Capillaries and Veins

Recall that when the left ventricle contracts, it forces blood into the aorta, the body's largest artery. From the aorta, blood travels through a network of smaller arteries, which in turn divide and form even smaller vessels, called arterioles. The arterioles branch into a network of tiny vessels, called capillaries. A capillary is shown in Figure 46-5.

figure 46-4

Notice the thick muscular layer of an artery. The layers of the artery wall are separated by elastic tissue. This tissue provides strength, preventing systolic pressure from bursting the artery.

figure 46-4

Notice the thick muscular layer of an artery. The layers of the artery wall are separated by elastic tissue. This tissue provides strength, preventing systolic pressure from bursting the artery.

figure 46-5

The diameter of a capillary is so small that red blood cells must move single file through these vessels, as shown in this photograph (1,200x). All exchange of nutrients and waste between blood and cells occurs across the thin walls of the capillaries.

The network formed by capillaries is so extensive that all of the approximately 100 trillion cells in the body lie within about 125 pm of a capillary. This close association between capillaries and cells allows for rapid exchange of materials. Capillary walls are only one cell thick; gases and nutrients can diffuse through these thin walls. Wherever the concentration of oxygen or nutrients is higher in the blood than in the surrounding cells, the substance diffuses from the blood into the cells. Wherever the concentrations of carbon dioxide and wastes are higher in the cells than in the blood, these substances diffuse from the cells into the blood.

Blood flows through capillaries that merge to form larger vessels called venules (VEN-yoolz). Several venules in turn unite to form a vein, a large blood vessel that carries blood to the heart. Veins returning deoxygenated blood from the lower parts of the body merge to form the inferior vena cava. Veins returning deoxygenated blood from the upper parts of the body merge to form the superior vena cava. Refer back to Figure 46-2, and locate the inferior vena cava and the superior vena cava.

As you can see in Figure 46-6, although the walls of the veins are composed of three layers, like those of the arteries, they are thinner and less muscular. By the time blood reaches the veins, it is under much less pressure than it was in the arteries. With less pressure being exerted in the veins, the blood could flow backward and disrupt the pattern of circulation. To prevent that, valves in the veins help keep the blood flowing in one direction. Many veins pass through skeletal muscle. When these muscles contract, they are able to squeeze the blood through the veins. When these muscles relax, the valves can close, thus preventing the blood from flowing backward. Figure 46-6 shows the structure of a valve in a vein.

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