Separate effects of gravity and venous pressure on regional and capillary blood flows in the human finger.

The objective of this study was to determine if and to what extent a veno-arteriolar reflex is responsible for the adjustments in regional and capillary blood flows that occur in response to gravitational stress in the human finger. Nine male subjects 20 to 40 years of age consented to have regional cutaneous blood flow measured in the index finger via a laser Doppler flowmeter (LDF) and blood cell velocity (CBV) measured in individual nailfold capillaries via video microscopy while placing the hand 20 then 40 cm below the heart and while a pneumatic cuff placed around the wrist was inflated to pressures of 20, 40 than 60 mm Hg. Both lowering the hand and selective elevation of venous pressure elicited significant decreases in LDF and CBV (P less than 0.03). The flow reductions that occurred with lowering the hand 20 cm below the heart were significantly greater (P less than 0.05) for both LDF and CBV when compared to cuff pressure elevation of 27 cm H2O (20 mm Hg), and significantly greater (P less than 0.095) for LDF in matching the 40 cm below the heart position to data obtained at a cuff pressure of 57 cm H2O (40 mm Hg). Analyzing the flow responses relative to precapillary perfusion pressure (arterial pressure - estimated capillary pressure) indicated the reductions in LDF and CBV that occurred in response to cuff pressure elevation were a passive effect of the increase in venous pressure itself. These results indicate that the reductions in regional and capillary blood flow that occur in response to gravitational stress in the fingers are due to myogenic vasoconstriction of arterioles secondary to a rise in arterial pressure and that a veno-arteriolar reflex mechanism is not operative in this region of the cutaneous circulation.