Influence of oxygen on perfused capillary density and capillary red cell velocity in rabbit skeletal muscle

The pattern of capillary blood flow changes was investigated in rabbit tenuissimus muscle as a function of oxygen tension in the solution suffusing the muscle. The density of perfused capillaries decreased from 269 ± 22 (mean ± SD) to 6 ± 10 cap/mm² when ambient pO₂ was elevated progressively from 5 to 100 mm Hg. In the same pO₂ range capillary red cell velocity decreased from 0.29 ± 0.14 (mean ± SD) to 0.18 ± 0.06 mm/sec in the capillaries which were perfused. During exposure of the muscle to atmospheric oxygen tension (pO₂ = 150 mm Hg) there were no capillaries perfused and hence flow velocity was zero in all capillaries. The results demonstrate that oxygen influences capillary blood flow over an extensive range of oxygen tensions. The anatomical structures responsible for the alterations in both capillary density and capillary flow velocity are the arterioles. This site of control determines actively the total blood flow through the capillary bed, whereas passive factors seem to determine the distribution of the flow between specific capillaries. The results do not support the existence of precapillary sphincters controlling perfused capillary density.