Control of peripheral vascular resistance: evidence based on the in vitro study of resistance arteries.

Resistance arteries, mainly small muscular arteries and arterioles, control the distribution of the cardiac output, capillary flow, pressure, and exchange. The active tone of these small vessels is influenced by a large variety of factors. Some less well known factors are discussed, including myogenic tone, variation in alpha 1-adrenoceptor number and affinity, and synaptic cleft width. The implications are that the adrenergic transmitter might be acting on low affinity non-alpha-adrenoceptors--extraceptors; that long-term trophic effects influence not only the function of blood vessels, but their number; and that a variety of types of vasodilator nerves exists. The adrenergic synaptic mechanism is different in small, compared with larger arteries; in particular there is an increased role of uptake. It is not known whether these influences are changed in hypertension.