When the body is injured, a complex series of events takes place in the blood to stop any bleeding that occurs. Through the interaction of coagulation proteins, blood vessels and platelets, a stable clot is produced. The process continues after blood vessels heal and the blood clot is dissolved.Blood Vessels Constrict. Blood vessels react quickly to injuries. As soon as bleeding starts, blood vessels constrict, slowing the flow of blood through the vessels and minimizing blood loss. Exposed tissue interacts with blood coagulation proteins, signaling the start of the coagulation process. This happens within seconds of the injury.

Platelets are tiny cells that congregate around ruptures in blood vessels to provide the backbone of a clot. The platelets essentially form a temporary ‘plug’ to stop bleeding. Active platelets also stimulate the activation of other coagulation proteins.

Coagulation Factors. Blood Clotting: Normal vs. Bleeding DisorderWhile the platelets form the temporary plug, other blood proteins congregate on the damaged blood vessels to reinforce the clot. Over twenty different proteins are required for the full coagulation process. Two of these proteins, factor VIII and factor IX, are especially important: they are part of the process that stimulates production of factor X, and eventually the blood protein thrombin, the essential final step in blood clotting.

Low levels of factor VIII and factor IX slow the coagulation process. Hemophilia, which is characterized by slow coagulation and extended bleeding, is caused by low levels of one of these two blood factors.

Thrombrin and Fibrin. The interaction of the different blood factors and platelets eventually produces prothrombin (factor II), which continues to develop into a protein called thrombin (factor IIa). Thrombin is activated on the surface of platelets, and transforms a protein called fibrinogen into fibrin.

Fibrin surrounds the platelet plug, creating a fibrin mesh. The fibrin mesh is stronger and more stable than the temporary platelet plug. Over the next few days the fibrin mesh, or blood clot, strengthens even more, protecting the blood vessels from further damage or blood loss.

After the injury heals, the body has to remove the fibrin clot. A protein called plasmin is formed that dissolves fibrin, and the blood clot is slowly removed.
Thrombin and Emphysema. Left unchecked, thrombin has the potential to generate dangerous levels of clot-forming fibrin. In order to control thrombin levels, the body produces four different thrombin inhibitors. One of these is called alpha-1 antitrypsin. The lack or mutation of alpha-1 antitrypsin not only affects thrombin activity, but has also been linked to the development of emphysema.

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