Effects of gravity on regional and capillary blood flows in the human toe

The purpose of this study was to investigate the relative effects of gravity on capillary and regional blood flows in human extremities. Eighteen male subjects 20 to 40 years of age consented to have capillary blood cell velocity (CBV) measured in nailfold vessels of the large toe via video densitometry, and to have total blood flow (TBF) to the toe measured via plethysmography. These measurements were made first with the foot at heart level (HL), then after the foot had been lowered an average of 56 cm below the heart (BH). In the HL position CBV and TBF averaged respectively 0.26 mm/sec and 2.0 ml/min/100 ml. Lowering the foot elicited a decrease in CBV in all vessels studied (P less than or equal to 0.001). Average CBV in the BH position was 0.11 mm/sec. By contrast TBF on the average increased to 2.9 ml/min/100 ml in the BH position (P less than or equal to 0.05). However, this was not a consistent finding since both increases and decreases in TBF were observed among the individual subjects. These results indicate that the immediate precapillary vessels in the cutaneous circulation of the toe will consistently vasoconstrict when the foot is placed in a dependent position. However, the net effect on vessels governing regional flow in this area, e.g., A-V shunts, appears to result from a competition between dilator and constrictor influences. The overall result of these adjustments is that during orthostasis blood is shifted away from the superficial capillaries of the lower extremities. The relevance of this to blood-tissue fluid balance is discussed.