Page 27, CV Review, Essentials, Develop. Physiol., Dr. D. Penney

Misconception 28. Blood volume is primarily affected by drinking

The normal blood volume is approximately 5.5 liters, 8% of whole body weight or 10% of lean body weight. Men have a somewhat greater blood volume on a body weight basis because women have greater adiposity; this eliminated, the blood volumes of the two sexes are similar.

If blood is lost, a condition known as hypovolemia exists, which is characterized by hypotension, low cardiac output, weakness, torpor, and circulatory shock. Shock becomes increasingly serious with greater blood loss. Abnormally great blood volume, or hypervolemia, is characterized by venous congestion, ascites, pulmonary congestion, hypertension, systemic edema, and heart failure. Both severe hypovolemia and severe hypervolemia can result in death. Thus blood volume must be accurately regulated.

A number of factors are involved in the regulation of blood volume (Table 1.05): Thirst leading to drinking is an important means of adding to blood volume, but it is not entirely under conscious control. On the other hand, water is lost in the urine and feces. Water is less visibly lost from the body surface as sweat and as insensible evaporation, and also from the pulmonary surfaces.

Plasma volume is regulated actively via the actions of angiotensin, aldosterone, and vasopressin on the kidney, adrenals, and brain, and passively via the Starling-Landis phenomenon and Arthur Guyton's pressure diuresis mechanism. Erythrocyte volume is regulated by the hormone erythopoietin acting on bone marrow and by the splenic storage/release of erythrocytes. Plasma proteins are synthesized by the liver. Thus, although thirst and drinking are important means of increasing plasma water content, it is only one of many mechanisms.

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