Local postural vasomotor reflexes arising from the limb veins.

Blood flow of the toes and fingers was studied plethysmographically with the level of the blood vessels below or above the heart, using the contralateral digit as control. The apparent flow is a maximum when the vessels are near the heart level. The decrease of flow when the vessels are raised is explained on the basis of narrowing and closure of vessels when the hydrostatic pressure within them falls. The decrease in flow when the limb is lowered is interpreted as evidence of a vasoconstriction elicited by distension of the local veins. This raises serious doubt about the classic flow method.