Why Bio?

Believe it or not, I took Biology because it was an academic... probably not the best reason but hey it's better than taking Pre-Calculus! I'm don't like sciences nor am I going into sciences in the future but ironically I am really interested in learning about the human body. Plus, I do find biology one of the easier sciences compared to Physics! In grade 8 we didn't go into depth about the functions of the numerous systems in our body and I'm really looking forward to learn more about how our body is able to do the miraculous things it does! My question is: (it may seem silly or obvious, but I don't have a clue to what the answer is!) why does our heart start to beat fast when we are nervous or scared?