Diaphragm

The diaphragm is a muscle that is at the bottom of the ribcage of mammals.

When breathing in, the diaphragm pulls down so that the size of the lungs increases, allowing air to enter the lungs.
When breathing out, the diaphragm rests and goes up making a dome shape, decreasing the size of the lungs and pushing air out.
This increases the surface area of the air so that there is more oxygen that the lungs can hold.

Other websites
Science Aid: Lungs 

 Related pages 

 Respiratory system

 References 

Category:Anatomy of the respiratory system
Category:Muscles