Plasma Volume After Posture Change in Hypertensive Subjects

Abstract

THE INTRAVASCULAR volume is not considered a major determinant of blood pressure in man, and measurements of the blood volume in hypertensive subjects have yielded consistently normal values.^1-3 Recently, however, considerable interest has focused on the role of volume sensing devices within the arterial tree in the elaboration of vasopressor substances. The possibility, therefore, that intermittent hypovolemia might play a supporting role in the vasoconstriction of human hypertensive disease prompted this investigation of the blood volume response to quiet standing in persons with essential hypertension. The hemodynamic responses to motionless standing in normal individuals have been largely elucidated.^4-9 These include a decrease in circulating blood volume, a decline in central venous pressure and cardiac output, and an increase in pulse rate and diastolic blood pressure. The mean arterial pressure is usually maintained at normal or slightly elevated levels, probably by compensatory baroreceptor reflexes which serve to increase sympathetic