Effects of Low O₂ on Intestinal Blood Flow, Tonus and Motility

¹ From the Department of Physiology, University of Michigan, Ann Arbor, Michigan

Effects of low O₂ on intestinal blood flow were studied in blood perfused segments of dog intestine, 1) completely isolated and the low O₂ administered by perfusion with hypoxic heparinized dog blood, or, 2) completely isolated except for extrinsic nerves and the low O₂ administered either as in 1 or by respiratory gas mixtures via trachea. The local administration of low O₂ (10–7%) to either preparation 1 or 2, increased intestinal blood flow remarkably. This, in the absence of any changes in intestinal tonus or motility, must be ascribed solely to dilatation of the intestinal vasculature per se which commonly predominates over the reflexogenic and/or centrogenic vasoconstrictive influence of low O₂. Local histotoxic anoxia (0.3 ml of a 1% solution of NaCN) evokes effects similar to those of 7% O₂. Low O₂ can also prominently affect intestinal blood flow indirectly, a) by its action on intestinal tonus whereby it may enhance or completely nullify the effects of vasodilatation per se; and b) by its depressive action on the rhythmic intestinal contractions and their propulsion of intestinal blood. In passing through the intestinal vasculature the blood acquires potent vasodilatatory properties which must be ascribed to some ‘substance,’ other than CO₂ or low O₂, whose release into the blood is apparently facilitated by low O₂. The data indicate that the intestine contributes to the reflex chemical control of breathing.