Examville.com - Anatomy - Blood Clotting Or Coagulation

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BLOOD CLOTTING OR COAGULATION Blood Clotting or Coagulation.: Blood clotting is a nature’s device to check the excessive loss of blood from an injury caused to the body. The process of clotting is initiated by platelets. The injured cells release substances that attract the platelets. They gather at and stick to the injured surface of the blood vessel. Their clumping is enhanced by ADP. The mass of aggregated platelets alone may physically plug a cut in a very small vessel. The contact of platelets with the collagen fibers exposed by the injury causes them to disintegrate and release two substances : serotonin and thromboplastin, which minimize loss of blood from the injury in two ways. Vasocontration. Serotonin causes the blood vessels at the site of bleeding contract. This reduces the blood loss, and also makes it less likely for the clot, formed to plug the injury, to be swept out by the flow of blood. Clot Formation. Thromboplastin, a lipoprotein, helps in clot formation. Clot formation occurs in three steps

(i)

Thromboplastin helps in the formation of an enzyme prothrombinase. This enzyme inactivates heparin and it also converts the inactive plasma protein prothrombin into its active form, thrombin. Both the changes require calcium ions.

(ii)

Thrombin acts as a proteolytic enzyme to separate two peptides from the soluble plasma protein fibrinogen molecule to form insoluble fibrin monomer.

(iii)

The fibrin monomers polymerize to form long, sticky fibers. The fibrin threads form a fine network over the wound and trap blood corpuscles (RBCs, WBCs, platelets) to form a crust, the clot. The latter seals the wound and stops bleeding. A clot is formed in about 2 to 8 minutes. The platelets also polymerise actin and myosin into a contractile apparatus. The latter later contracts, and the clot becomes relatively firm and solid. The part of blood plasma in the clotted area is squeezed out as a clear fluid called blood serum. The serum is plasma except that it lacks fibrinogen and corpuscles. It is, therefore, unable to clot. Role of Vitamin K in Blood Clotting. Vitamin K is necessary for the synthesis of prothrombin in the liver. If vitamin K is inadequate in the diet or is not absorbed in the intestine, blood clotting becomes inefficient. This produces symptoms similar to those of haemophilia. Natural Anticoagulants. A substance that checks the clotting of blood is called anticoagulant. Blood does not clot in uninjured vessels due to the presence in if of a strong natural anticoagulant, heparin, or antiprothrombin, produced in the liver. It checks the change of prothrombin into thrombin. Thrombosis. Sometimes a clot is formed inside an intact vessel. Such a clot is known as thrombus, and its formation as thrombosis. Thrombosis blocks the blood flow, and proves very serious if it involves the blood vessels of heart or brain. Another natural anticoagulant, named hirudin, occurs in the saliva of leech. It prevents clotting of victim’s blood in the leech’s crop. Sodium and potassium oxalates are other anticoagulants. They precipitate calcium ion which prevents clotting.