Cerebral hematocrit in normal man

The cerebral mean circulation times of plasma and erythrocytes and the cerebral hematocrit were determined by injection of I 131 -labeled albumin and Cr 51 -labeled erythrocytes into an antecubital vein, drawing multiple blood samples simultaneously from the carotid artery and the internal jugular vein, and calculating the arteriovenous concentration differences. The total body hematocrit was determined from equilibrium concentrations. In ten normal subjects the average cerebral hematocrit was 92.2% of the large vessel hematocrit, this difference being highly significant (P < 0.001). The total body hematocrit averaged 88.6% of the large vessel value and was significantly lower (P < 0.01) than for the brain. As the cerebral vascular bed has protein-tight capillaries, it is concluded that this area contains an extra plasma volume inside the vessel walls. It is suggested that the oft-repeated finding of a body hematocrit of about 90% of the large vessel hematocrit is mainly due to excess intravascular plasma localized to all the minor vessels in the body. cerebral mean circulation time; cerebral blood volume; body hematocrit Submitted on January 6, 1964