Why blood must clot
Blood clotting (coagulation) is the process that plugs a damaged blood vessel. It has two vital jobs: to stop excessive loss of blood, and to seal the wound so bacteria and other pathogens cannot enter. Without clotting, even a small cut could be dangerous. The dried clot, called a scab, also protects the healing tissue underneath while new skin forms, and it slowly dissolves once the wound is repaired.
The clotting sequence
When a vessel is injured, the wall becomes rough and platelets stick to it and to each other, forming a temporary plug. The clumped platelets and damaged tissue release an enzyme called thromboplastin (thrombokinase). In the presence of calcium ions and vitamin K, thromboplastin converts the inactive plasma protein prothrombin into the active enzyme thrombin. Thrombin then changes the soluble plasma protein fibrinogen into insoluble threads of fibrin. These fibrin threads form a mesh that traps red blood cells and platelets, and the whole tangle dries into a hard clot or scab.
Key idea
Platelets → thromboplastin → (prothrombin → thrombin) → (fibrinogen → fibrin) → mesh traps cells → clot. Calcium ions and vitamin K are needed for the middle steps.
Example
A person who lacks vitamin K, or whose diet is very poor in it, bruises easily and bleeds for longer, because prothrombin cannot be converted efficiently into thrombin.
When clotting fails
Haemophilia is an inherited disorder in which a clotting factor is missing, so blood clots very slowly and even minor injuries cause prolonged bleeding. On the other hand, an unwanted clot (a thrombus) forming inside an intact vessel can block blood flow and cause a heart attack or stroke, so clotting must be tightly controlled.
Remember
Fibrinogen is soluble and does the "waiting"; fibrin is the insoluble net that does the "trapping".