Why are blood clots in arteries dangerous?

The fact that blood can clot is a very good thing. Clotting is what stops us from bleeding excessively when we get injured. But as you age, you can develop blood clots where you don't want them -- namely, on the walls of the arteries. And fat eventually builds in the walls of the arteries, slowing the flow of blood and causing platelet pileups -- blood vessel traffic jams -- that slow the flow of blood even more. These platelet pileups can form small clots in the arteries. If a clot gets too big, it can fill the entire artery, and blood can't get through at all, causing the tissue supplied by that artery to be at risk of dying. The heart has to work harder to push the blood to where it's supposed to go, increasing blood pressure and stressing the arteries even more. Indeed, just as a major traffic jam can affect a whole city, cardiovascular disease can stress your whole body.