Vascular Adjustments to Increased Transmural Pressure in Cat and Man with Special Reference to Shifts in Capillary Fluid Transfer

Abstract

Abstract When a vascular bed is exposed to an increased transmural pressure, as occurs in the lower extremities in man on standing, adjustments of the vascular tone take place which tend to protect the organism against excessive transcapillary loss of circulating fluid into the extravascular space. Thus, first, by closure of a number of the precapillary sphincter vessels, the size of the capillary surface area directly available for blood flow and hence for fluid exchange is reduced, and the rate of filtration correspondingly decreased. Second, by an increase of the ratio of precapillary to postcapillary flow resistance which increases the pressure drop across the precapillary resistance vessels, the rise in capillary pressure will be less than expected from the observed increase in arterial and venous pressures. From a quantitative point of view, the reduction of the capillary surface area seems to be by far the more important protective mechanism, as it was shown that in the human foot the rate of filtration per unit transcapillary pressure difference was decreased to only 1/3 to 1/8 of normal when the regional transmural pressure was increased by shifting from the supine to the erect posture. The vascular rejponse parterns described are evidently brought into play mainly by a reinforcement of the local inherent automaticity of the vascular smooth muscle induced by the raised trans. nura1 pressure, but, in all probability, can be intensified to some extent by superimpored extrinsic vasomotor fiber influence. This active vascular adaptation to hydrostatic load seems to be of greater importance than extravaicular mechanisms for preventing edema formation in dep‐ndent regions in normal subjects during quiet standing.