Haemostasis

Haemostasis is a complex process which causes the bleeding to stop, meaning to keep blood within a damaged blood vessel (the opposite of haemostasis is hemorrhage). It refers to the process of keeping blood within a damaged blood vessel. It is the first stage of wound healing. This involves coagulation, blood changing from a liquid to a gel. Intact blood vessels are central to moderating blood's tendency to form clots. 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 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 fibrin clot. These processes seal the hole until tissues are repaired.

Vasoconstriction

Vasoconstriction is the body's first response to a break in the vessel wall. It reduces blood flow to the area and therefore limits the loss of blood. This is followed by platelet plug formation and then by the blood coagulation to form a stable clot.

Platelet plug formation

Platelet plug formation is the second step in the process of vessel repair. Platelets are cells that circulate in the blood and bind together when they recognize damaged blood vessels. When the lining of a vessel is damaged, collagen is exposed. Platelets bind to the collagen with collagen receptors. This adhesion is strengthened further by von Willebrand factor (vWF), which is released from the endothelial cells and from the platelets; the vWF binds between the collagen and the platelet.

Blood coagulation

Blood coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It potentially results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The mechanism of coagulation involves activation, adhesion, and aggregation of platelets along with deposition and maturation of fibrin.

See also

• Coagulation
• Platelet
• Vasoconstriction
• Von Willebrand factor

Haemostasis Resources
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