Pathways of Coagulation

The coagulation pathway is a set of events that results in hemostasis. The coagulation cascade is also known as secondary hemostasis. The coagulation cascade involves the activation of a series of clotting factors, which are further involved in blood clotting. It is a chain of events that occurs during the formation of a blood clot after a tissue injury. The main objective/aim of coagulation is to form a blood clot so that there is no bleeding and the tissue has enough time to repair itself. Get to know the 3 pathways of Coagulation. 

Intrinsic Pathway

The intrinsic pathway of coagulation of blood is the first and the longest pathway of blood coagulation in hemostasis and thrombosis. The intrinsic pathway consists of factors I, II, IX, X, XI, XIA. The intrinsic pathway starts with the activation of Factor XII and then it becomes XIIA after being exposed to endothelial collagen. It is only exposed when there is endothelial damage. We also know that Factor XIIA acts as a catalyst in activating Factor XI to Factor XIA. Furthermore, Factor XIA goes on to activate Factor IX and Factor IXA. The Factor IXA goes on to act as a catalyst in turning X to Xa, this is also known as a cascade. 

When each of the factors is activated, it goes on to activate many more factors later on. When we move further down the cascade, we can see that the concentration of factors in the blood increases by a lot. For example, the concentration of Factor IX is more than Factor XI. Furthermore, each one of them is named, fibrinogen, prothrombin, Christmas factor, Stuart-Prower factor, plasma thromboplastin, and Hageman factor. Factor XII is less critical and patients can clot their blood well without Factor XII

Extrinsic Pathway

The extrinsic pathway of coagulation of blood is the second and the shortest pathway of blood coagulation in hemostasis and thrombosis. The extrinsic pathway consists of factors I, II, VII, and X. Factor VII is called a stable factor. Once the damage is dealt with the vessel, the endothelial cells release tissue factor that activates the Factor VII and VIIa. Factor VIIa activates X and Xa. This is the point of time when the intrinsic and the extrinsic pathway become one.

The activity of the extrinsic pathway may be assessed in the laboratory using a simple test known as the prothrombin time.

Common Pathway

The common pathway begins at Factor X, which is activated by factor XA. The method of activating Factor XA is a complicated reaction. It is the third pathway of blood coagulation in hemostasis and thrombosis. The complex that divides X and Xa is known as tenace. Tenace has two forms, intrinsic and extrinsic. The extrinsic form consists of factor VII, factor III (tissue factor), and Ca2+. The intrinsic form consists of cofactor factor VIII, factor IXA, a phospholipid, and Ca2+. Once Factor Xa is activated, it goes on to activate factor II (prothrombin) into factor IIa (thrombin). Also, we know that factor Xa requires factor V as a cofactor to divide prothrombin into thrombin. Factor IIa (thrombin) will go on to activate fibrinogen into fibrin. The Fibrin subunits usually come together and form fibrin strands, and factor XIII acts on fibrin strands to form a fibrin mesh. This mesh will rather help to stabilize the platelet plug. 

Organ Involved 

The organ that is intimately involved in the coagulation process is the liver. The organ which is responsible for making the Factors of I, II, V, VII, VIII, IX, X, XI, XIII, and proteins C and S is the liver. The liver is a very essential organ in performing the main steps in the coagulation of blood. Pathology to the liver is very dangerous as it can cause a lack of coagulation which can finally lead to hemorrhage. If there is a decrease in coagulation factors then it indicates severe liver damage. Factor VII has the shortest half-life, which in turn lea to elevated PT first in liver disease.

In Coagulation, liver diseases are treated with fresh frozen plasma. 

Main Function of the Coagulation Pathway

The main function of the coagulation pathway is to keep hemostasis, which is the blockage of bleeding or hemorrhage. There are two main hemostasis, primary hemostasis, and secondary hemostasis.

• Primary hemostasis is an aggregation of platelets that forms a plug at the damaged site of exposed endothelial cells. 
• Secondary haemostasis includes the two common pathways, which are intrinsic and extrinsic. The intrinsic and the extrinsic pathways both lead to the common pathway. Ultimately, the common pathway activates fibrinogen into fibrin.

Mechanism

Do you want to know what are the main steps in coagulation of blood? Keep reading to know about it. The mechanism which allows coagulation for hemostasis is an intricate process that is done through a series of clotting factors. 

• The factors of intrinsic and extrinsic pathways circulate through the bloodstream as zymogens and are activated into serine proteases. 
• These serine proteases act as a catalyst to divide the next zymogen into more serine proteases and ultimately activate fibrinogen. 
• The intrinsic pathway is then activated through exposed endothelial collagen, and the extrinsic pathway is activated through tissue factor which is then released by endothelial cells after external damage. 

The Main Factors of an intrinsic pathway are factors I, II, IX, X, XI, XIA, and the main Factors of an extrinsic pathway are factors I, II, VII, and X.

Conclusion

To sum it all up there are 3 pathways of coagulation of blood, intrinsic, extrinsic, and common pathways. The intrinsic is the longest and the extrinsic is the shortest respectively. The intrinsic and the extrinsic pathways lead to the common pathway.  The most important organ is directly involved with the coagulation of blood in the liver. The coagulation of blood/blood clot forms when there is bleeding due to damage in the tissue, the blood clot gives the tissues enough time to heal themselves instead of losing excessive blood.