Small Arteries Arterioles and Capillaries Account for 90 of Vascular Resistance

The relative importance of the various segments contributing to the systemic vascular resistance is appreciated by ob serving a profile of the pressure drop along the vascular tree (Fig. 15.8). Little change in pressure occurs in the aorta and large arteries. Approximately 70% of the pressure drop occurs in the small arteries and arterioles, and another 20% occurs in the capillaries. Contraction and relaxation of the smooth muscle in the walls of small arteries and arterioles cause changes in vessel diameter, which, in turn, influence blood flow.

When medium and large arteries are affected by disease, they may become major sources of increased resistance and significantly reduce blood flow to regions of the body (see Clinical Focus Box 15.1).

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