Control of bat wing capillary pressure and blood flow during reduced perfusion pressure

Regulation of blood flow depends on changes in the sum of arterial (Ra) and venous (Rv) resistances, whereas regulation of capillary pressure (Pc) depends on the ratio of Rv to Ra. If the myogenic response of the arterial system (i.e., delta Ra) is the primary mechanism for controlling pressure and flow when perfusion pressure is lowered, then Pc and total flow should be regulated to the same degree under these conditions. This hypothesis was tested by making direct measurements of Pc and flow in skin and skeletal muscle in the wings of unanesthetized bats. The box method was used to reduce perfusion pressure to the wing. Pressures were measured with a servo-null system; flows were computed from measurements of vascular diameters and red cell velocities using intravital microscopy. All branching orders of arterioles dilated significantly during decreases in box pressure (Pb). For 0 less than Pb less than or equal to -30 mmHg, total flow (1st-order arteriolar flow) remained nearly constant, whereas Pc was "regulated" only approximately 60%. These results cannot be explained by changes in arteriolar resistance alone and suggest that changes in Rv may be important. The possible consequences of flow redistribution, capillary recruitment, and micropressure sampling procedures are discussed in relationship to local regulation of capillary pressure and flow.