Question

Explain why blood does not clot within the blood vessel in a healthy person

Answer 1

When blood vessels are injured, the endothelium is damaged and the tissue underneath the endothelium, called the sub- Vena cava A large vein that carries blood from the tissues to the heart, and then on to the lungs to pick up oxygen. ... Coagulation The process of blood clotting. Endothelium The lining of a blood vessel.

Answer 2

Blood coagulation starts when vessel wall is damaged! There is collagen layer beneath the endothelium which triggers coagulation cascade through a mechanism known as intrinsic mechanism. Plus, tissue factor (a clotting factor, a protein molecule which is required for clotting) enters circulation through damaged blood vessel. This factor triggers clotting mechanism through one more way known as extrinsic mechanism.

Normally, the collagen layer is not exposed to inside of the vessel. This fact together with smooth endothelial surface and flow of blood keeps blood from clotting. (that's very short summary of a long process of coagulation!)