Coagulation of Blood
Coagulation of Blood
- The process in which blood loses its fluidity and becomes a jelly-like mass few minutes after it is shed out or collected in a container.
Clotting Factors
- Coagulation occurs through a series of reactions due to the activation of a group of substances. The substances necessary for clotting are called Clotting factors.
- Cloting factors were named after the scientists who discovered them/as per their activity (except factor IX)
- Roman numerals of clotting factors are based on the order in which they were discovered.
Clotting Factors
| Name | Factor |
| I | Fibrinogen |
| II | Prothrombin |
| III |
Thromboplastin (Tissue Factor) |
| IV | Calcium |
| V | Labile Factor (Proaccelerin/ Accelerator globulin) |
| VI | Factor has not been proved |
| VII | Stable Factor |
| VIII | Antihemophilic Factor (Antihemophilic globulin) |
| IX | Christmas Factor |
| X | Stuart-Prower factor |
| XI | Plasma thromboplastin antecedent |
| XII | Hageman factor (contact factor) |
|
XIII |
Fibrin-stabilizing factor (fibrinase) |
Stages of Blood Clotting
Normally during circulation, the blood does not clot, because the enzymes involved in clotting are in inactive form. Slight initial activation causes clotting in which each enzyme activates another one in a sequential manner till the conversion of fibrinogen into fibrin. In general, clotting occurs in 3 stages namely
- Formation of Prothrombin Activator
- Conversion of Prothrombin into Thrombin
- Conversion of Fibrinogen into Fibrin
Stage 1- Formation of Prothrombin
Activator Formation of prothrombin activator occurs through 2 pathways
A. Intrinsic pathway - initiated by platelets within the blood.
- An injury, as the blood vessels get damaged, the blood comes in contact with the exposed collagen of the damaged vessel wall. This contact activates factor-XII (Hageman Factor) and also the platelets, which subsequently initiates the clotting mechanism. As in this case the clotting mechanism is initiated by the components intrinsic to the blood, i.e., platelets and clotting proteins, this pathway is called intrinsic pathway.
- The intrinsic pathway of blood coagulation causes the blood to clot within the vessel. In the following steps of intrinsic pathway, the activated factor XII leads to the activation of factor-XI, activated factor-XI, along with Ca++ ions and factor-IV, leads to the activation of factor-IX and activated factor-IX, along with Cat++ ions, factor-VIII and platelet factor-III leads to the activation of factor-X.
B. Extrinsic pathway – initiated by tissue thromboplastin formed from injured tissues.When a blood vessel is ruptured, the surrounding tissues are damaged. The damaged tissues release a complex of substances referred to as tissue thromboplastin. The tissue thromboplastin further complexes with factor-VII and Ca++ ions to activate factor-X directly. Here the tissue thromboplastin (Factor-III) is derived from tissue, which is outside the blood. Therefore, this mechanism of blood clotting is called extrinsic pathway.
Stage 2- Conversion of Prothrombin into Thrombin
Blood clotting is all about thrombin formation. Once thrombin is formed, it definitely leads to clot formation. Once formed, thrombin initiates the formation of more thrombin molecules. Factor V in turn accelerates formation of both extrinsic and intrinsic prothrombin activator, which converts prothrombin into thrombin. This effect of thrombin is called positive feedback effect.
Stage 3- Conversion of Fibrinogen into Fibrin
- Thrombin converts fibrinogen into activated fibrinogen which is called fibrin monomer.
- Fibrin monomer polymerizes with other monomer molecules and form loosely arranged strands of fibrin.
- Later these loose strands are modified into dense and tight fibrin threads by fibrin- stabilizing factor (factor XIII) in the presence of calcium ions.
- All the tight fibrin threads are aggregated to form a meshwork of stable clot.