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One of a pair of purple-brown organs situated at the back (retroperitoneal area) of the abdominal cavity. Each kidney 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 (4.5 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 (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. Nephrons are the microscopic structural and functional units of the kidney. Each nephron 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.
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KIDNEY A. Urinary Structures; B. Cross section of the kidney

The nephron consists of a renal corpuscle and renal tubule. The renal corpuscle is made of a capillary network called a glomerulus surrounded by the Bowman capsule. The renal tubule extends from the Bowman 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.
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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 the Bowman capsule (this fluid is 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 (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 person 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.

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 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.

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; SEE: glomerulonephritis; SEE: nephropathy; SEE: nephritis; SEE: 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 (as for electrolytes, blood urea nitrogen, and creatinine) and by urinalysis or timed collections of urine.

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