system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it healthy to strain your heart through exercise, but unhealthy to strain it through stress, caffeine, nicotine etc? What is the difference between these kinds of cardiac strain?
gpt
Exercise causes natural vascular dialation allowing the increase in heart rate to provide your circulatory system with more oxygen.  
  
Caffeine and nicotine cause vascular constriction along with the increase in heart rate. This puts more stress on your heart since it has to work even harder to achieve the same level of blood oxygenation.