Regarding the previous poster's use of Arnica, the thymol in its roots is a vasodilator, which means it widens blood vessels. In the wikipedia
article on vasodilators, it says, that this results in: "relaxation of smooth muscle cells within the vessel walls, particularly in the large arteries, smaller arterioles and large veins..." So it might seem to do with the arteries. Perhaps small clots passing through that area?
This is as far as I can figure, for now.