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The coordination of blood flow control in exercising skeletal muscle is exemplified by the interaction among muscle fibers, nerves, and the smooth muscle and endothelial cells which comprise the resistance vasculature. During functional hyperemia in active muscle, maximal flow exceeds resting values by 10- to 50-fold, according to muscle fiber type and recruitment pattern. The control of muscle blood flow is coordinated among many vessel branches, encompassing the resistance arteries external to the muscle and the arteriolar network embedded within the tissue. As motor unit recruitment and metabolic demand increase with exercise intensity, the locus of blood flow control "ascends" from distal arterioles, which govern capillary perfusion and flow distribution, into the proximal arterioles and feed arteries, which control the volume of flow into a muscle. The organization of vasomotor responses within and among resistance vessels can be explained by the spread of electrical, 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 pressure and luminal flow. Thus, from several perspectives, cell-to-cell communication coordinates blood flow control in accord with the metabolic demands of active muscle fibers. |
