RESTING OXYGENATION OF RAT AND RABBIT INTESTINE - ARTERIOLAR AND CAPILLARY CONTRIBUTIONS

Counter-current exchange of oxygen may occur between inflow and outflo w microvessels of the small intestine and greatly influence the domina nt sites of tissue oxygenation. To determine the location and magnitud e of potential exchange, percent saturation of hemoglobin with oxygen (%S-Hb) was measured in microvessels throughout the intestine of rats and rabbits. Oxygen losses from systemic arterial blood through large and intermediate arterioles (second order, 2A) was 5-7%S-Hb in both sp ecies, and there was no evidence of an increase in percent saturation along intermediate and large venules. A larger loss of oxygen from art erioles and an increase in venous saturation would be evident if signi ficant arteriolar to venular counter-current exchange of oxygen occurr ed in the submucosa. From 2A to the villus tip, arteriolar saturation decreased similar to 10%S-Hb in rabbits and similar to 15%S-Hb in rats ; the villus tip percent saturation was 72.9 +/- 39%S-Hb in rabbits an d 69.9 +/- 2.9%S-Hb in rats. An additional decrease of 5%S-Hb in rabbi ts and 15%S-Hb in rats occurred across the villus capillaries and smal lest venules. Although the total reduction in percent saturation acros s the villi was different between the two species, 70-90% of the total arteriovenous oxygen losses occurred in the capillaries and small art erioles of the villi. We found no evidence of counter-current exchange of oxygen in villi or any other vascular region. Rather, as appears t o occur in most organs, small arterioles in conjunction with capillari es dominate resting oxygen exchange to tissue.