Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It is part of a larger process called hemostasis, which is the way that the body makes bleeding stop when it needs to. The coagulation process involves activation, adhesion, and aggregation of platelets, as well as deposition and maturation of fibrin. The coagulation pathway is a cascade of events that leads to hemostasis, allowing for rapid healing and prevention of spontaneous bleeding. There are two paths, intrinsic and extrinsic, that originate separately but converge at a specific point, leading to fibrin activation. The purpose is to ultimately stabilize the platelet plug with a fibrin mesh.

When coagulation works the way it is supposed to, it is an important step on the way to healing a wound. When a blood vessel breaks, as with a cut or other injury, coagulation helps form a clot, made from a substance called fibrin. The clot seals the hole until the tissues can repair themselves. However, when something happens that keeps coagulation from happening, it can lead to bruising, which happens when bleeding happens from small blood vessels under the skin, or to a hemorrhage, which is uncontrolled bleeding. When coagulation happens where it is not supposed to, it can form a clot that blocks a blood vessel instead of fixing it. Thrombosis is the formation of a blood clot, called a thrombus, inside a blood vessel, that blocks the flow of blood through the circulatory system.

The coagulation process is under the inhibitory control of several inhibitors that limit the clot formation, thus avoiding the thrombus propagation. This delicate balance is interrupted whenever the procoagulant activity of the coagulation factors is increased, or the activity of naturally occurring inhibitors is decreased. Coagulation disorders occur when there is a deficiency in a clotting factor involved, and the most common disorders are hemophilia and vitamin K deficiency.