fluidistic

Hi. I don't understand why stress is "bad" for the heart while exercising is "good". For example I'm having a final exam tomorrow worth 100% of my grade for a course and I'm having a huge stress (I feel adrenaline rushes almost permanently). I'm also used to run 10 km twice a week and so I know how painful running is (extremely painful). In both cases the heart beats much faster than when at rest, but I don't understand why stress would be different from say running, for the heart.