Circulation of the blood
The blood is the fluid bringing nourishment to all the living structures or tissues of the body, and carrying away the waste matter formed there.
Blood is composed of numerous and extremely small separate particles of living matter called blood cells, suspended in a fluid called plasma. The majority of the blood cells are called red cells because of their reddish colour, and they carry the oxygen absorbed from the air into the lungs through breathing to the tissues. The other blood cells are called white cells because they contain no colouring. They are not oxygen carriers. Their main purpose is to serve as the scavengers of the blood and as a first line of defence against germs of infection which may enter the body. Nutrients are dissolved in the plasma and carried to the tissues. The blood also contains platelets that help with coagulation.
In order to nourish the tissues, the blood must constantly circulate through all parts of the body, and the organs which maintain the circulation are the heart and the blood vessels. These are therefore called the organs of circulation (fig 2.15).
The heart is a hollow, self-acting muscular pump supplying the activating power to the circulation. It is fitted with valves, so that it drives the flow of blood always in one direction, on through the blood vessels to the tissues and back again from the tissues to the heart. The blood vessels form a continuous closed system of tubes in which the blood is carried. They are divided into three classes:
arteries - blood vessels carrying blood from the heart to the tissues
capillaries - very fine blood vessels through which the blood circulates in the tissues
veins - blood vessels carrying blood from the tissues back to the heart
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