Oxygen exchange in the microcirculation of hamster retractor muscle

We determined percent hemoglobin oxygen saturation (SO2) in arterioles and venules of the hamster retractor muscle at rest. We found that SO2 decreased from 69.9 +/- 1.4% (SE) in large input arterioles (first order, 1A, ID = 60 +/- 3 micron) to flow-weighted values of 56.7% in small arterioles (4A, ID = 20 +/- 1 micron), 51.3% in small venules (4V, ID = 28 +/- 1 micron), and to 50.6 +/- 1.0% in large venules (1V, ID = 147 +/- 13 micron). Thus approximately two-thirds of the net decline in SO2 for this tissue occurred by diffusion of oxygen from arterioles, whereas only about one-third occurred by diffusion from capillaries. Furthermore, no net shunting of oxygen from the arterioles to the venules was detected as evidenced by the absence of any significant change in venular SO2. By determining the SO2 at upstream and downstream ends of arterioles in four consecutive branching orders (1A-4A), we found that the decrease in SO2 per unit length (delta SO2/L) increased approximately 20-fold from 1A to 4A. This increase in delta SO2/L was directly proportional to estimated luminal minus tissue oxygen tension and inversely proportional to red blood cell flow.