Coagulation of Blood

Blood clotting (coagulation) is the body's emergency repair mechanism, stopping blood loss from an injured vessel within minutes. The process begins the moment the vessel's inner lining (endothelium) is torn and the connective tissue beneath it is exposed to flowing blood: platelets stick to the exposed collagen fibres and release chemical signals that recruit more platelets, forming a temporary platelet plug that gives immediate, if fragile, protection. Substances released by these clumped platelets and by the damaged tissue cells then mix with clotting factors already present in the plasma, producing a prothrombin-activator substance. This activator converts the inactive plasma protein prothrombin into its active enzyme form, thrombin, a reaction that requires both calcium ions and vitamin K. Thrombin then acts on a second plasma protein, fibrinogen, converting its soluble molecules into long insoluble threads of fibrin. These fibrin threads weave into an interlocking mesh at the wound site that physically traps blood cells and platelets, sealing the injury - this whole cascade is illustrated in Figure 7.4 as the intrinsic clotting mechanism. Once the mesh has formed, the fibrin fibres slowly contract, squeezing out a pale straw-coloured fluid called serum (which is simply plasma with the …