Leo Sher, M.D.

Blood is a complex fluid which consists of plasma and of formed elements: red blood cells, white blood cells, and platelets. The volume of blood in an average human adult is about 5 liters. Females generally have less blood volume than males. People who live at high altitudes, where the air contains less oxygen, may have more blood than people who live in low altitude regions. The extra blood delivers additional oxygen to body cells.

Blood volume is controlled by the amount of water and sodium consumed, excreted by the kidneys into the urine, and lost through the gastrointestinal system, lungs and skin. The amounts of water and sodium ingested and lost are very changeable.  To sustain blood volume within a normal limit, the kidneys regulate the quantity of water and sodium lost into the urine. Multiple control systems are known to contribute to this regulation. The remarkable degree of constancy under normal conditions indicates that the excretion rates of water and sodium ions are constantly regulated by very sensitive, comparatively independent mechanisms.

The primary mechanism by which the kidneys adjust blood volume is by regulating the quantity of water and sodium lost into the urine. At various sites along the proximal tubules, thick ascending limb of the loop of Henle, distal and collecting tubules, sodium transport is controlled by angiotensin II which increases sodium transport thus leading to sodium retention. In the collecting tubules, aldosterone, increases sodium transport from the tubular fluid into the interstitium. Jointly, angiotensin II and aldosterone provide a potent mechanism for raising sodium retention and therefore fluid volume in the body. Antidiuretic hormone (vasopressin) increases water permeability in the late distal tubules and collecting tubules. This permits the reabsorption of water.