Human Body

I need to choose the theme and write an essay for biology class. This theme should be related to human body. For example:

1.Muscules: there structure and function

2.Nervouse system

You can choose some other theme, but send it to me before starting the work. 

So, I need the work, that answers the next questions:

1.Why did you choose this theme?

2.Why is it so important for people?

3.What is your methodology?

Please pay attention to the introduction espesially, it should be very detailed (what is it, why is it important and so on). The work shouldn't be too long (approximately 3 pages). It also shouldn't be too complicated. Of course, I need references.

Don't forget to send me the theme you choose, please.