The chest expands and then relaxes during breathing primarily due to the contraction and relaxation of the diaphragm and intercostal muscles. When the diaphragm contracts, it flattens and moves downward, increasing the volume of the chest cavity. This expansion lowers the pressure inside the chest cavity, allowing air to flow into the lungs. The intercostal muscles also contract to lift and expand the rib cage, further increasing lung volume. When the diaphragm and intercostal muscles relax, the chest cavity volume decreases, pushing air out of the lungs and causing the chest to relax back to its original size.

This process allows the lungs to take in oxygen during inhalation and expel carbon dioxide during exhalation, which is essential for respiration and gas exchange in the body.