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Coagulation is the biological process by which blood hardens into solid form. There are two related and critical functions of coagulation. First, it seals any breach of a human body’s primary organ of protection against harmful, external agents such as germs — the skin. Second, it maintains the closed circuit integrity necessary for the body to deliver oxygen and nutrients to all of its tissues via the circulatory system. The process is common to many nonhuman animals as well, and can be thought of as nature’s version of a bandage.
Blood is a viscous stew of several different specialized cells and many complex chemicals flowing through tubular blood vessels. If a vessel’s wall is damaged, such as when punctured through the skin or from the blunt force of a swung object, blood flow is compromised by hemorrhaging, more commonly called bleeding. The damaged site needs to be immediately plugged somehow to give the blood vessel a chance to initiate hemostasis, the return to a state of normal flow without blood loss.
From start to finish, this is a complex process involving both unique blood cells and chemical proteins, but it is fundamentally a linear cascade of triggering events. When the endothelial tissue, or inner wall of a blood vessel, senses trauma, it contracts to both restrict blood flow and minimize the damaged site. The tissue also releases a protein called thrombokinase to signal its distress. This triggers a reaction in two blood components: cell fragments called platelets and a protein called fibrinogen.
A multitude of platelets are attracted to, and aggregate at, the source site of thrombokinase. Fibrinogen’s response to thrombokinase is to restructure itself into a different protein called fibrin, which adheres to platelets. In a very short time, enough fibrin will enmesh the clump of platelets to create a solid barrier called a clot. While there are many other known intermediate chemicals and steps in the process, this is summarily the main function of coagulation — to form a clot, to plug a damaged blood vessel.
If coagulation is successful, the blood vessel relaxes and its endothelial tissue begins regenerative repair. When completely healed, a chemical enzyme called plasmin in the blood breaks the clot apart by dissolving the fibrin that held it together. If the damage was caused by a skin wound, the skin also heals and the extruded portion of the blood clot sloughs off. Disorders of coagulation include an inability to clot, such as the rare bleeding disorder hemophilia, which leaves victims at high risk for contracting potentially fatal infections. The more common disorder is thrombosis, when a blood clot dislodges and moves through the circulatory system, potentially to plug and disrupt blood flow to critical organs such as the heart or brain.