Physiology of haemostasis

Haemostasis is a complex and sophisticated process that requires the interplay of multiple physiological pathways. Cellular and molecular mechanisms interact to seal damaged blood vessels with localized clot formation preventing significant bleeding. Once vascular integrity is restored, clot breakdown occurs and normal haemostasis is reinstated. Thrombohaemorrhagic imbalance may occur in the perioperative period or during critical illness, leading to an increased risk of thrombosis, bleeding or in some instances both. Therefore an understanding of the normal physiological processes is important for the anaesthetist as: (i) it allows us to identify targets for the therapeutic modulation of bleeding and thrombosis; (ii) many commonly encountered medications alter the normal haemostatic pathways and it is important to recognize their effects; and (iii) it enables enhanced understanding of the dynamic tests of haemostasis and clotting.