Active and passive capillary control in red muscle at rest and in exercise.

Capillary control was quantified in dog gracilis muscles from in situ. About 550 capillaries/mm2, one-third the total number present, were perfused with erythrocytes simultaneously at rest; two-thirds the total could be perfused during maximal vasodilation. The functional capillary reserve was about 600/mm2. Capillary distribution at rest reflects a passive, random process at individual capillaries and an active process that coordinates perfusion of small groups of capillaries. The latter creates long diffusion distances. These are unaltered by denervation, or flow per se, but are abolished by adenosine. Twitch contraction at 4/min recruited about 400 capillaries/mm2 without any change in flow. Capillaries opened selectively where diffusion distances were longest. The same changes occurred within 5 s during work at 4/s, even if flow was held constant. If flow could increase, about 200 additional capillaries/mm2 were slowly recruited, without change in capillary distribution. Conclusions are that 1) hemodynamics and active vasomotion contribute equally to capillary density at rest; 2) active papillary control in exercise is ungraded and solely responsible for eliminating metabolically significant diffusion paths; 3) flow and capillary density can be controlled independently by proximal and terminal arterioles, respectively.