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INTRODUCTION

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 is estimated at between 100,000 and 150,000 km. The blood vascular system, or cardiovascular system (Figure 11–1), consists of the following structures:

  • 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, are 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.

FIGURE 11–1

The cardiovascular system.

The cardiovascular system consists of the heart, arteries, veins, and microvascular beds and moves blood throughout the body along two routes: the systemic circulation and the pulmonary circulation. In the larger systemic circulation (black arrows), blood is pumped from the left side of the heart into the aorta and then through other arteries supplying the multiple microvascular beds with systemic capillaries in most organs and regions of both the upper and lower body, and finally back to the right side of the heart in veins.

In the pulmonary circulation (yellow arrows), the right side of the heart pumps blood through pulmonary arteries into the lungs for oxygenation in the local microvasculature of pulmonary capillaries, and then back to the left side of the heart via pulmonary veins.

When the body is at rest, approximately 70% of the blood is moving through the systemic circulation, about 18% through the pulmonary circulation, and 12% through the heart.

As shown in Figure 11–1, two major divisions of arteries, microvasculature, and veins make up the pulmonary circulation, where blood is oxygenated in the lungs, and the systemic circulation, where blood brings nutrients and removes wastes in tissues throughout the body.

The lymphatic vascular system, introduced with the discussion of interstitial fluid in Chapter 5, begins with the lymphatic capillaries, which are thin-walled, closed-ended tubules carrying lymph that merge to form vessels of steadily increasing size. The largest lymph vessels connect with the blood vascular system and empty into the large veins near the heart. This returns fluid from tissue spaces all over ...

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