The Blood Clotting Mechanism

Blood Clotting is one of three mechanisms that reduce the loss of blood from broken blood vessels.

The three mechanisms are:

Following damage to a blood vessel, vascular spasm occurs to reduce blood loss while other mechanisms also take effect:

• Blood platelets congregate at the site of damage and amass to form a platelet plug.
  This is the beginning of the process of the blood 'breaking down' from its usual liquid form in such a way that its constituents play their own parts in processes to minimise blood loss.
• Blood normally remains in its liquid state while it is within the blood vessels but when it leaves them the blood may thicken and form a gel (coagulation).
• Blood clotting (technically 'blood coagulation') is the process by which liquid blood is transformed into a solid state.
  It is a complex process involving many clotting factors incl. calcium ions, enzymes, platelets and damaged tissues, activating each other.

The three stages of this process are:

1. Formation of Prothrombinase
   
   Prothrombinase can be formed in two ways, depending of which of two 'systems' or 'pathways' apply.
   
   These are

   Intrinsic System	is initiated by liquid blood making contact with a foreign surface, i.e. something that is not part of the body; or
   Extrinsic System	is initiated by liquid blood making contact with damaged tissue.

   Both the intrinsic and the extrinsic systems involve interactions between coagulation factors. The coagulation factors have individual names but are often referred to by a standard set of Roman Numerals, e.g. Factor VIII (antihaemophilic factor), Factor IX (Christmas factor).

2. Prothrombin converted into the enzyme Thrombin
   
   Prothrombinase (formed in stage 1.) converts prothrombin, which is a plasma protein that is formed in the liver, into the enzyme thrombin.
   
   
3. Fibrinogen (soluble) converted to Fibrin (insoluble)
   
   In turn, thrombin converts fibrinogen (which is also a plasma protein synthesized in the liver) into fibrin.
   Fibrin is insoluble and forms the threads that bind the clot.

Consequences of Blood Clotting Problems:

If blood clots too quickly / easily then thrombosis may occur. This is blood clotting in an unbroken blood vessel, which is dangerous and can lead to strokes or heart-attacks.

Conversely, if blood takes too long to clot hemorrhage may occur. In this case much blood may be lost from the blood vessels, which is also dangerous.

The hereditary disorder haemophilia is a condition in which certain coagulation factors are missing from the blood, as a result of which the blood cannot form clots (without medical intervention).

See also pages about the structure and functions of blood, blood vessels, blood pressure, the structure of the heart, the functions of the heart, and the systemic circulation and the vascular system generally.