The resistance to blood flow differs at each level in the vasculature. Resistance in the aorta and the network of large conducting arteries is minimal because of their large radii. On the other hand, resistance is quite high in the arteriolar network because of the small radius of each vessel, even though there are many arterioles arranged in parallel. This illustrates the major importance of vessel radius. The capillaries offer much less resistance than the arterioles. Although the radius of each capillary is less than that of an arteriole, the capillaries as a group have a low resistance because there are so many of them in parallel. Each arteriole supplies many capillaries. This hierarchical arrangement lowers capillary resistance and at the same time increases the surface area for exchange of metabolites. Because of their large radii and their parallel arrangement, the veins offer minimal resistance to blood flow.
Although both length and radius of the vessel contribute to its resistance, it is the radius that is altered to modulate flow. Smooth muscle cells in the vessels, especially the arterioles, are arranged such that their contraction or relaxation results in reductions and increases in vessel radius, resp., causing major changes in the resistance to blood flow. Figure 16 illustrates the change in resistance in a single vascular bed that occurs during stimulation of its sympathetic nerves. The perfusion pressure is held constant. Note that at increasing stimulation rates the flow falls, indicating an increase in resistance. Now let's extrapolate this experiment to the intact body under two conditions. First, if there were generalized sympathetic stimulation to all the organs, and if cardiac output (CO) remained constant, then mean arterial blood pressure would increase. That is because the total peripheral resistance
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