The respiratory system is composed of airways, lungs, blood vessels, and muscles working together to supply oxygen to all parts of your body. The diaphragm, a muscle located beneath the lungs, plays an important role. When you inhale, the diaphragm squeezes, or contracts, and moves downward. As a consequence, there is more room for the lungs to expand as they fill with air. When you exhale, the diaphragm relaxes and moves upward, which leads to air being pushed out of the lungs.