The vast majority of arterioles, whether large or small, are tubes of endothelial cells surrounded by a connective tissue basement membrane, a single or double layer of vascular smooth muscle cells, and a thin outer layer of connective tissue cells, nerve axons, and mast cells (Fig. 16.1). The vascular smooth muscle cells around the arterioles are 70 to 90 |xm long when fully relaxed. The muscle cells are anchored to the basement membrane and to each other in a way that any change in their length changes the diameter of the vessel. Vascular smooth muscle cells wrap around the arterioles at approximately a 90° angle to the long axis of the vessel. This arrangement is efficient because the tension developed by the vascular smooth muscle cell can be almost totally directed to maintaining or changing vessel diameter against the blood pressure within the vessel.
In the majority of organs, arteriolar muscle cells operate at about half their maximal length. If the muscle cells fully relax, the diameter of the vessel can nearly double to increase blood flow dramatically (flow increases as the fourth power of the vessel radius, see Chapter 12). When the muscle cells contract, the arterioles constrict, and with intense stimulation, the arterioles can literally shut for brief periods of time. A single muscle cell will not completely encircle a
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