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 kilometers. The blood vascular system, or cardiovascular system (Figure 11–1), consists of the following structures:
The heart propels blood through the system.
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 that continue enlarging as they approach the heart, toward which they carry the blood to be pumped again.
Diagram of the cardiovascular system.
The system consisting of the heart, arteries, veins, and microvascular beds is organized as the pulmonary circulation and the systemic circulation. In the pulmonary circulation the right side of the heart pumps blood through pulmonary vessels, through the lungs for oxygenation, and back to the left side of the heart. The larger systemic circulation pumps blood from the left side of the heart through vessels supplying either the head and arms or the lower body, and back to the right side of the heart.
When the body is at rest, approximately 70% of the blood moves 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 the body to the blood.
The internal surface of all components of the blood and lymphatic systems is lined by a simple squamous epithelium called endothelium. As the interface between blood and the organs, cardiovascular endothelial cells have crucial physiologic and medical importance. Not only must endothelial cells maintain a selectively permeable, ...