Determinants of resting and passive intestinal vascular pressures in rat and rabbit

Microvascular pressures in the intestinal arteries, submucosal arterioles, and mucosal venules were measured in rats and rabbits at rest and during maximum dilation. From these data and Doppler velocimetry measurements of relative changes in whole organ blood flow on maximum dilation, it was possible to determine to what extent microvascular pressures at rest depend on the active control and passive hemodynamic characteristics of specific vascular segments. New Zealand White rabbits (2-3 kg body wt) had a mean arterial pressure of 70-75 mmHg. However, pressures in arterioles of both species became equivalent at the second order of arteriolar branching within the bowel wall, and pressures in the smallest mucosal venules were 13.7 +/- 0.6 (SE) mmHg in rabbits and 14.9 +/- 0.3 mmHg in rats. Maximum vasodilation to approximately 300% of the control blood flow increased mucosal venule pressures approximately 10 mmHg in rats compared with approximately 4 mmHg in rabbits. The increased mucosal venule pressure during vasodilation was primarily due to increased pressures within the submucosal small arterioles, which immediately precede the villus vasculature in both species. The increased blood flow during vasodilation was due primarily to a decreased resistance of the small arteries and large arterioles, even though pressures in these larger vessels changed only approximately 10%. This situation allows a major decrease in intestinal vascular resistance to substantially increase blood flow with a minimal increase in mucosal microvascular pressures.