Pressures in cat mesenteric arterioles and capillaries during changes in systemic arterial blood pressure.

An interpretive model of the myogenic autoregulatory response of cat mesenteric arterioles has appeared in the literature; it describes arteriolar dimensional changes during changes in mean systemic arterial blood pressure in terms of tension-radius diagrams (Laplace relationship). The original model assumes that arteriolar pressure is a simple, linear function of systemic blood pressure and that arteriolar pressure is always 40% of systemic blood pressure. The purpose of the present study was to test the validity of these assumptions. Using the isolated, autoperfused mesentery of 14 cats, direct, simultaneous measurements of diameter and pressure were made in microvessels during changes in systemic blood pressure. Diameters and micropressures were recorded with a video filar micrometer system and a servonull transducer, respectively. Control microvascular pressure, dimensional distributions, and stress distributions were established when systemic blood pressure was set at 100 ± 10 (SD) mm Hg. Three regions of the arteriolar vasculature were defined and identified for study in terms of these control states. Pressures in the arteriolar regions were a linear function of systemic blood pressure, thus confirming the validity of the previous assumption. Capillary pressures tended not to follow this pattern; pressure in five of eight capillaries sampled was relatively independent of changes in systemic blood pressure between 100 and 40 mm Hg. The apparent constancy of capillary pressure during changes in systemic blood pressure was examined in terms of the modular arrangement of mesenteric microvessels and resistance changes, and the question of physiological regulation of capillary pressure was considered. Data are presented to suggest that a constant capillary pressure in mesentery during changes in systemic blood pressure is primarily the consequence of the vascular geometry peculiar to this tissue.