Expansion
Sympathetic tone at rest, metabolic vasodilatation during exercise
Expansion
At rest
- Flow is low, about 2 to 5 ml per 100 g per minute, and only a minority of capillaries are perfused
- Dominated by sympathetic vasoconstrictor tone
- Muscle nonetheless accounts for about 20 per cent of cardiac output because of its total mass
During exercise
- Metabolic vasodilatation from adenosine, potassium, hydrogen ions, carbon dioxide and rising osmolality
- Capillary recruitment increases the surface area for exchange and shortens diffusion distance
- Flow can rise twentyfold, and muscle receives over 80 per cent of cardiac output
- Functional sympatholysis: local metabolites blunt the response to sympathetic constriction in active muscle, so vasoconstriction elsewhere can coexist with vasodilatation here
During sustained (isometric) contraction above about 20 per cent of maximum force, intramuscular pressure exceeds perfusion pressure and flow actually falls, forcing anaerobic metabolism and rapid fatigue. Rhythmic contraction instead aids flow through the muscle pump.