THE BLOOD FLOW IN THE BRAIN AND THE LEG OF MAN, AND THE CHANGES INDUCED BY ALTERATION OF BLOOD GASES 1

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Abstract
In 50 observations on unanesthetized persons, the O and the CO2 content of blood from an artery and from an internal jugular and femoral vein were measured. Blood samples were taken before and during the 50 experimental periods, which consisted of breathing of various gas mixtures designed to alter the composition of alveolar air and hence the gaseous content of arterial blood. Increases in the difference in the O content of arterial and venous blood were taken to represent a diminution in the speed of blood flow through the brain or the leg, Decreases in A-V difference were taken to represent an increase in speed of blood flow. Changes in speed were presumably due to a constriction of a dilatation of arterioles. It was found that alteration in the composition of alveolar air was attended by a marked change in the speed of blood flow. With an increased tension of CO2 in arterial blood, the speed of flow through the brain was increased and through the leg was decreased. With a decreased tension of CO2 the flow through the brain was decreased and that through the leg increased. Alteration of the O content of arterial blood produced less pronounced results. Increase in the 0 content resulted in a slight decrease in speed of blood flow through the brain and increase through the leg. Pronounced anoxemia resulted in increased flow in both brain and leg. An increase in the flow in the brain was more readily produced than a decrease. Alterations in the flow were more consistent in the brain than in the leg. Measurements of the respiratory quotient indicated that in conditions of anoxemia the tissues of the brain and the leg absorbed less than the usual amount of CO2. It is thus seen that, without the necessity of changes in the systemic blood pressure, under conditions of decreased O or increased CO2 tension, the brain receives preferential treatment, so far as the allocation of arterial blood is concerned.

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