Mechanics of breathing and interpretation of a spirometer trace

During inhalation the intercostal muscles and diaphragm contract to expand the thoracic cavity volume, so air rushes in. During exhalation the intercostal muscles relax, the rib cage returns to its resting position and air is expelled. During exercise the lungs can expand more using the pectoralls and sternocleidmastoid. Expiration is aided by the abdominals pulling the ribs down quicker to force air out. A spirometer trace is a graphical representation of lung volume changes over time during breathing.