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:6 : Circulatory System
Basic Functions of the System
Blood is 'the essence of life', for this vital fluid carries to every individual cell in the body the nutrients necessary for producing energy and the raw materials necessary for tissue growth, maintenance and repair. It is vital too in clearing away all the waste products of the cells' various activities. It also acts as the body's policeman, liquidating any potentially dangerous invaders. Chemical messengers and other vital substances, too, need a transporting medium to distribute them through the body to keep the cells alive and active. The blood circulatory system with its intricately branching and inter-connecting tubes provide these services. Thus, the basic functions of this system are in the area of transport-carrying anything and everything that needs to be shifted from one part of the body to another. Oxygen from the air that is breathed into the lungs and nutrients absorbed from the intestines are carried and distributed to all the body cells. Hormones-chemical messengers-that regulate and co-ordinate the activities of the body, are carried by the blood, from the glands that secrete them, to the target cells on which they act. It also provides a continual supply of water for the needs of the body. The wasteproducts such as carbon dioxide (CO2), urea, etc. removed by it are carried to the lungs, kidneys and liver, where they are processed and excreted. Finally it is an important link in the body's system of temperature-regulation. Organs of the System
The propulsive force that keeps the blood moving is the steady beating of a powerful pump-the heart. The contractions of the heart drive blood into arteries which branch and Tebranch into smaller tubes, arterioles
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