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Platelets, also called thrombocytes, are small, colorless, irregularly-shaped bodies that are present in the blood. Their sticky surface lets them, along with other substances, form clots to stop bleeding. Platelets play a vital role in hemostasis, the prevention of blood loss. When the lining of a blood vessel is traumatized, platelets are stimulated to go to the site of the injury where they form a plug that helps reduce blood loss.

Platelets are not cells, but are tiny fragments of other cells. The life span of a platelet, once it is released into the circulatory system, is approximately five to nine days. Normal platelet counts range from 150,000 to 400,000 per cu/ml. Certain stem cell disorders may cause an increase in platelets, a condition called thrombocytosis. Increased bleeding or clotting can occur in thrombocytosis. A decrease in the platelet count in thrombocytopenia. It can be caused by diseases that affect platelet production or by excessive platelet destruction. Thrombocytopenia makes the individual more susceptible to bruising and/or increased bleeding.

Young platelets are more effective in achieving and maintaining hemostasis, the process carried out by the body to maintain blood in the vascular system. Old, damaged and nonfunctional platelets are removed by the spleen.

The hemostatic process prevents blood loss from ruptured blood vessels through the following processes:

The first and most immediate hemostatic response to blood vessel injury is vascular spasm, a rapid constriction of small arteries and arterioles. The second response is the formation of a platelet plug. The final response is the initiation of the coagulation cascade, which leads to fibrin production and the formation of a fibrin clot. Together, all the responses help prevent blood loss.

Platelets perform two functions in coagulation. First, a clot begins to form when blood is exposed to air. The platelets sense the presence of air, and begin to break apart and react with a protein called fibrinogen to form a temporary plug that covers the break in the blood vessel lining. Next, the platelets bind to each other, releasing agents that recruit and activate surrounding platelets. The result of these two processes is the formation of fibrin, which stabilizes the platelet plug, stops bleeding and allows injuries to heal.

Calcium and vitamin K must be present in blood to support the formation of clots. If the blood is lacking these nutrients, it will take longer for the blood to clot. A healthy diet provides most people with enough vitamins and minerals although, in some instances, vitamin supplements are sometimes needed.

Not all blood clots are good. If a small clot forms in an unbroken blood vessel in the brain, blocking the flow of blood to the cells, these brain cells may begin to die, causing a stroke.

Platelets are transfused to:

Platelets are prepared using two methods. They are collected for transfusion from a unit of whole blood or by Apheresis. In an Apheresis is donation, only the platelets are collected from the donor with the rest of the donor's blood being returned to him or her.

© 2006. American Red Cross. Connecticut Blood Services Center.