Physiological changes in blood volume.

Measurement of blood volume is an important clinical tool in establishing a diagnosis as in polycythemia vera, in assessing the true significance of a low blood count in a patient with splenomegaly, and in evaluating a bleeding patient. In theory, blood volume measurements should be of great value in treating acute blood loss, but this is often too time-consuming to be practicable. The main value of blood volume studies so far has been in clinical research where it is essential for correct interpretation of the peripheral blood measurements. Estimates using the hematocrit (packed cell volume) could be misleading since this measurement does not take into account total blood volume and changes in the plasma volume. A variety of isotopes that tag the red blood cell have been used to measure total blood volume and red cell mass. Most commonly used at present is autologous 51-Cr labeled red cells, thus eliminating the risk of hepatitis in transfusion to the subjects. Radio labeled albumin and other plasma proteins were used to measure plasma volume. A large molecular protein is ideal to eliminate overestimation from extravascular diffusion of labeled material for accurate determination of plasma volume. Physiologic changes in the newborn, in pregnant women, during exercise, bed rest, and altitude in the nonpathologic state are discussed. Studies in the normal subject's adaptation in different physiologic conditions have revealed a variety of factors that regulate blood volume.