Arteries are not simply conduits for carrying blood from the heart to the entire body. They are complex living structures and the most delicate part is their inner lining or endothelium. This is one cell thin on the inner lining of the artery.
This endothelium is so thin that it can be seen with a microscope with difficulty. It seals the inside of the artery and prevents blood from sticking on the inside of the artery and forming colts.
A damaged endothelium becomes leaky and if it is lost totally the arterial wall becomes exposed to blood which immediately forms clots and becomes occluded.