The Blood Clotting Mechanism

Introductory Note: Knowledge of the structure and functions of blood and other aspects of the heart and vascular system are part of training in many therapies, such as Massage, Aromatherapy, Shiatsu, and others. This page is intended as Revision Notes for Basic / First Level Courses in these therapies, and some ITEC Diplomas.

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

The three mechanisms are:

Vascular Spasm - The smooth muscle in blood vessel walls contracts immediately the blood vessel is broken. This response reduces blood loss for some time, while the other hemostatic mechanisms become active.

Platelet Plug Formation - When blood platelets encounter a damaged blood vessel they form a "platelet plug" to help to close the gap in the broken blood vessel.

The key stages of this process are:

platelet adhesion,

platelet release reaction, and

platelet aggregation

Blood Clotting (Coagulation) - As described below:

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 is 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.

This blood clotting is a complex process involving many clotting factors (incl. calcium ions, enzymes, platelets, damaged tissues) activating each other.

The three stages of this process are:

Formation of Prothrombinase

Prothrombinase can be formed in two ways, depending of which of two "systems" or "pathways" apply.

These are

Intrinsic System

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

Extrinsic System

This is initiated by liquid blood making contact with damaged tissue.

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

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.

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.

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