Clotting is the process that changes blood in the body from a free-flowing liquid into a thick, jellylike substance that stops bleeding.
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Almost as soon as blood starts to flow from a cut finger or scraped knee, platelets begin to gather. Platelets are clear blood cells that start the process of clotting. But like a towel used to wrap a leaky drainpipe, the disc-shaped platelets cannot completely stop the flow of blood.
As many as 20 different proteins in the blood come together in the clotting process. The proteins are in the plasma, which is the yellow-tinted portion of the blood that also contains water and other substances and carries blood cells and nutrients. These proteins in plasma are called "clotting factors." The protein clotting factors react with calcium and with other substances in the body's tissues and platelets to create a thick mass of jellylike material that plugs up the cut.
Clotting helps to stop bleeding from wounds. But blood may clot in the wrong places and times, such as the veins in the leg. This causes a condition known as "thrombosis." Clots in veins may break free and travel to the heart and lungs, where they may cause a pulmonary embolism * , which can cause death.
* pulmonary embolism is a blockage of the pulmonary artery or one of its branches that is frequently caused by thrombosis, or formation of a blood clot, in the lower extremities,
* hemophilia (hee-mo-FIL-e-a) is a hereditary disease that results in abnormal bleeding because the blood fails to clot. It occurs almost exclusively in males.
Some people have blood that does not clot well. Perhaps the best known clotting disorder is hemophilia * . People with liver disorders also may have blood that does not clot well since the liver produces some of the most important clotting factors found in the blood.