Why does the blood clot outside the body and not within the blood vessels?
Clotting of blood is brought about by an enzyme called prothrom-bin in the blood. Another substance called heparin or anti-pro- thrombin keeps check on its production and hence the blood does not coagulate in the blood vessels.
But whenever an injury takes place and blood flows out, the blood platelets release a substance called platelet factor- 3 which combines with blood proteins and convert them to prothrombo plastin. This changes to thromboplastin on reaction with calcium ions. In the presence of thromboplastin, the inactive prothrombins changes into active thromb in changing the protein fibrinogen in the blood to fibrin. It is this fibrin which becomes a network of fine fibres to form a blood clot.
