kidney, artificial, mechanical device capable of assuming the functions ordinarily performed by the kidneys. In treating cases of kidney failure a tube is inserted into an artery in the patient's arm and blood is channeled through semipermeable tubes immersed in a bath containing all the normal blood chemicals except urea and other metabolic waste products. Since the concentration of harmful metabolic wastes are higher in the blood than in the bath, they pass through the walls of the tubes into the bath and purified blood is returned to the body. This process of blood purification, called hemodialysis (see dialysis), is continuous or intermittent, depending on the residual kidney function in the patient. Kidney transplants usually make hemodialysis unnecessary.

artificial kidney: see kidney, artificial.

or renal transplant

Replacement of a diseased or damaged kidney with one from a living relative or a legally dead donor. The former's tissue type is more likely to match, reducing the chance of rejection; but removal puts the donor at risk, and a kidney from a dead donor is more likely to be available. The new kidney is implanted and its blood vessels and ureter sewn in place. A near-normal life may be resumed within two months, but the drugs that prevent rejection leave the patient vulnerable to infection. Seealso transplant.

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or renal calculus

Mass of minerals and organic matter that may form in a kidney. Urine contains many salts in solution, and low fluid volume or high mineral concentration can cause these salts to precipitate and grow, forming stones. Large stones can block urine flow, be a focus for infection, or cause renal colic (painful spasms). They can obstruct the urinary system at various points. Treatment deals with any underlying problem (e.g., infection or obstruction), tries to dissolve stones with drugs or ultrasound (lithotripsy), or removes large ones surgically.

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or renal failure

Partial or complete loss of kidney function. Acute failure causes reduced urine output and blood chemical imbalance, including uremia. Most patients recover within six weeks. Damage to various kidney structures can result from chemical exposure, major blood loss, crush injury, hypertension, severe burns, severe kidney infections, diabetes mellitus, renal artery or urinary tract blockage, and liver diseases. Complications include heart failure, pulmonary edema, and high potassium levels. Chronic failure usually results from long-term kidney diseases. The blood becomes too acidic, bones can lose calcium, and nerves can degenerate. The kidneys can sustain life until they lose about 90percnt of their function. If one is removed, the other increases in size and function to compensate. Failure of both usually requires dialysis or kidney transplant.

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or hemodialysis

Process of removing blood from a patient with kidney failure, purifying it with a hemodialyzer (artificial kidney), and returning it to the bloodstream. Many substances (including urea and inorganic salts) in the blood pass through a porous membrane in the machine into a sterile solution; particles such as blood cells and proteins are too large to pass. This process controls the acid-base balance of the blood and its content of water and dissolved materials.

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One of a pair of organs that maintain water balance and expel metabolic wastes. Human kidneys are bean-shaped organs about 4 in. (10 cm) long, in the small of the back. They filter the entire 5-quart (about 4.5-liter) water content of the blood every 45 minutes. Glucose, minerals, and needed water are returned to the blood by reabsorption. The remaining fluid and wastes pass into collecting ducts, flowing to the ureter and bladder as urine. Each kidney has over 1 million functional units (nephrons) involved in the process of filtration and reabsorption. The kidneys also secrete renin, an enzyme involved in blood pressure regulation. Disorders include kidney failure, kidney stones, and nephritis. Seealso urinary system.

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