Blood doesn't clod inside the blood vessel because the chemical which causes the blood to clot in the cut area gets activated only when it comes in contact with air (oxygen actually) to form the clotting substance.Detailed description:There are two major facets of the clotting mechanism the platelets, and the thrombin system.The platelets are tiny cellular elements, made in the bone marrow, that travel in the bloodstream waiting for a bleeding problem to develop. When bleeding occurs, chemical reactions change the surface of the platelet to make it sticky. Sticky platelets are said to have become activated. These activated platelets begin adhering to the wall of the blood vessel at the site of bleeding, and within a few minutes they form what is called a white clot. (A clump of platelets appears white to the naked eye.)The thrombin system consists of several blood proteins that, when bleeding occurs, become activated. The activated clotting proteins engage in a cascade of chemical reactions that finally produce a substance called fibrin. Fibrin can be thought of as a long, sticky string. Fibrin strands stick to the exposed vessel wall, clumping together and forming a web-like complex of strands. Red blood cells become caught up in the web, and a red clot forms.A mature blood clot consists of both platelets and fibrin strands. The strands of fibrin bind the platelets together, and tighten the clot to make it stable.

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Inside the body liver secretes a substance called heparin. This prevents clotting of blood in blood vessels. But if there is internal injury, platelets break to release a thermoplastic substance. It initiates clotting.
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