This is how are lungs operate. Inside our bodies, at the base of our lungs there is a membrane called the diaphragm.
When you breathe, your diaphragm contracts and expands. When you inhale, your diaphragm contracts to make room for your lungs, which fill up with air the way balloons do when you blow them up. This demonstrates this action when you pull down on its balloon bottom.
When you exhale, your diaphragm expands, forcing the air out of your lungs. You can see this action when I push up on its latex bottom.
The muscles of the rib cage, which protect the lungs, are also used in respiration, allowing the lungs to fill up with air. The main purpose of respiration is the intake of air, which contains oxygen, an element vital to our survival, and the removal of the waste product carbon dioxide.