During human breathing, both the intercostal muscles and the diaphragm act in a coordinated manner to change the volume of the chest cavity in a rhythmic pattern. To draw air into the lungs the intercostal muscles move the ribs upwards and outwards while the diaphragm pulls the floor of the chest cavity downwards. This increases the volume of the chest, lowers the air pressure within the chest, and atmospheric air at a higher pressure rushes into the lungs.
