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The circulatory system pumps and directs blood cells and substances carried in blood to all tissues of the body. It includes both the blood and lymphatic vascular systems, and in an adult the total length of its vessels reaches an estimate between 100,000 and 150,000 kilometers. The blood vascular system, or cardiovascular system (Figure 11–1), consists of the following structures:
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The heart propels blood through the system and is surrounded by the pericardium, a fibrous sac lined by serous mesothelium.
Arteries, a series of vessels efferent from the heart that become smaller as they branch into the various organs, carry blood to the tissues.
Capillaries, the smallest vessels, correspond to the sites of O2, CO2, nutrient, and waste product exchange between blood and tissues. Together with the smallest arterial and venous branches carrying blood to and from them, capillaries in almost every organ form a complex network of thin, anastomosing tubules called the microvasculature or microvascular bed.
Veins result from the convergence of venules into a system of larger channels which continue enlarging as they approach the heart, toward which they carry the blood to be pumped again.
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As shown in Figure 11–1, two major divisions of arteries, microvasculature, and veins make up the pulmonary circulation, where blood receives oxygen in the lungs, and the systemic circulation, where blood delivers nutrients and removes wastes in tissues throughout the body.
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The lymphatic vascular system, introduced with the discussion of interstitial fluid in Chapter 5, begins with the lymphatic capillaries, which consist of thin-walled, closed-ended tubules carrying lymph and merging to create vessels of steadily increasing size. The largest lymph vessels connect with the blood vascular system, emptying into the large veins near the heart. This returns fluid from tissue spaces all over ...