Regulation of muscle blood flow.

Nervous control of muscle blood flow, exerted mainly by the sympathetic adrenergic fibres, is important under resting conditions and also mainly during haemorrhage (when both alpha and beta receptors are involved), during fight and flight and in flow redistribution to muscles during exercise. The role of other fibres (cholinergic, histaminergic and peptidergic) is discussed. Myogenic control is responsible for the high basal tone and, consequently, the relatively low resting blood flow. The main regulatory mechanism is, however, the adaptation of blood flow to metabolic demands. Thus at rest, flow is higher in muscles with a high proportion of oxidative fibres. The regulation of flow in contracting muscles is exerted by metabolites, and various metabolites seem to play different roles to different extents in different types of contractions, with several factors probably involved at any one time. Potassium seems to be more important in mixed, predominantly glycolytic, muscles during tetanic or long-lasting isometric contractions; inorganic phosphate plays a role in short-lasting contractions and is probably more involved in highly oxidative muscles. Adenosine may play a role in long-lasting contractions in muscles with a high proportion of oxidative fibres. Dilatation of arterioles enables so-called capillary recruitment during muscle contractions, which seems to be mainly the result of changes in the velocity of red blood cells and a variable percentage of capillaries with intermittent flow (with very few cells with zero velocity and a more homogeneous flow in contracting muscles) rather than opening of unperfused capillaries.