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blood clotting

blood clotting
When an injury causes a blood vessel wall to break, platelets are activated. They change shape from round to spiny, stick to the broken vessel wall and each other, and begin to plug the break. They also interact with other blood proteins to form fibrin. Fibrin strands form a net that entraps more platelets and blood cells, producing a clot that plugs the break. © Merck
A protective mechanism that prevents excessive blood from being lost after an injury. The clotting process is set in motion when blood comes into contact with tissues outside its ruptured vessel. These tissues contain a factor, thromboplastin, which activates a sequence of changes in the plasma clotting factors (12 enzymes). Factor II (prothrombin, formed in the liver), with calcium ions and a platelet factor, is converted to thrombin. This converts factor I (fibrinogen) to fibrin, a tough, insoluble polymerized protein which forms a network of fibers around the platelets that have stuck to the edge of the wound and to each other. The network entangles the blood cells, and contracts, squeezing out the serum and leaving a clot, which dries to form a scab. This prevents further loss of blood, and also prevents bacteria getting into the wound. Normal clotting takes place within five minutes.

In some diseases, such as hemophilia, the clotting mechanism is impaired. See also excessive blood clotting.

Related entries

   • anticoagulant
   • embolism
   • hemorrhage
   • thrombosis

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