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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


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


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.
References in periodicals archive ?
That problem is compounded, he said, because of the medications essential to kidney transplant success.
She had a kidney transplant when she was just six years' old and developed a rare form of cancer which affects only 2percent of patients.
Murray, a pathfinder in kidney transplants at Brigham and Women's Hospital in Boston, and bone-marrow specialist E.
This means every patient who receives a kidney transplant saves the NHS pounds 100,000 over the first five years after the transplant.
The findings indicate that having lupus should not keep individuals from seeking a kidney transplant if they need one.
The Centre started kidney transplants for adults in 1986, developing a liver transplant programme early in 2012 and now our paediatric kidney transplant programme has been launched in December," Dr al-Maslamani said adding: "We are very proud of our surgical, medical and support teams and we are looking forward to further develop our programmes and introduce other vital and life saving procedures including pancreas transplantation.
medical director of the Kidney Transplant Program at Intermountain Medical Center "It's the right time to reach out to those patients to minimize their travel.
Last year was the first time in 22 years that less than 100 kidney transplants were carried out here.
Lewis, 17, from Caerphilly, underwent a kidney transplant in 2004, with his own mother Tracy as the donor after being diagnosed with chronic renal failure, a condition where Lewis' kidneys had failed to develop.
Researchers analyzed and compared the group's risk for a heart attack, angioplasty and bypass surgery rates, and death from heart disease after a kidney transplant with Caucasian, black and East Asian men and women.
The average waiting time for an adult kidney transplant is 841 days, but many people die before they receive an organ.
Researchers have identified a distinct pattern of gene expression in the largest reported group of kidney transplant recipients who have not rejected the transplant kidneys even though they stopped taking anti-rejection drugs.