Development and adaptability of microvasculature in skeletal muscle.

In skeletal muscle, the size of the capillary bed is adapted to the type of muscle metabolism and can be altered by adaptation to different environments or increased activity. Muscle fibres with high aerobic metabolism have more capillaries, and an increase in aerobic metabolism is usually followed by capillary growth. It is assumed that local hypoxia - created by increased demand for oxygen during growth, cold exposure or increased activity - can stimulate proliferation of capillaries. Capillary density is reduced in parallel with enhanced glycolytic metabolism. The size of the capillary bed can also increase without any apparent change in the oxidative metabolism (e.g. in the early stages of chronic electrical muscle stimulation or as a result of long-term administration of vasodilating drugs), and it is argued that the growth of capillaries in these cases may be due to various mechanical factors connected with increased blood flow.