Blood volume in health and disease.

THOUGH physiologic and pathologic alterations in circulating blood volume do not generally come within the E 1 A zpurview of the hematologist, neither are they in the specific province of any other specialty. The subject of this presentation then may be appropriately considered in a program devoted to abnormalities of the blood. Early interest in blood volume was expressed in its measurement by the exsanguination of condemned criminals. Aside from the fact that this method did not promise to be applied with much benefit at the bedside, its accuracy was open to question. Interestingly enough, in spite of considerable refinement in techniques, the accuracy of blood volume determinations still remains a matter of some dispute. It behooves us, therefore, to examine, in some detail, the manner in which blood volume is estimated, before considering what the estimations purport to reveal in the various clinical states. Most recent methods have utilized the tracer dilution principle, wherein some detectable label is introduced into the blood stream and, after a suitable interval for mixing throughout the vascular system, its concentration is determined. For example, if five million units of label are injected and the concentration is found to be iooo/ml., after mixing, the apparent volume of dilution is 5ooo ml. Aside from nontoxicity, the label should have the following characteristics. It must remain within the vascular system for a time sufficient to insure complete mixing, it must not be destroyed during the same period and it must mix uniformly in the volume to be measured. If there is a leak out of the compartment or destruction within the compartment, the concentration of the label in the sample specimens will be lower than if it is entirely retained within the compartment and falsely high values for volume will result. If mixing is not uniform the samples will