Intrinsic regulation of functional blood flow and water absorption in canine colon.

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Abstract
Autoregulation of total and absorptive site blood flow and H2O fluxes were studied in canine colon during arterial pressure decreases and venous pressure elevations. Reductions in arterial pressure caused proportional decreases in total colonic blood flow indicating no autoregulatory ability. Absorptive site blood flow was constant over an arterial pressure range of 40-140 mmHg, but decreased sharply below 40 mmHg. Venous pressure elevation decreased total blood flow and increased resistance, but had no effect on absorptive site blood flow over a range of 0-30 mmHg. Net H2O absorption was unchanged by alteration of arterial or venous pressure but the unidirectional H2O fluxes decreased with arterial pressure reduction and were unchanged by venous pressure elevation. Adenosine infusion increased total and absorptive site blood flow and the unidirectional H2O fluxes yet had no effect on net H2O flux. Absorptive site blood flow is apparently very well autoregulated in the colon but total blood flow is not. Net H2O absorption is maintained during decreased blood flow by maintenance of blood flow to the mucosa and by decreasing the back flux of H2O across the mucosa.