Haemostasis Mechanisms

There are four main mechanisms which control haemostasis. These are: Vascular Spasm, Platelet Plug Formation, Coagulation and Clot Retraction.

Vascular Spasm – this occurs in response to the immediate injury and stimulates the closure of the blood vessel wall by a sudden contraction of the smooth muscles within the vascular lumen; nervous system reflexes, chemicals mediators are then used to cause further vascular spasm while other mediators move into the area, such as platelets and thromboxan. This response occurs rapidly, but ceases after the inflammatory response.

Platelet Plug Formation – Platelets are developed during blood development and circulate around normal blood within the vasculature. Once an injury has caused the smooth surface of the vascular wall to open, platelets are no longer able to easily and smoothly slide around in the blood and immediately start to bind and accumulate into an aggregation of platelets that will eventually form a plug in order to fill the hole.

Coagulation (The Formation of a Clot) – although vascular spasm and platelet plugs may close most small wounds, a large would will require full coagulation, which is the formation of a proper clot. Coagulation factors are usually found within the plasma in normal blood, but are inactive until the vascular wall is damaged and the coagulation factors are activated.

Clot Retration and Dissolution – once the clot has been formed and has stopped the leakage of blood and the wound starts to heal, the clot starts to retract as a result of a hardening of the clot, as a result of contactiel proteins called actin and myosin, which pull the clot together. As the clot condenses, the serum, which is made up of fibrinogen and some clotting factors are literally squeezed out of the clot, which pulls the damaged vessels together. Disolution occurs as the clot dissolves and within a few days fibrinolysis (break up of fibrin) as a secondary result of plasmin release occurs.