Capillary pressures in rat intestinal muscle and mucosal villi during venous pressure elevation.

Whole-organ experiments designed to estimate the capillary filtration coefficient require information about the numerical relationship between capillary pressure and venous pressure. Indirect estimates using isogravimetric and isovolumetric methods indicate that 62-85% of a step change in venous pressure reaches the intestinal capillaries, taken as a whole. We have made direct measurements of capillary pressure with a servo-null micropressure system in the microcirculation of both the intestinal muscle and the mucosal villi of rats during local elevation of venous pressure. Consistent regional differences in the relationship between capillary pressure and venous pressure were observed. During increased venous pressure, submucosal arterioles constricted, while muscularis arterioles dilated. The diameter changes of the small arterioles were consistent with blood flow redistribution from mucosa to muscle during venous pressure elevation, but inconsistent with a pure myogenic response. These data raise questions about the exact role for the expression of the myogenic response during venous pressure elevation in the intestine and about previous interpretations of whole-organ experiments concerned with intestinal blood flow and fluid exchange.