Skeletal muscle: microcirculatory adaptation to metabolic demand

The issue of whether skeletal muscle is master or slave of the cardiovascul ar system depends on frame of reference. Acute manipulations of convective O-2 delivery clearly show that O-2 supply sets the upper limit of muscle (V ) over dot O-2max. However, studies of adaptation to chronic conditions suc h as training and hypoxia show that skeletal muscle has a remarkable capaci ty to meet changes in metabolic demand. Moreover, there are several lines o f evidence that these adaptations are essential to changes in (V) over dot O-2max. Studies show that with training, electrical stimulation, and chroni c hypoxia, the ratio of capillary surface per fiber surface and fiber mitoc hondrial volume/fiber length is preserved, suggesting a primary regulated f eature in skeletal muscle is matching the structural capacity for O-2 flux to mitochondrial metabolic demand. Adaptations in both capillarity and mito chondrial respiratory capacity have also been shown to be important compone nts in the adaptive increase in (V) over dot O-2max. With training. Collect ively, this evidence argues against skeletal muscle being simply a slave to the cardiovascular system.