The Circulatory System
Fighting infection and foreign invaders is one of the primary functions of the leucocytes.
Platelets, or thrombocytes, are much smaller than the red blood cells. They are round or biconcave disks and are normally about 30 to 40 times more numerous than the white blood cells. The platelets' primary function is to stop bleeding. When tissue is damaged, the platelets aggregate in clumps to obstruct blood flow.
The plasma is more than 90 percent water and contains a large number of substances, many essential to life. Its major solute is a mixture of proteins. The most abundant plasma protein is albumin. The globulins are even larger protein molecules than albumin and are of many chemical structures and functions. The antibodies, produced by lymphocytes, are globulins and are carried throughout the body, where many of them fight bacteria and viruses.
An important function of plasma is to transport nutrients to the tissues. Glucose, for example, absorbed from the intestines, constitutes a major source of body energy. Some of the plasma proteins and fats, or lipids, are also used by the tissues for cell growth and energy. Minerals essential to body function, although present only in trace amounts, are other important elements of the plasma. The calcium ion, for example, is essential to the building of bone, as is phosphorus. Calcium is also essential to the clotting of blood. Copper is another necessary component of the plasma.
A major source of body energy, transported to the cells by the plasma, is ________.
adenosine triphosphate (ATP)
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