FUNCTIONS OF THE CIRCULATORY SYSTEM
The blood vessels of the body form a network more complex than an interstate highway system. The blood vessels carry blood to within two or three cell diameters of nearly all the trillions of cells that make up the body. Blood flow through them is regulated, so that cells receive adequate nutrients and so that waste products are removed. Blood vessels remain functional, in most cases, in excess of 70 years, and when they are damaged, they repair themselves.
Blood vessels outside the heart are divided into two classes: (1)the pulmonary vessels, which transport blood from the right ventricle of the heart through the lungs and back to the left atrium, and (2) the systemic vessels, which transport blood through all parts of the body, from the left ventricle of the heart and back to the right atrium. Together, the pulmonary vessels and the systemic vessels constitute the circula-tory system.
The heart provides the major force that causes blood to circu-late, and the circulatory system has five functions:
1. Carries blood. Blood vessels carry blood from the heart toall the tissues of the body and back to the heart.
2. Exchanges nutrients, waste products, and gases with tissues. Nutrients and oxygen diffuse from blood vessels tocells in essentially all areas of the body. Waste products and carbon dioxide diffuse from the cells, where they are produced, to blood vessels.
3. Transports substances. Blood transports hormones,components of the immune system, molecules required for coagulation, enzymes, nutrients, gases, waste products, and other substances to all areas of the body.
4. Helps regulate blood pressure. The circulatory system andthe heart work together to regulate blood pressure within a normal range.
5. Directs blood flow to the tissues. The circulatory systemdirects blood to tissues when increased blood flow is required to maintain homeostasis.
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