How the Body Works: Blood Clotting

Blood clotting takes place in a sequential process to prevent excessive blood loss from a wound. Normally, circulating blood contains red cells, platelets, plasma-clotting factors and fibrinogen, a soluble protein. Tissue-clotting factors lie trapped within cells surrounding each blood vessel. When an injury occurs, blood escapes from the broken vessel. Platelets aggregate at the site and partially plug the break. Tissue-clotting factors are released. The interaction of the platelets with plasma and tissue-clotting factors results in the conversion of fibrinogen into insoluble fibrin threads, which form a mesh across the break in the vessel wall. Blood cells and platelets become trapped in the mesh. The semisolid mass shrinks and a yellow fluid, serum, is pushed out, leaving the familiar dark red clot.