Why does blood clot outside our body? Why doesn't it clot inside our body?
When blood is exposed to new surface or air it clots.
Blood clotting factors are intrinsic and extrinsic. Intrinsic factors when are in cells, tissues and plasma blood do not clot. Some of the factors as surface activation factor stimulates blood to clot when exposed.
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.
Heparin is secreted in liver. It is strong anticoagulent. Small quantities of heparin granules are present in basophils. Blood clots after injury prevents blood flow in smaller blood vessels. Basophils reach injury site and releases heparin granules. This removes smaller clots to restore blood flow.
In short heparin prevents formation of internal blood clots.
Heparin prevents internal blood clots to form