When you breath or inhale,the diaphragm muscle contracts.Inhaling moves the diaphragm down and expands the chest cavity.Simultaneously,the ribs move up and increase the size of the chest ca.There is now more space and less air pressure inside the lungs. Air pushes in from the outside where there is higher air pressure. It pushes imo the lungs where there is a lower air pressure . When you breath out or exhale , the diaphragm muscle relaxes . The diaphragm and ribs return to their original place. The chest cavity returns to its original size. There is now less space and greater air pressure inside the lungs . It pushes the air outside where there is a lower air pressure.