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1. either of two bean-shaped organs at the back of the abdominal cavity in man, one on each side of the spinal column. They maintain water and electrolyte balance and filter waste products from the blood, which are excreted as urine
2. the corresponding organ in other animals
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005


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 capsuleenlarge picture
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 Urinary system

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.


Either of a pair of organs involved with the elimination of water and waste products from the body of vertebrates; in humans they are bean-shaped, about 5 inches (12.7 centimeters) long, and are located in the posterior part of the abdomen behind the peritoneum.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Characterized by reduced kidney function or kidney damage, chronic kidney disease affects approximately 10 percent of adults in the US.
"Kidney Diseases Dictionary Index." National Kidney and Urologic Diseases Information Clearinghouse.
The single step of universally immunizing every patient with chronic kidney disease against influenza has the potential to save millions of dollars annually in hospitalization costs, said John M.
Most part of the kidney is already damaged by the time symptoms of disease appear while Pakistan is the eight largest countries with high prevalence of kidney diseases causing 20 thousand deaths annually.
'There are cost-effective models of screening; such as diabetic nephropathy test, screening other high-risk patients and treatment of kidney diseases that can be integrated within the health care.
Mark Solero, chief of the PRO's Regional Health Service XI, said that kidney diseases have become prevalent in the police force with 130 cases recorded last year.
He said male are mostly affected of Kidney diseases as compared to women in our country.
He said that CKD was a silent killer as most of the kidney was already damaged by the time the symptoms of kidney diseases appeared.
The experts said the leading cause of chronic kidney diseases were primarily diabetes, kidney glomerular diseases and high blood pressure in addition to excessive use of painkillers, uncertified herbal medicines, genetic disorders, defects at birth, stone disease, infections, environmental pollution, unhygienic conditions and old age.
They said that the leading cause of chronic kidney diseases are primarily diabetes, kidney glomerular diseases and high blood pressure.
Nephrologist and head of the Kidney Transplant Unit at Pakistan Institute of Medical Sciences (PIMS), Dr Khawar Sultan said consuming junk and low quality food, self-medication or excessive use of medicine, low water intake, obesity, diabetes, hypertension and renal stones are few common causes of kidney diseases.