The harder a muscle is working, the higher is its demand for oxygen. The amount of oxygen available to a muscle is limited by the rate at which oxygen can be delivered.
Oxygen delivery depends on convective transport by the blood and diffusion within the tissue.
We use a Krogh-type model to investigate oxygen transport from capillaries to exercising skeletal muscle under conditions of high oxygen demand. Oxygen consumption is found to depend sensitively on the resistance to radial oxygen diffusion both within the capillary and within the tissue. Under conditions of high oxygen demand, a large part of the radial decline in PO 2 occurs within the capillary, and PO 2 is very low in most of the tissue. Predicted oxygen consumption rates under conditions of high demand are consistent with experimental measurements of maximal oxygen consumption in exercising humans.