Blood Clotting Mechanism

The human blood clotting mechanism is a puzzle for evolutionists.The blood clotting mechanism centers around FIBRINOGEN.This FIBRINOGEN is a BLOOD PROTEIN that forms the fibres that make up the clots.Normally this fibronogen is dissolved in the blood plasma.When bleeding begins,a protein called THROMBIN cuts firbrinogen to make strings of proteins called as FIBRIN.

Now these Fibrin filaments stick together forming a network that catches blood cells,thus stopping the flow of blood from the wound.At first this network is not very strong.It sometimes break,allowing the blood to flow out from the wound again.To prevent this a protein called the FIBRIN STABILISING FACTOR (FSF),creates cross links between fibrin filaments,strengthening the network.

Meanwhile,thrombin is cutting more fibrinogen into more fibrin,which forms more clots.At a certain point,the thrombin has to stop cutting fibrinogen or else much of fibrin would be produced that it would clot up the whole blood system and the person would die!
Thrombin originally exists as an inactive form called as PROTHROMBIN.So for the clotting process to start,prothrombin must be converted to thrombin.Otherwise, a person bleeds to death.And once the proper clotting is formed,thrombin has to be turned back into prothrombin.Otherwise the clotting continues until all the blood stops flowing.

A protein called as STUART FACTOR is involved in the activation of prothrombin,turning it into thrombin,so that the clotting process can start.The thrombin cuts fibrinogen,forming fibrin,which actually forms clots.Now we can turn to the question of how to stop the clotting process once it starts.Runaway clotting would clog up the organism's blood vessels.After thrombin molecule have formed,a protein called antithrombin binds them,thus inactivating them and stopping the clotting.In an injured blood vessel the clotting can continue.In this way,the clotting goes on only at the site of the wound and not in the uninjured blood vessels.

Once the injured vessels are repaired the clotting will stop there.After sometime ,when the wound has healed,the clot itself must be removed.A protein called PLASMIN cuts the fibrin network that makes up the clot.Plasmin exists in its unactive form as PLASMINOGEN and must be activated at the proper time to remove the clot.Its activation of course,involves complex interactions with other protein.