Evidence Against Local Control of Intestinal Tissue Oxygenation by Myogenic Mechanisms
The proposed basis for the myogenic theory of local blood flow control is that vascular smooth muscle will actively contract if stretched by increased transmural pressure. Critical experiments to test the myogenic hypothesis have employed elevated venous pressure to raise transmural pressures in the microvasculature. In isolated loops of small bowel, elevated venous pressure has been reported to cause an increase in total resistance to blood flow (1). Subsequent studies of capillary filtration showed that the increase in resistance occurred on the precapillary side. Such resistance changes in response to elevated venous pressure are thought to result from a myogenic constriction of resistance vessels (2). In addition to resistance vessels, precapillary sphincters are thought to respond to elevated venous pressure. By measuring the rate of fluid filtration at various venous pressures, and by determining the isogravimetric capillary pressure with a stop-flow technique, Johnson (3) was able to calculate capillary filtration coefficients for small intestine. His study showed that, although the filtration rate increased with increased venous pressure as expected, the cal-cualted capillary filtration coefficients fell with increased venous pressure. The proposed mechanism to explain the reduction in capillary filtration coefficients was a myogenic constriction of precapillary sphincters in response to the increased venous pressure.
KeywordsVenous Pressure Oxygen Extraction Myogenic Response Washout Curve Capillary Filtration
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