Question

Question #4223f

Answer 1

Heparin

Explanation:

Brilliant question! Blood has a chemical called heparin which is an anticoagulant, meaning it stops blood from clotting.

Answer 2

Well the question should be why blood doesn't clot in an intact blood vessel,because blood can clot inside an injured blood vessel inside the body,and the pathway through which it occurs is called Intrinsic Pathway of blood coagulation.

For this process,the blood needs to come in contact with the ruptured endothelium of blood vessels,which activates factor X|| of intrinsic pathway,and the coagulation process begins.

But this can't happen until blood vessel is injured,and this happens to be a problem in patients with long standing hypertension,where blood exerts massive amount of pressure to the wall of blood vessels,and injury occurs,leading to coagulation inside the blood vessels,which leads to the obliteration of the lumen of blood vessels,and as a result of that,blood can't reach vital organs,and they stop acting properly.

Then,another important step for coagulation inside blood vessels is platelate aggregation,which needs statis(slow blood flow) and also some derivatives of arachidonic acid called TXA_2(Thromboxane A_2) secreted from activated platelates,but normal hemostatis mechanism produces equal amount of platelete antiaggregatory substances called PGI_2 (prostacyclins) from endothelium of blood vessels,which prevents platelate aggregation,thereby prevents platelate plug formation.

And as normally blood keeps on flowing through blood vessels,statis doesn't occur considerably to help platelate aggregation and this becomes a problem with chronic debilitated patients who have to lie down for long (like patients with plaster cast on their legs) and this immobility for long leads to statis of blood causing venous embolism.

Finally,blood contains several anticoagulants which acts to suppress coagulation pathway to maintain hemostatis.Some of the examples are Antithrombin |||, Heparins etc. which prevents selectively steps of coagulation.

That's why blood does never clot inside an intact blood vessel.