Blood volume is a complex problem not fully understood. The volume of blood in circulation is not a static quantity, rather, it is the net effect of counteracting forces in dynamic equilibrium. There are constant additions to the circulation and losses from the circulation, which in healthy persons tend to balance each other. Water, salts and nonprotein nitrogenous products pass back and forth through the capillary endothelium, into and out of the blood stream, with considerable freedom. Plasma proteins, which escape slowly, are matched by fresh accretions from lymph and from protein stores. Blood cell losses are replaced by newly formed cells. The result of this ceaseless activity is, within limits of physiological variation, a fairly constant blood volume. But, under abnormal conditions of hemorrhage, trauma and many diseases, that balance is upset and the volume of blood is altered. If the imbalance is severe, the efficiency of the entire