Increased metabolic rate (MR) and arterial hypoxemia are independent stimuli to increase blood flow and delivery of oxygen (O2) into the active muscle tissue and may differ in location of their action. We speculated that blood gas analysis could provide the answer whether a given stimulus acts on the recruitment of capillaries, manifested by the apparent density of capillaries. We used the Krogh cylinder model, modified for the description at low partial pressure of O2 (pO2) that lead to O2 deficit. Using data of Goodman et al. (Circ Res 43: 769–76, 1978) we calculated the apparent density of capillaries, which changed proportionally to MR, but was independent of O2 saturation of the arterial blood, suggesting that the increased radial O2 diffusion gradient at higher MR might be responsible for it.