Here is the answer of the given question above. So when Sam breathes in, his chest expands, causing air to fill his lungs. The structure that causes his chest to expand is the diaphragm. The diaphragm pulls your lungs downwards and your chest wall pulls outwards and upwards – this causes your lungs to stretch and pull air inside. When you relax, the elasticity of the lungs pushes air out. Hope this answer helps.