COORDINATION OF BLOOD-FLOW CONTROL IN THE RESISTANCE VASCULATURE OF SKELETAL-MUSCLE

The coordination of blood flow control in exercising skeletal muscle i s exemplified by the interaction among muscle fibers, nerves, and the smooth muscle and endothelial cells which comprise the resistance vasc ulature. During functional hyperemia in active muscle, maximal now exc eeds resting values by 10- to 50-fold, according to muscle fiber type and recruitment pattern. The control of muscle blood flow is coordinat ed among many Vessel branches, encompassing the resistance arteries ex ternal to the muscle and the arteriolar network embedded within the ti ssue. As motor unit recruitment and metabolic demand increase with exe rcise intensity, the locus of blood flow control ''ascends'' from dist al arterioles, which govern capillary perfusion and flow distribution, into the proximal arterioles and feed arteries, which control the vol ume of flow into a muscle. The organization of vasomotor responses wit hin and among resistance vessels can be explained by the spread of ele ctrical, chemical, and physical signals between endothelial and smooth muscle cells. These signals are triggered by substances released from muscle fibers and nerve terminals and by changes in transmural pressu re and luminal flow. Thus, from several perspectives, cell-to-cell com munication coordinates blood now control in accord with the metabolic demands of active muscle fibers.