Blood Coagulation Mechanisms

Blood Coagulation Blood coagulation is the main characteristic of blood. The ability to clot within a short time after exposure to air prevents an impervious cover for damaged tissue to improve the blood loss. The normal blood clotting time is 3-10 minutes. The phenomenon or the process of blood clotting is called blood coagulation or clotting of blood. Conventional concept of blood coagulation: Best and Taylor's Theory: According to this theory, four substances are responsible for blood coagulation. Prothrombin: It is the glycoprotein and a small amount is found in plasma. Thromboplastin: It is the fatty compound found in tissue. Calcium ion: It is the inorganic compound or substance found in plasma. Blood Coagulation Blood coagulation is the main characteristic of blood. The ability to clot within a short time after exposure to air forms an impervious cover for damaged tissue to prevent blood loss. The normal blood clotting time is 3-10 minutes. The phenomenon or the process of blood clotting is called blood coagulation or clotting of blood. Conventional Concept of Blood Coagulation: Best and Taylor's Theory According to this theory, four substances are responsible for blood coagulation: Prothrombin: It is a glycoprotein, and a small amount is found in plasma. Thromboplastin: It is a fatty compound found in tissue. Calcium ion: It is an inorganic compound or substance found in plasma. Fibrinogen: It is a soluble plasma protein and remains in the dissolved form. According to this theory, when an injury takes place, thromboplastin is liberated from the tissue and it converts the prothrombin into thrombin in the presence of calcium ions. The active thrombin converts the soluble fibrinogen into insoluble fibrin, which develops the cellular portion of blood. Blood does not coagulate in the absence of thromboplastin or prothrombin (inactive). Diagram: The diagram shows the process of blood coagulation according to Best and Taylor's theory, illustrating the conversion of fibrinogen (soluble) to fibrin (insoluble) via the action of thrombin. Howell's Theory: According to this theory, the prothrombin is converted into thrombin only in the presence of Ca++. Thromboplastin has no role in the blood coagulation. In the blood vessel, the blood does not coagulate due to the presence of antithrombin. Antithrombin prevents the activation of prothrombin in blood vessel. Diagram showing the Howell's Theory of blood coagulation: In blood vessel: Prothrombin + Antithrombin → Prothrombin (Inactive) In injured tissue: Injured tissue + cephalin + prothrombin + antithrombin + Ca++ → Thrombin Fibrinogen (soluble) → (Thrombin) → Fibrin (insoluble) Fuld and Spiro's Theory: According to this theory, when an injury takes place, the blood platelets release an enzyme called Thrombokinase which converts the prothrombin into thrombin. Prothrombin (Inactive) + Thrombokinase + Ca++ → Thrombin (Active) Fibrinogen (Soluble) → (Thrombin) → Fibrin (Insoluble) [Diagram showing the Fuld and Spiro's Theory of blood coagulation] Modern concept of blood coagulation: The International commission recommended that the 13 factors are involved in the blood coagulation. These are designated as Roman nomenclature. Those are as follows: Factor-I (Fibrinogen): It is the glycoprotein and its molecular weight is about 340,000. It is made up of 2 subunits each consists of non-identical polypeptide chains; these are α, β, and γ. It is synthesized in the liver and remains in dissolved form in the plasma. Factor-II (Prothrombin): It is the glycoprotein and its molecular weight is about 70,000. It is synthesized in the liver due to the presence of vitamin K. The inactive prothrombin is activated into thrombin in the presence of thromboplastin during blood coagulation. Factor-III (Thromboplastin): It is the lipoprotein derived from two sources: intrinsic in the plasma and extrinsic in the tissue. Factor-IV (Calcium ion): It is regarded as a co-factor in the blood coagulation. It is essential for the formation of thromboplastin and converts the prothrombin into thrombin. Factor-V (Labile factor or proaccelerin): It is the plasma glycoprotein and its molecular weight is about 330,000. It is synthesized in the liver and helps in the conversion of prothrombin into thrombin. Factor-VI (Accelerin): It is the hypothetical product of proaccelerin. It has no longer use in the blood coagulation. Factor-VII (Proconvertin or stable factor): It is the plasma protein and its molecular weight is about 45,000. It stimulates the formation of thromboplastin. It is synthesized in the liver due to the presence of vitamin K. Factor-VIII (Antihaemophilic): It is a glycoprotein and its molecular weight is about 1.1 million. Factor-IX (Christmas Factor): It is a glycoprotein and its molecular weight is about 55,400. It is synthesized in the liver due to the presence of vitamin-K. Factor-X (Stuart Factor): It is a glycoprotein and its molecular weight is about 55,000. It is synthesized in the liver having non-identical polypeptide chains. Its deficiency causes noise bleeding. Factor-XI (Plasma Thromboplastin): It is the glycoprotein and its molecular weight is about 156,000. It contains 2 similar polypeptide chains. Factor-XII (Hageman): It is the glycoprotein and its molecular weight is about 76,000. Its inactive form is activated by contact or surface. It promotes the mechanism of blood coagulation. Factor-XIII (Fibrin Stabilizing Factor): It is the glycoprotein and its molecular weight is about 3,20,000.