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

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 whereas blood flow in subcutaneous tissue of the segment studied and anterior tibial muscle remained constant. Based on this average CFC was about 0.0012 ml X min-1 X 100 g-1 X mmHg-1. Further lowering of the calf caused a decrease in blood flow in subcutaneous tissue and skeletal muscle by about 50% corresponding to an increase in total vascular resistance by about 100%. During these circumstances the increase in infiltration rate was reduced by about 33%. The resulted indicate that the local veno-anterior reflex responsible for the observed vasoconstriction in the tissue under study together with intrinsic vascular mechanisms reduces transcapillary fluid filtration in the lower position. The mechanism appears to be mainly due to a reduction of the increase in mean capillary pressure whereas CFC seems to remain almost constant.