All but the very largest arterioles consist of a single layer of smooth muscle cells (SMCs) wrapped circumferentially around the vessel, which is lined by a single layer of endothelial cells (ECs) oriented axially along the vessel. In addition to these major cell types, the adventitial (ablumi-nal) aspect of the arteriole supports perivascular nerves, pericytes, fibroblasts, and a substantial extracellular matrix. All of these elements have the capacity to contribute to arter-iolar control; the role of neural inputs, both central and perivascular, is best documented, but it is also now established that signals generated by the extracellular matrix can influence arteriolar tone through integrin coupling to Ca2+ channels, although how this might contribute to control of skeletal muscle arterioles is not known. Similarly, the contribution of pericytes (or other cell types located in close apposition to the arteriolar wall) to responses of arterioles in skeletal muscle remains largely undefined. The arteriolar wall is exposed to inputs from both the vessel lumen and the tissue space. It is established that ECs respond to mechanical inputs, of which the most prominent is flow, but other luminal stimuli have been identified. For example, red blood cells, in addition to their role in oxygen delivery, have been postulated to act as modifiers and/or sources of ATP and NO. On the abluminal side of the arteriole, skeletal muscle myocytes are a primary source of vasoactive agents, but additionally, the arteriole can respond to external mechanical stimuli, such as compression of microvessels induced by muscle contraction, and will be of course be influenced by neural inputs. Overall, these arterioles are exquisitely responsive to a very wide range of inputs; redundancy of control mechanisms clearly confers survival advantages on this important system but requires that the microcircula-tory network also have the capacity to integrate all this information to produce appropriately coordinated responses. Thus, an important emerging issue is that of identifying the mechanisms by which many inputs are integrated locally, and perhaps of even more significance, how they are integrated over the length of individual arteriolar segments, and across the microvascular network.
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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.