When there is an injury and blood flows, a mechanism is provided within the body whereby blood loss is prevented. This is termed as the coagulation or clotting of blood.
The actual mechanism of blood coagulation is a complicated one, but the general principles are simple and important. Before clotting, there are some substances which must be present in the blood. They are prothrombin, calcium, fibrinogen and thromboplastin.
Prothrombin, calcium and fibrinogen are all normal constituents of blood. But thromboplastin is released only when there is a damage in a blood vessel or tissue cell the release of thromboplastin from thrombocytes or blood platelets brings about a series of changes or events which finally produces a blood clot.
Normally, prothrombin (protein present in the plasma) as such is inactive, but when acted upon thromboplastin in the presence of calcium is converted to an active substance thrombin.
Thrombin in turn acts on fibrinogen, another plasma protein, to produce an in soluble thread-like substance called fibrin.
The fibrin threads entrap blood cells to form a solid mass, the clot. After sometimes the clot shrinks’, and a clear stickle fluid, serum is released, (Serum=plasma-fibrinogen).
The mechanism of clotting can be expressed in a simple formula:
1. Prothrombin + Calcium+thromboplastin = Thrombin (active) (inactive) (from damaged tissue cells and platelets)
2. Thrombin + fibrinogen = Fibrin (fine threads) (inactive)
3. Fibrin + blood cells = CLOT
Certain factors or conditions hasten the clotting of blood, while others retards
Factors hastening clotting
1. Calcium salts acts as good coagulants.
2. Vitamin K has a coagulant action because it helps in the formation of prothrombin, which is necessary for blood clotting.
3. Injury to the tissues or vessel wall helps in coagulation, so that a clean cut with a sharp knife bleeds more freely than a crushed wound in which there is bruising and damage to the surrounding tissues.
4. Contact with a foreign body as the application of surgical dressings help in the speedy formation of a clot and arrest of hemorrhage.
5. Slightly higher temperature than that of the body helps clotting and hence the use of hot swabs to stop surgical bleeding.
Factors retarding clotting:
1. Heparin is a protein normally present in the blood is formed in the liver and prevents blood clotting in the vessels.
2. Addition of sodium citrate and potassium Oxalate to the blood will combine with calcium and form insoluble salts, thereby make it inactive. These prevent coagulation. In this method, blood is preserved in the blood banks without coagulation.
3. Contact with oil, grease or paraffin wax, would retard clotting.
4. Local Cold.
5. Snake venom is another anticoagulant.