The Circulatory System
 

Transportation Of Blood  The Cardiac Muscle  Impluses  Transportation In Blood  Plasma  Red Blood Cell White Blood CellPlatelets Blood Vessles  Arteries Capillaries  Vein Distribution Of Blood  

Platelets

Platelets are not complete cells, but fragments of large cells called megakaryocytes. Megakaryocytes remain in the bone marrow, pinching off membrane-enclosed pieces of their cytoplasm, called platelets. Platelets then enter the blood and play a central role in blood clotting. Like red blood cells, platelets lack a nucleus, and their life span is even shorter: about 10 to 12 days. Clot formation is a complex process. It starts when platelets and other factors in the plasma contact an irregular surface, such as a damaged blood vessel. Platelets tend to stick to irregular surfaces, and they may build up and plug the damaged vessel if it is narrow enough. This mechanism is supplemented by blood coagulation, or clotting, which is the most important of the body's defenses against bleeding. The ruptured surface of an injured blood vessel not only causes platelets to adhere, but it also initiates a complex sequence of events among circulating plasma proteins. These events culminate in production of the enzyme thrombin. Thrombin catalyzes the conversion of the plasma protein fibrinogen into stringlike molecules called fibrin. Fibrin molecules adhere to one another, end to end and side to side, forming a fibrous matrix.. This protein web immobilizes the fluid portion of the blood, causing it to solidify in much the same way that gelatin does as it cools. The web traps red blood cells, further increasing the density of the clot. Platelets then adhere to the fibrous mass and send out sticky projections that attach to one another. Within half an hour, the platelets contract, pulling the mesh together and forcing liquid out. This action creates a denser, stronger clot and also constricts the wound, pulling the damaged surfaces closer together in a way that promotes healing.