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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 ?
In conjunction, the sNDA is based on data from the company's multinational phase 3 study LCP-Tacro 3002, which evaluated the safety and efficacy of Envarsus XR versus immediate-release tacrolimus capsules for the prevention of acute allograft rejection in 500 de novo adult kidney transplant recipients.
Dr Mercy Korir said: "Why lead surgeons from India yet we have competent Kenyan surgeons (urologists) who have been doing kidney transplants in this country for many years?
It is good news for Kenyans that Mbagathi Hospital can now perform kidney transplants.
The ONT has registered a slight decrease in living donor kidney transplants in the last three years (with a total of 332 in 2017), probably due to the large number of deceased donor grafts, a trend that both institutions try to correct for the benefit of kidney patients, especially the youngest, because our deceased donors are getting older.
She required an early kidney transplant, but there were no blood group compatible donors.
This is the third successful surgery to be carried out at Bahrain's largest public hospital ever since the kidney transplant programme was re-launched.
Since a donor from her immediate family is not available right now, the kidney transplant might take some more time.
The renal needle biopsies, though a mainstay for kidney transplant diagnosis of acute allograft rejection, are invasive and are compromised by sampling heterogeneity and subjective interpretation.
DAMMAM: A total of 627 kidney transplant surgeries were performed in the Kingdom last year, bringing the number of kidney transplants, performed since the beginning of the organ donation program, to 9,000 surgeries, said Dr.
The new NHS Blood and Transplant campaign launches with the screening of a 14-hour film capturing a day in the life of a patient waiting for a kidney transplant.
All Blacks rugby union champion Jonah Lomu, who died at just 40 years old this week, was on the waiting list for a kidney transplant.