HUMAN ANATOMY AND PHYSIOLOGY

 COAGULATION OR CLOTTING OF BLOOD 

Formation of a blood clot is conversion of a hydrosol into a hydrogel the blood clot is a semi gel like substance that retracts on standing it works like a sticky cork that stops the bleeding when the cork retracts a little liquid serum loozing out at the end 

clot structure:

Electron microscope reveals the structural changes leading to clot formation:(1) Platelets disintegrate to give granules (2) Fibrin appears as needles or threads (3) Meshwork of threads is formed (4) WBCs and RBCs get entangled in the meshwork (5) Retraction occurs and finally blood clot is formed 
    RBC and WBC are not actually concerned with clotting they get trapped afterwards but platelets (TC) play an important role to initiate clotting of the blood it take 3-8 minutes for the blood to clot the average coagulation time being 5 minutes 

Mechanism of clotting :

clotting of blood involves basically three steps::
1. Activation of prothrombin which is present in the plasma 
2. Conversion of prothrombin into thrombin in the presence of calcium and other factors 
3. Conversion of fibrinogen to fibrin (clot) by active thrombin 

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