When diaphragm contracts and moves downwards, the chest cavity enlarges and the pressure in lungs decreases. The air is breathed in to equalise pressure. When diaphragm relaxes and moves upwards, the lungs push out the air.

When diaphragm contracts and moves downwards, the chest cavity enlarges and the pressure in lungs decreases. The air is breathed in to equalise pressure. When diaphragm relaxes and moves upwards, the lungs push out the air.