Arteries and arterioles have abundant elastic and muscle fibers

The walls of the large arteries have many collagen and elastic fibers, which enable them to withstand the high pressures of blood flowing rapidly from the heart (Figure 49.10). The elastic fibers have another important function as well: During systole, they are stretched, and thereby store some of the energy imparted to the blood by the heart. During diastole, they return this energy to the blood by elastic recoil, squeezing it and pushing it forward. As a result, even though the flow of blood through the arteries pulsates with the beating of the heart, it is smoother than it would be through a system of rigid pipes.

Smooth muscle cells in the walls of the arteries and arte-rioles allow those vessels to be constricted or dilated. When the diameter of the vessels changes, their resistance to blood flow changes as well, and the amount of blood flowing through them changes as a result. By influencing the contraction and relaxation of the smooth muscle in the vessel walls, neuronal and hormonal mechanisms can control the resistance of the vessels and therefore the distribution of blood to the different tissues of the body. (We'll see how these mechanisms work later in this chapter.) The arteries and ar-terioles are referred to as the resistance vessels because their resistance can vary.

Because veins operate under low pressure, some veins have valves to prevent backflow of blood.

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