (1 in) thick. The kidneys in the newborn are about three times as large in proportion to body weight as they are in the adult.

Each kidney is surrounded by adipose tissue and by the renal fascia, a fibrous membrane that helps hold the kidney in place. On the medial side of a kidney is an indentation called the hilus or hilum, at which the renal artery enters and the renal vein and ureter emerge. The microscopic nephrons are the structural and functional units of the kidney; each consists of a renal corpuscle and renal tubule with associated blood vessels. In frontal section, the kidney is composed of two areas of tissue and a medial cavity. The outer renal cortex is made of renal corpuscles and convoluted tubules. The renal medulla consists of 8 to 18 wedge-shaped areas called renal pyramids; they are made of loops of Henle and collecting tubules. Adjacent to the hilus is the renal pelvis, the expanded end of the ureter within the kidney. Urine formed in the nephrons is carried by a papillary duct to the tip (papilla) of a pyramid, which projects into a cuplike calyx, an extension of the renal pelvis. See: illustration


The nephron consists of a renal corpuscle and renal tubule. The renal corpuscle is made of a capillary network called a glomerulus surrounded by Bowman's capsule. The renal tubule extends from Bowman's capsule. The parts, in order, are as follows: proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting tubule, all of which are surrounded by peritubular capillaries. See: illustration

Formation of Urine

Urine is formed by filtration, reabsorption, and secretion. As blood passes through the glomerulus, water and dissolved substances are filtered through the capillary membranes and the inner or visceral layer of Bowman's capsule; this fluid is now called glomerular filtrate. Blood cells and large proteins are retained within the capillaries. Filtration is a continuous process; the rate varies with blood flow through the kidneys and daily fluid intake and loss. As the glomerular filtrate passes through the renal tubules, useful materials such as water, glucose, amino acids, vitamins, and minerals are reabsorbed into the peritubular capillaries. Most of these have a renal threshold level, i.e., a limit to how much can be reabsorbed, but this level is usually not exceeded unless the blood level of these materials is above normal. Reabsorption of water is regulated directly by antidiuretic hormone and indirectly by aldosterone. Most waste products remain in the filtrate and become part of the urine. Hydrogen ions, creatinine, and the metabolic products of medications may be actively secreted into the filtrate to become part of the urine. The collecting tubules unite to form papillary ducts that empty urine into the calyces of the renal pelvis, from which it enters the ureter and is transported to the urinary bladder. Periodically the bladder is emptied (a reflex subject to voluntary control) by way of the urethra; this is called micturition, urination, or voiding. If a normally hydrated individual ingests a large volume of aqueous fluids, in about 45 min a sufficient quantity will have been excreted into the bladder to cause the urge to urinate. See:


Urine is about 95% water and about 5% dissolved substances. The dissolved materials include minerals, esp. sodium, the nitrogenous waste products urea, uric acid, and creatinine, and other metabolic end products. The volume of urine excreted daily varies from 1000 to 2000 ml (averaging 1500 ml). The amount varies with water intake, nature of diet, degree of body activity, environmental and body temperature, age, blood pressure, and many other factors. Pathological conditions may affect the volume and nature of the urine excreted. However, patients with only one kidney have been found to have normal renal function even after half of that kidney was removed because of cancer. There is no evidence that forcing fluids is detrimental to the kidneys.

Nerve Supply

The nerve supply consists of sympathetic fibers to the renal blood vessels. These promote constriction or dilation, esp. of arteries and arterioles.


Frequently encountered diseases of the kidney include infection (pyelonephritis), stone formation (nephrolithiasis), dilation (hydronephrosis), protein loss (nephrosis), cancer (hypernephroma), and acute or chronic renal failure. See: dialysis; glomerulonephritis; nephropathy; nephritis; renal failure


The kidneys are examined by palpation, intravenous pyelography, ultrasonography, computed tomography scan, cystoscopy, retrograde cystoscopy, or magnetic resonance imaging. Kidney function is also frequently examined with blood tests (e.g., for electrolytes, blood urea nitrogen, and creatinine) and by urinalysis or timed collections of urine.

amyloid kidney

An enlarged, firm, smooth kidney usually associated with systemic amyloidosis. Synonym: waxy kidney


Infected persons typically lose large quantities of protein in the urine and may present with edema or symptoms of fluid overload, nephrosis, or renal failure.

artificial kidney


cake kidney

Congenitally fused kidneys.

contracted kidney

The abnormally small kidney found in end-stage renal disease.

cystic kidney

A kidney that has undergone cystic degeneration.
See: polycystic kidney disease

embolic contracted kidney

A kidney in which embolic infarction of the renal arterioles produces degeneration of renal tissue and hyperplasia of fibrous tissues produces irregular contraction.

fatty kidney

A kidney with fatty infiltration or degeneration of tubular, glomerular, or capsular epithelium, or of vascular connective tissue.

flea-bitten kidney

A kidney with small petechiae covering the surface, a pathological finding in bacterial endocarditis and some other systemic illnesses.

floating kidney

A kidney that is displaced and movable.

fused kidney

A condition in which the kidneys are joined into one anomalous organ.

