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Hemostasis is the process by which blood is changed to a solid state. It is what stops the bleeding after an injury to the blood vessels occurs. The blood vessels are protected by cells that prevent the formation of thrombin, a coagulation protein that catalyzes reactions in the bloodstream. When an injury permeates the cells and gets to the vessels, hemostasis occurs.
There are two phases of hemostasis. During the first, primary hemostasis, the vascular muscle contracts temporarily as soon as the cells are disturbed. This contraction slows the blood flow and either activates or speeds up the adhesion of platelets. During adhesion, proteins on the surface of each platelet stick to the von Willebrand factor, a protein found in blood plasma.
As platelets collect across the surface, they make contact with collagen, the main proteins in humans, and are thus activated. These platelets cover the surface and fibers, and the receptors of the platelet membranes grip the fibrinogen, a protein found in plasma and synthesized by the liver. When platelets and fibrinogen build up, they form a plug. This all happens within 20 seconds of the injury.
During secondary hemostasis, the clot is stabilized, but platelet secretions continue the contractions of the vascular muscle. Through the interaction of enzymes, platelet membranes and various coagulation processes, the plug becomes solid. The coagulation processes occur in the liver but circulate inactively throughout the body until something called a coagulation cascade begins. Throughout the cascade, a series of steps occurs in which one reaction leads to another until fibrinogen is converted to fibrin, a protein that forms the hemostatic plug or clot over an injury. The fibrin is mesh-like in texture at first, but when platelets and red blood cells combine with a dense grouping of fibers, a blood clot is formed.
Hemostasis and thrombosis are closely related, as thrombosis is the formation of the blood clot in a blood vessel. Thrombosis can occur in a vein or artery, and the clot itself is called a thrombus, which is Greek for lump or clump. Thrombosis in a vein can cause deep vein thrombosis, a condition that affects blood clotting in the legs. Coronary thrombosis is thrombosis that affects the arteries and can cause a heart attack when a blood clot cuts off the blood supply to the heart. A thrombus can be caused by an injury to a blood vessel, the disruption of regular blood flow, an inflammation or atherosclerosis.