What happens when you inhale and exhale?
When you inhale the diaphragm muscles contract and move down. In the same time your ribs move up to make more space for the lungs. Air pushes into the lungs where air pressure is lower than outside them.
When you exhale, everything is in the opposite. Diaphragm muscles relax and return to their original size. Ribs are also moving down. The chest cavity is also returning to its natural size. When you exhale the air pressure is higher inside the lungs than outside.
