Effect of orthostatic blood pressure changes upon capillary filtration-absorption rate in the human calf

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Abstract
The effect of postural changes upon fluid filtration rate in the calf was studied in 3 healthy subjects placed in supine position. Volume changes in the calf were measured by a water plethysmograph. Relative changes in blood flow in subcutaneous tissue and anterior tibial muscle were measured by the local 133Xe washout technique. Lowering the calf from 5-25 cm caused a linear increase in filtration rate; blood flow in subcutaneous tissue of the segment studied and anterior tibial muscle remained constant. Based on this average CFC [capillary filtration coefficient] was .apprx. 0.0012 ml .cntdot. min-1 .cntdot. 100 g-1 .cntdot. mmHg-1. Further lowering of the calf caused a decrease in blood flow in subcutaneous tissue and skeletal muscle by .apprx. 50% corresponding to an increase in total vascular resistance by .apprx. 100%. During these circumstances the increase in filtration rate was reduced by .apprx. 33%. The local veno-arteriolar reflex responsible for the observed vasoconstriction in the tissue under study together with intrinsic vascular mechanisms reduces transcapillary fluid filtration in the lowered position. The mechanism appears to be mainly due to a reduction of the increase in mean capillary pressure; CFC seems to remain almost constant.