Haemostasis is the physiological process that helps to maintain blood in the fluid state and prevent the escape of blood from damaged blood vessels through clot formation .
The haemostatic system is an important and tightly controlled component of the body’s defence system. It has features that are similar to other components of the host defence system, including inflammation and immune function . Understanding the process of haemostasis has allowed a better understanding of the major disease states associated with thrombosis—the formation of blood clots inside blood vessels—and their prevention and treatment.
The circulatory system maintains blood at pressure within the arterial system, and therefore a powerful, rapid, yet localised procoagulant response is required at sites of vascular injury to stem blood loss without compromising blood flow in general . Intimately related to this response to injury is the system of anticoagulant and clot-dissolving components present in the blood that curtail the procoagulant response beyond the vicinity of vascular injury. Thus, the haemostatic system is complex, with activating and inhibitory feed-back or feed-forward pathways that integrate five major components:
Recent advances have clarified how coagulation is initiated . What is now known is that a transmembrane protein called ‘tissue factor’ initiates blood coagulation. Tissue factor forms what is regarded as a ‘protective envelope’ around blood vessels and organs, and is ready to initiate clotting as soon as blood leaks out of vessels. It is the intravascular exposure of tissue factor by any route that can result in pathological thrombosis .
Figure: tissue factor initiates blood coagulation; intravascular exposure of tissue factor by any route can result in pathological thrombosis.