Haemostasis - the prevention of excessive blood loss from a damaged blood vessel — is a fundamental homeostatic mechanism. There are three main processes:

• a local vasoconstriction at the site of injury which restricts blood flow;
• the formation of a temporary plug of platelets which seals the injury;
• the conversion of this loose aggregate of platelets into a more stable structure - the blood clot (thrombus).

This latter process is known as coagulation and involves a complex series of reactions culminating in the conversion of a soluble plasma protein, fibrinogen, to an insoluble protein, fibrin. Integral to the haemostatic mechanisms are the blood platelets or thrombocytes.