Goldblatt kidney

See: Goldblatt, Harry

granular kidney

A slow form of chronic nephritis characterized by diminishing size; by redness; and by a hard, fibrous, and granular texture. Synonym: red contracted kidney

horseshoe kidney

A congenital malformation in which the superior or inferior extremities are united by an isthmus of renal or fibrous tissue, forming a horseshoe shape.

hypermobile kidney

A freely movable kidney. Synonym: wandering kidney

medullary sponge kidney

A congenital condition characterized by the presence—seen best during urography—of spongy or porous appearing renal collecting tubules. The disease may be asymptomatic or may cause urinary bleeding, stone formation with renal colic, or recurrent urinary tract infections.
Synonym: Cacchi-Ricci syndrome

movable kidney

A kidney that is not firmly attached owing to lack of support of fatty tissue and perinephric fascia. Synonym: nephroptosis

myeloma kidney

Cast nephropathy.

polycystic kidney

A kidney bearing many cysts. See: kidney disease, polycystic

red contracted kidney

Granular kidney.

sacculated kidney

A condition in which the kidney has been absorbed and only the distended capsule remains.

small indented calcified kidney

Abbreviation: SICK
The computed tomographic (CT) appearance of the kidneys in patients who develop chronic kidney disease from the overuse of analgesic medications like acetaminophen, aspirin, ibuprofen, or phenacetin.

syphilitic kidney

Kidney with fibrous bands running across it, and caseating gummata, as a result of syphilis.

wandering kidney

Hypermobile kidney.

waxy kidney

Amyloid kidney.
Medical Dictionary, © 2009 Farlex and Partners


One of the paired, reddish brown, bean-shaped structures lying in pads of fat on the inside of the back wall of the ABDOMEN on either side of the spine, just above the waist. The kidneys filter the blood, removing waste material and adjusting the levels of various essential chemical substances, so as to keep them within necessary limits. In so doing, they produce a sterile solution of varying concentration known as urine. This passes down the ureters to the bladder where it is stored until it can be conveniently disposed of. The kidneys are largely responsible for regulating the amount of water in the body and controlling the acidity of the blood. Most drugs or their products are eliminated through the kidney. Kidneys control fluid and chemical levels by both filtration and selective reabsorption under the control of various hormones such as ALDOSTERONE from the adrenal gland, the ANTIDIURETIC HORMONE from the pituitary gland and PARATHYROID hormone from the parathyroid glands. Sodium, potassium, calcium, chloride, bicarbonate, phosphate, glucose, amino acids, vitamins and many other substances are returned to the blood and conserved. Proteins, fats and all the cells of the blood remain in the circulation. The kidneys produce ERYTHROPOIETIN, which stimulates the rate of formation of blood cells in the bone marrow. When blood pressure falls below normal the kidneys release the enzyme renin into the blood. This results in the formation of a further hormone, angiotensin, which rapidly causes blood vessels throughout the body to constrict and raise the blood pressure.
Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005
Kidneyclick for a larger image
Fig. 200 Kidney . General structure.


an organ, found in pairs in a dorsal situation in vertebrates, that serves the dual purposes of EXCRETION and OSMOREGULATION. See Fig. 200 . Ultrafiltration takes place in the glomerulus of the Bowman's capsule, where the glomerular filtrate contains all the constituents of blood except for blood cells and plasma proteins. Pores of about 0.1 μm diameter in the basement membrane lining the Bowman's capsule allow the passage of the filtrate under pressure. The high pressure is produced by the action of the heart and by the efferent blood vessels from the glomerulus being narrower than the afferent vessels, together with the branching of vessels in the glomerulus. The filtrate, which is modified in its flow along the tubule, eventually emerges from the kidney as urine. The LOOP OF HENLE employs the principle of a hairpin COUNTERCURRENT MULTIPLIER. Active transfer of salt (NaCl) takes place from the ascending limb to the descending limb, so raising the concentration in the latter. This results in a region of high salt concentration deep in the medulla of the kidney through which the collecting duct passes. Water is extracted by osmosis from the distal-convoluted tubule and collecting duct, so concentrating the urine (see ADH). Over 99% of kidney fluid is thus reabsorbed by the kidney tubules. see NEPHRON.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005


Either of two organs in the lumbar region that filter the blood, excreting the end products of the body's metabolism in the form of urine and regulating the concentrations of hydrogen, sodium, potassium, phosphate and other ions in the body.
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.


(kid'nē) [TA]
One of the paired organs that excrete urine, remove nitrogenous wastes of metabolism, reclaim important electrolytes and water, contribute to blood pressure control, and erythropoiesis.
Medical Dictionary for the Dental Professions © Farlex 2012

Patient discussion about kidney

Q. Why do i get kidney stones? I am 38 and have had three stones pass so far. Is it the coffee, the meat, the stress, or the damned DNA?! My uncle is in his 50s and has passed over 30 stones!

A. Kidney stones are very common and even without the genetic or familial background people tend to get them. Of course, the more family predisposition you have, the higher are your chances of developing them, which is probably why you did. Also, a diet rich with dairy and calcium can cause your body to store excess calcium, that tends to calcify and create stones. Not drinking enough fluid is also one of the reasons.

Q. How can i overcome kidney ache? in the morning it appears.after wake up.

A. First of all, it is important to distinguish kidney ache from lower back ache. If you have ever suffered from kidney problems or infections, it might be advisable to see a doctor, and rule out an infection. However, if you are otherwise healthy, and have been experiencing back pain after you wake up, it is very much possible your pain is not from the kidneys, but from the muscles of your back. In this case, some exercise on a daily basis to help strengthen your lower back can very much ease the pain you're experiencing.

Q. what cause pain around kidney uncomfortable pressure swelling right side back

A. thanx....the pain is dull and there's no fever: muscular pain perhaps? If it worsens, persists or fever developes; I will head to the Doctor. thamx again....

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