Skeletal muscle arterioles, like those of other organs, display myogenic tone, that is, in response to the transmural (distending) pressure exerted on the wall by the blood, the SMCs increase their degree of constriction. How this is manifest at any level of the microcirculation depends on the transmural wall stress, which is a function of both the pressure and the vessel radius. Thus the myogenic effect should be less in smaller vessels; despite this, smaller arterioles typically have higher tone those located more proximally. This reflects longitudinal differences in the ability of arterioles to respond to other regulatory mechanisms (local, neural). Differential sensitivities of larger versus smaller arterioles to a variety of stimuli is an important way in which integration of responses is achieved in the arteriolar microcirculation. In skeletal muscle microcirculation, perivascular nerves and varicosities are found in association with arterioles of all sizes, but arteriolar sensitivity to adrenergic inputs is not uniform across the network. Both a1 and a2 adrenergic receptors have been identified on large arterioles, while a2 receptors appear to predominate on smaller arterioles. Furthermore, most studies suggest that larger vessels (sometimes classified as first- and second-order vessels, in contrast to the smaller third- through fifth-order vessels) are more susceptible to neural inputs than are smaller arterioles; third-through fifth-order arterioles are in turn more responsive to metabolic stimuli than are larger arterioles. In like manner, oxygen sensitivity appears to predominate in smaller arteri-oles, while flow-dependent responses are more prominent in first- and second-order vessels. Notwithstanding these gradations in response characteristics as one moves distally through the network toward smaller arterioles, most arteri-oles do exhibit some degree of responsiveness to all these inputs. An important unanswered issue is to identify which of these differences are an acute reflection of the local conditions, and which reflect phenotypic differences in response capability.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.