The smallest arteries and all arterioles are primarily responsible for regulating vascular resistance and blood flow. Vessel radius is determined by the transmural pressure gradient and wall tension, as expressed by Laplace's law (see Chapter 14). Changes in wall tension developed by arteriolar smooth muscle cells directly alter vessel radius. Most arte-rioles can dilate 60 to 100% from their resting diameter and can maintain a 40 to 50% constriction for long periods. Therefore, large decreases and increases in vascular resistance and blood flow are well within the capability of the microscopic blood vessels. For example, a 20-fold increase in blood flow can occur in contracting skeletal muscle during exercise, and blood flow in the same vasculature can be reduced to 20 to 30% of normal during reflex increases in sympathetic nerve activity.
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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.