OK, forget the top bit, that's just explaining why what he said happens with a heart attack doesn't work. So, on to the bottom bit (this is simplified, but means I've sort of skipped some important stuff). Basically, you get those atheromatous plaques in your coronary arteries - the ones that supply blood, and therefore that important oxygen to your heart muscle. The plaques are actually quite complex structures, which have an outside and an inside. As long as the outside is touching the blood, there is no problems, however, the stuff on the inside has stuff in it which tricks the body into thinking you are bleeding. What happens when you bleed? Your body makes a blood clot, to stop the bleeding. Unfortunately, the outside layer isn't as strong as we would like, doing things like getting your blood pressure up can cause the outside bit to rupture, thus letting the blood get to the inside bit. Suddenly, it thinks it's bleeding, and the body tries to stop the bleeding. But because you're not actually bleeding, the clot that would be on the outside, is not on the inside, it fills up the coronary artery so nothing can get past, and suddenly, all that blood with it's oxygen can't get through, and supply the heart muscle, and viola you have a heart attack. RS Final Year Medical Student