Coagulation is known as extrinsic system as

Coagulation Factors:

The circulating plasma and tissue surrounding the blood vessel contain twelve coagulation factors. These coagulation factors are various organic compounds, proteins, co-enzymes and inorganic element—calcium (Ca).

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Under normal condition, in circulation these coagulation factors are present in an inactive form. Whenever there is injury, these are converted to active form.

The twelve blood coagulation factors are indicated by Roman numbers. These are:

Factor – Name of Factor

I – Fibrinogen

II – Prothrombin

III – Tissue extract/ Tissue thromboplastin

IV – Calcium

V – Proaccelerin / Liable factor

VI – Combined with factor V

VII – Proconvertin / stable factor

VIII – Antihaemorphilic factoe A / Antihaemophilic factor (AHF)

IX – Antihaemophilic factor B / Plasma thromboplastin component (PTC) (Christmas factor)

X – Stuart factor/Prower factor

XI – Plasma thromboplastin Antecedent/Antihae­mophilic (Factor C)

XII – Contact factor/Hageman factor

XIII – Fibrin stabilizing factors (Laki-L or/and Factor)

Platelets are not included in the list of coagulation factors, because platelets release several factors that help in clotting process. In addition, thromboplastin and thrombin are not included in the list, because these are intermediates of coagula­tion process, and not present normally in the body unless bleed­ing occurs.

Normal fresh plasma contains all factors listed above, except factor IV (Calcium), which is removed by anticoagulant.

Mechanism of Coagulation:

The mechanism of blood coagulation is necessary and signi­ficant to understand routine coagulation tests. Any of the deficiency of coagulation factors disturbs coagulation process.

Stage 1:

The tissue extract or tissue thromboplastin (III) enters the blood vessel through the site of injury and combines with factor VII and Ca++ (IV).

This is known as extrinsic system as the reaction is going on out of the blood vessel. The extrinsic system then joins with the intrinsic system, i.e. the blood vessel.

The factors that participate in intrinsic system are all present in the blood circulation. The factors are—XII, XI, IX, and VIII.

Now, the extrinsic and intrinsic system joins together and follows a common path. These react with factor X, V and Ca++ to produce plasmathromboplastin.

The end product of Stage 1 is plasma thromboplastin, which enters in Stage 2.

Any defect in intrinsic system of Stage 1 is recognized by activated partial thromboplastin time (APTT).

Stage 2:

In Stage 2, prothrombin (Factor II) is activated by plasma thromboplastin, which is the end product of Stage 1. This results in the formation of active thrombin. Any defect in Stage 2, i.e. deficiency of factor II, will be recognized by prolonged prothrombin time (PT).

Stage 3:

In the final stage of coagulation process, inactive fibrinogen is activated by the end product of Stage 2, i.e. thrombin. The fibrinogen is first converted in soluble fibrin. Soluble fibrin polymerizes with the help of factor XIII to form insoluble fibrin clot. In this way, for the formation of blood clot, twelve coagulation factors work in co-ordination with one another.