In an animal at rest, the major mechanism determining the peripheral resistance is myogenic or basal tone [1]. This is the tone that arises in vascular smooth muscle cells as a result of their stretch by the intravascular pressure: the arterial pressure and its excursions. This stretch of the wall is related, albeit indirectly, to the function of the heart. At rest the influence of the sympathetic nervous system and the adrenal gland on the vasculature via the α-adrenoceptor, although present, is not dominant. However, during activity, and particularly during stress of various kinds such as exercise, emotions, changes in altitude and temperature etc., other factors, one of which is the α-adrenoceptor-mediated influence, become relatively more important and form a major pathway of homoeostatic adaptation. It must be borne in mind that the α-adrenoceptor mechanism is only one of a number of possible influences on the vasculature, which include β-adrenoceptor activation, non-catecholamines, peptide, purine and other putative or modulator transmitter systems, and also circulating substances such as vasopressin, angiotensin and other peptides.

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