Oxygen- and pressure-dependent functional capillary density in rabbit tenuissimus muscle

Perfusion of capillaries was investigated in the tenuissimus muscle of young anesthetized New Zealand White rabbits during control conditions, complete aorta occlusion, and reactive hyperemia at various local oxygen tensions. Capillaries were visualized with bright-field microscopy. The number of capillaries perfused under experimental conditions was compared with that during control conditions. Capillary diameter was measured to assess whether the interventions caused changes in luminal diameter. During control conditions at a local pO2 of about 20 mm Hg, capillary perfusion fluctuates; instantaneous capillary density is smaller than anatomical capillary density. When the aorta is (partially) occluded, capillary perfusion becomes continuous and instantaneous capillary density equals anatomical capillary density. The latter is also observed during the early phase of reactive hyperemia, prior to the reappearance of flowmotion. Capillary diameter is not invariant during these interventions, but decreases by 8% during occlusion and increases by 12% during reactive hyperemia. The concomitant change in perimeter and cross-sectional area should be factored in with functional capillary density, when tissue exchange surface area or volume flow are considered. When during control conditions, the muscle becomes locally (under the microscope lens) exposed to a higher oxygen tension, capillary diameter does not change. However, the relative number of capillaries perfused at complete aorta occlusion is unity at low local oxygen, and diminishes with increasing local oxygen to become 0 at an oxygen tension of about 70 mm Hg. In preparations in which capillary diameter is not invariant under the experimental conditions, functional capillary density can only be used to compare the number of perfused capillaries with the number of capillaries anatomically present. Capillary diameter has to be factored in when tissue perfusion or exchange surface area are considered.