Blood flow to skeletal muscle is important for oxygen and nutrient delivery and for removal of metabolic waste products. With the large increases in muscle metabolism that occur during exercise, muscle blood flow must increase dramatically in order for activity to be maintained for more than a few seconds. Skeletal muscle blood flow is relatively low at rest (3 to 5 mL/min/100 g), but can increase up to 100fold (400-500mL/min/100g in some muscles) during maximal, large muscle, rhythmic exercise (such as running or cycling) . The rate of blood flow to skeletal muscle is determined by the perfusion pressure (roughly equivalent to arterial blood pressure) and the vascular resistance to flow within the muscle. Diastolic blood pressure changes little during this type of exercise while systolic blood pressure increases in an intensity-dependent manner, meaning that mean arterial blood pressure (and therefore mean perfusion pressure) increases only modestly during exercise. Thus the major mechanism for increasing blood flow to skeletal muscle during exercise is a reduction in vascular resistance within the exercising skeletal muscle. Elegant control mechanisms are required to mediate this large increase in blood flow and to direct it to the capillaries in regions within the muscle where it is needed without causing dramatic reductions in systemic blood pressure or excessive transcapillary fluid loss. These mechanisms include sympathetic neural control and a host of local humoral, metabolic, and mechanical factors. The purpose of this chapter is to review the role of these control mechanisms in the microcirculation in mediating exercise-induced increases in skeletal muscle blood flow.
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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.