Hemostasis

Hemostasis or haemostasis (from the αἱμόστασις haimóstasis "styptic (drug)") is a process which causes bleeding to stop, meaning to keep blood within a damaged blood vessel (the opposite of hemostasis is hemorrhage). Most of the time this includes blood changing from a liquid to a solid state. Intact blood vessels are central to moderating blood's tendency to clot. The endothelial cells of intact vessels prevent blood clotting with a heparin-like molecule and thrombomodulin and prevent platelet aggregation with nitric oxide and prostacyclin. When endothelial injury occurs, the endothelial cells stop secretion of coagulation and aggregation inhibitors and instead secrete von Willebrand factor and tissue thromboplastin which initiate the maintenance of hemostasis after injury. Hemostasis has three major steps: 1) vasoconstriction, 2) temporary blockage of a break by a platelet plug, and 3) blood coagulation, or formation of a clot that seals the hole until tissues are repaired.

Overview
Hemostasis is maintained in the body via three mechanisms:
 * Vascular spasm - Damaged blood vessels constrict.
 * Platelet plug formation - Platelets adhere to damaged endothelium to form platelet plug (primary hemostasis) and then degranulate.
 * Blood coagulation - Clots form upon the conversion of fibrinogen to fibrin, and its addition to the platelet plug (secondary hemostasis).

Steps
Eventually, as the damaged vessel repairs itself, the clot retracts and is slowly dissolved by the enzyme plasmin.
 * The first step is immediate constriction of damaged vessels caused by vasoconstrictive paracrine released by the endothelium. Vasoconstriction temporarily decreases blood flow and pressure within the vessel. When you put pressure on a bleeding wound, you also decrease flow within the damaged vessel.
 * Vasoconstriction is rapidly followed by the second step,mechanical blockage of the hole by a platelet plug. The plug forms as platelets stick to the exposed collagen(platelet adhesion) and become activated, releasing cytokines into the area around the injury. Platelet factors reinforce local vasoconstriction and activate more platelets which stick to one another( platelet aggregation) to form a loose platelet plug.
 * Simultaneously, exposed collagen and tissue factor (a protein-phospholipid mixture) initiate the third step, a series of reactions known as the coagulation cascade. The cascade is a series of enzymatic reactions that ends in the formation of a fibrin protein fiber mesh that stabilizes the platelet plug. The reinforced platelet plug is called a clot. Some chemical factors involved in the coagulation cascade also promote platelet adhesion and aggregation in the damaged region.