If the arterial blood pressure to an organ is decreased to the extent that blood flow is compromised, the vascular resistance decreases and blood flow returns to approximately normal. If arterial pressure is elevated, flow is initially increased, but the vascular resistance increases and restores the blood flow toward normal,- this is known as autoregulation of blood flow. Autoregulation appears to be primarily related to metabolic and myogenic control, as well as an increased release of NO if the tissue oxygen availability decreases. The cerebral and cardiac vasculatures, followed closely by the renal vasculature, are most able to autoregu-late blood flow. Skeletal muscle and intestinal vasculatures exhibit less well-developed autoregulation.
A phenomenon related to autoregulation is reactive hyperemia. When blood flow to any organ is stopped or reduced by vascular compression for more than a few seconds, vascular resistance dramatically decreases. Absence of blood flow allows vasodilatory chemicals to accumulate as hypoxia occurs, the vessels also dilate due to decreased myogenic stimulation (low microvascular pressure). As soon as the vascular compression is removed, blood flow is dramatically increased for a few minutes. The excess blood in the part is called hyperemia, it is a reaction to the previous period of ischemia. A good example of reactive hyper-emia is the redness of skin seen after a compression has been removed.
An example of autoregulation, based on data from the cerebral vasculature, is shown in Figure 16.8. Note that the arterioles continue to dilate at arterial pressures below 60 mm Hg, when blood flow begins to decrease significantly as arterial pressure is further lowered. The vessels clearly cannot dilate sufficiently to maintain blood flow at very low arterial pressures. At greater-than-normal arterial pressures, the arterioles constrict. If the mean arterial pressure is elevated appreciably above 150 to 160 mm Hg, the vessel walls cannot maintain sufficient tension to oppose passive distension by the high arterial pressure. The result is excessive blood flow and high microvascular pressures,
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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.