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Venous pressure-volume curves obtained by venous congestion may be useful for the study of venous tone in human extremities. However, such pressure-volume curves should be measured from a constant reference point or baseline of venous volume and effective venous pressure. The purpose of this paper is to show that such a constant baseline is obtained when an external pressure equal to or greater than the natural local venous pressure is applied to the leg. This has been demonstrated by measuring the volume of blood in the human leg, in vivo, at graded external pressures. The data to be presented are not only of physiological interest, but also have a practical bearing on the use of elastic compression to accelerate the linear velocity of blood flow in the leg veins for the prevention of thromboembolic complications in hospitalized patients (1-3). There have been no quantitative measurements of the volume and distribution of blood in the vessels of the leg, in vivo, with the circulation under normal conditions of rest. Several investigators have reported a few measurements of the blood volume of the leg, using the Evans blue dye (4) or the carbon monoxide (5) method, but their results do not give accurate information about the distribution of blood in the vessels of the leg. Neither method is adaptable for our purpose. The method employed in the present study is to express the blood from the leg in a water-filled plethysmograph and then to record the increase in leg volume which occurred when the circulation was restored. This was repeated over a wide range of external water pressures applied to the leg. The rationale of the method depends upon the as-

THE VOLUME AND DISTRIBUTION OF BLOOD IN THE HUMAN LEG MEASURED IN VIVO. I. THE EFFECTS OF GRADED EXTERNAL PRESSURE 12