The body part that helps you breathe is the diaphragm. The diaphragm, which separates your stomach from your heart and lungs is dome-shaped when it relaxes. When your diaphragm contracts and flattens, air is breathed in (your lungs inflate just like balloons). When your diaphragm loosens and returns to a dome-shape, the air in your lungs is breathed out; the diaphragm deflates the lungs to force out the waste air. The movement of such muscles are caused by nerve impulses being sent from the respiratory center in the medulla oblongata, which is stimulated by the high concentration of carbon dioxide in the bloodstream.

Breathing in! Beathing out! 		
The air that you breathe in contains around 20.9% oxygen and 0.04% carbon dioxide, while the top air that you breathe out contains around 16.3% oxygen an 4.5% carbon dioxide. You can see the amount of carbon dioxide you breathe out is 100 times more than what you breathe in as a result of gaseous exchange. 	

An average person's breathing rate is 20 times per minute. The volume of air per breath is about 500 cubic centimetres. You may find that you breathe faster when you exercise. When you exercise, your body needs more oxygen which can be met by higher breathing rate.