Pulmonary Blood Volume and “Pulmonary Hematocrit” during Hypervolemia in Rats

In rats erythrocytes and plasma albumin were labelled by radioisotopes, and circulatory arrest was then achieved by rapidly freezing the intact animal in liquid nitrogen. Pre‐mortal blood content in different organs could then be evaluated by measuring the radioactivity in tissue samples from the frozen animals. In one group of rats the circulating blood volume was increased 10 % by blood transfusions. The pulmonary blood volume in these hypervolemic animals was found to increase by 80%. Most of this increase was due to an augmentation of the pulmonary plasma content, consequently “pulmonary hematocrit” had decreased markedly. It could be demonstrated that this increase in pulmonary blood volume was caused by the hypervolemia per se, and not by substances released from or by the transfused blood. When the left thoracic vago‐sympathetic nerve trunk had been cut beforehand, no increase in lung blood volume was found in hypervolemic rats. It is suggested that the increase in lung blood volume normally found after a blood transfusion, could stem from a constriction of small vessels. This could then lead to a passive distension of vascular sections upstreams to the constricted ones.