When you inhale or breathe in air, your diaphragm will contract or tighten and it will move downward. This will result to an increase in space in your chest cavity where your lungs will expand. The intercostal muscles located between your ribs will also help in the enlargement of the chest cavity. When your lungs expand, air will flow through your nose or mouth. The air will travel through your wind pipe and into your lungs. It will pass through your bronchial tubes and it will eventually reach and enters the alveoli or the air sacs.
When you exhale or breathe out on the other hand, your diaphragm will relax and it will move upward into your chest cavity. The intercostal muscles will likewise relax resulting to the reduction of space in the chest cavity. When the space in the chest cavity gets smaller, air that is carrying carbon dioxide will be forced out of your lungs and windpipe and it will eventually go out of your nose or mouth.