The overall function of the kidneys is to maintain the normal composition and volume of the blood. This is accomplished by four interrelated activities.

(1) Wastes of metabolism are excreted. These include urea, uric acid, creatinine ammonia, sulfates and other products of metabolism, and toxic substances.

(2)  The amounts of fluid and electrolytes that are reabsorbed or excreted are exactly regulated so that fluid-electrolyte balance is maintained. The antidiuretic hormone (ADH) secreted by the pituitary gland influences the amount of water that will be reabsorbed from the tubules. Aldosterone secreted by the adrenal gland controls the relative amounts of sodium that will be reabsorbed or excreted.

(3)  The final adjustment in acid-base balance is made by the kidney. If there is an increased production of metabolic acids, more acid urine is secreted. Also, the kidney synthesizes ammonia which neutralizes acid so that there is less loss of base. (4) The kidney synthesizes certain enzymes and hormones. Renin is an enzyme that influences the production of aldosterone. Renin also has a powerful effect on blood pressure. Erythropoietin, a hormone produced by the kidney, stimulates the bone marrow to produce red blood cells. The kidney converts vitamin D to vitamin-D hormone. This hormone is necessary for the absorption of calcium from the intestinal tract, and for the regulation of calcium levels in the blood.

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