What is Blood clotting Process?
Coagulation or blood clotting process is a physico-chemical process by which the blood loses its fluidity and turns into a semisolid jelly. If it is examined under high power microscope, blood clot appears to be composed of a network of fine thread-like fibrils among which the blood cells are entangled. These fibrils are composed of a protein called fibrin. Clotting is a very important property of blood because it helps in hold of bleeding by plugging the ruptured blood vessel.
Mechanism of blood clotting process
Blood clotting process is a complex process, the basic mechanism of which is formation of insoluble fibrin threads from the soluble plasma protein called fibrinogen. The fibrin threads then form a network within which the red and white blood cells become entangled. Formation of fibrin from fibrinogen is catalyzed by an enzyme named thrombin. This enzyme is in general not present in blood. During clotting, thrombin is formed from another plasma protein called prothrombin by the action of the enzyme called thromboplastin. Calcium ion is essentially required for coagulation. For this reason, clotting is prevented in absence of Ca++ in blood. Vitamin K helps in clotting by taking part in synthesis of the plasma protein prothrombin in liver. In deficiency of this vitamin or in liver diseases, blood clotting is impaired.
Thromboplastin is also not present in blood usually and is formed only when a blood vessel is ruptured or injured. From the following two sources thromboplastin may originate: -
i) From platelets which disintegrate when they come in contact with the injured surface.
ii) From the damaged tissue-cells of the injured area.
The blood clotting process includes three main following phases: -
i) Formation of thromboplastin,
ii) Formation of thrombin from prothrombin,
iii) Formation of fibrin from fibrinogen.
After the formation of clot, the clot retracts and a straw colored fluid called serum is separated. Thus, the serum is blood minus the clot. Serum can also be broadly considered as plasma minus the proteins fibrinogen and prothrombin that are used up in coagulation.
Anticoagulants of blood clotting process
Some chemical substances which on being mixed with blood prevent coagulation are called anticoagulants. Oxalate and citrate salts of sodium and potassium, and heparin are most commonly used anticoagulants. When oxalates or citrates are mixed with blood, they remove the Ca++ ions by precipitating them as calcium oxalate or calcium citrate and as a result the blood clotting process is prevented. In blood banks, sodium citrate is the preferred anticoagulant for storage of blood because it can be easily metabolized in the body of the recipient through citric acid cycle. Heparin is muco-polysaccharide secreted from liver, basophil leucocytes and mast cells. It prevents clotting by inactivating thromboplastin which in turn prevents development of thrombin from prothrombin.
Intravascular blood clotting process or Thrombosis
Usually, the circulating blood does not clot within the blood vessels. The main cause behind it is the absence of thromboplastin which is not formed unless there is an injury and rupture of blood vessel. Silkiness of the endothelial lining prevents rupture of platelets and formation of thromboplastin from them. In addition to this, presence of heparin in blood and a continuous motion of the blood also prevent clotting of blood within blood vessels. But, in some pathological conditions e.g., atherosclerosis, in which the inner lining of the blood vessels become damaged, the platelets stick on to it and disintegrate to release thromboplastin. This may lead to clotting of blood within the blood vessel which is referred to as intravascular clotting or thrombosis. Thrombosis is a blood clotting process in vital organs like the heart called coronary thrombosis or the brain called cerebral thrombosis, both of which is fatal because it may result in blockage of circulation and thus restriction of oxygen supply to these organs.
What is Coagulation time?
This is the time taken for blood to clot after it is withdrawn from the body and taken in a tube. Increase of coagulation time indicates decreased blood clotting process. It can be measured by a number of methods that give different results. The normal coagulation time is 2-8.
What is bleeding time?
Bleeding time (BT) is the time up to which blood oozes from a pricked point on the skin, preferably on the ear lobe. Its normal value ranges between 2-5 minutes.