The kidneys are bean-shaped excretory organs in vertebrates. Part of the urinary system, the kidneys filter wastes (especially urea) from the blood and excrete them, along with water, as urine. The medical field that studies the kidneys and diseases affecting the kidney is called nephrology, from the Greek name for the kidney; the adjective meaning "kidney-related" is renal, from the Latin.


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 spleen. Above each kidney is an adrenal gland (also called the suprarenal gland).

The kidneys are retroperitoneal, which means they lie behind the peritoneum, the lining of the abdominal cavity. They are approximately at the vertebral level T12 to L3, and the right kidney usually lies slightly lower than the left in order to accommodate the liver.

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 fat and the pararenal fat) which help to cushion it.



In a normal human adult, each kidney is about 11 cm long and about 5 cm thick, weighing 150 grams. 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 outermost portion of the kidney is called the renal cortex, which sits directly beneath the kidney's loose connective tissue 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 calyx, which empties into the renal pelvis. The pelvis transmits urine to the urinary bladder via the ureter.


The basic functional unit of the kidney is the nephron, of which there are more than a million in each normal adult human kidney. Nephrons regulate water and soluble matter (especially electrolytes) in the body by first filtering the blood, then reabsorbing some necessary fluid and molecules while secreting other, unneeded molecules. Reabsorption and secretion are accomplished with both cotransport and countertransport mechanisms established in the nephrons and associated collecting ducts.

Collecting duct system

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. Failure of the organism to produce ADH (or inability of the collecting ducts to respond to it) may cause excessive urination, called diabetes insipidus.

Conversely, 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 may lead to water retention and dangerous dilution of body fluids, which in turn may cause severe neurological damage. 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.


Excretion of waste products

The kidneys excrete a variety of waste products produced by metabolism, for example, urea (from protein catabolism) and uric acid (from nucleic acid metabolism).


The kidneys regulate the pH, mineral ion concentration, and water composition of the blood.

By exchanging hydronium ions and hydroxyl ions, the blood plasma is maintained by the kidney at pH 7.4. Urine, on the other hand, becomes either acidic at pH 5 or alkaline at pH 8.

Sodium ions are controlled in a homeostatic process involving aldosterone which increases sodium ion absorption in the distal convoluted tubules.

Any rise or drop in blood osmotic pressure due to a lack or excess of water is detected by the hypothalamus, which notifies the pituitary gland via negative feedback. A lack of water causes the posterior pituitary gland to secrete antidiuretic hormone, which results in water reabsorption and urine concentration. Tissue fluid concentration thus returns to a mean of 98%.

Hormone secretion

The kidneys secrete a variety of hormones, including erythropoietin, renin, and vitamin D.

Diseases and disorders Congenital


Dialysis and kidney transplants

Generally, humans can live normally with just one kidney. Only when the amount of functioning kidney tissue is greatly diminished will renal failure develop. If renal function is impaired, various forms of medications are used, while others are contraindicated. Provided that treatment is begun early, before a serum creatinine of 2 mg/dl, it may be possible to reverse chronic kidney failure due to diabetes or high blood pressure. If creatinine clearance (a measure of renal function) has fallen very low ("end-stage renal failure"), or if the renal dysfunction leads to severe symptoms, dialysis is commenced. Dialysis is a medical procedure, performed in various different forms, where the blood is filtered outside of the body.

Kidney transplantation is the only cure for advanced chronic renal failure; dialysis, while correcting the abnormalities to a degree, is seen as a form of "buying time" to bridge the inevitable wait for a suitable organ.

The first successful kidney transplant was announced on March 4, 1954 at Peter Bent Brigham Hospital in Boston. The surgery was performed by Dr. Joseph E. Murray, who was awarded the Nobel Prize in Medicine in 1990 for this feat.

There are two types of kidney transplants: living donor transplant and a cadaveric (dead donor) transplant. When a kidney from a living donor, usually a blood relative, is transplanted into the patient's body, the donor's blood group and tissue type must be judged compatible with the patient's, and extensive medical tests are done to determine the health of the donor. Before a cadaveric donor's organs can be transplanted, a series of medical tests have to be done to determine if the organs are healthy. Also, in some countries, the family of the donor must give its consent for the organ donation. In both cases, the recipient of the new organ needs to take drugs to suppress their immune system to help prevent their body from rejecting the new kidney.

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