What is the primary muscle responsible for breathing?

• A. Diaphragm
• B. Intercostal muscles
• C. Abdominal muscles
• D. Pectoralis minor

Answer: (A)

Breathing is driven most by the diaphragm. When it contracts, the diaphragm pulls downward and flattens, which expands the vertical size of the thoracic cavity. This creates a drop in intrathoracic pressure, pulling air into the lungs during quiet inhalation. The rib cage also expands thanks to the intercostal muscles, but they contribute less to the overall volume change than the diaphragm does. Exhalation at rest is largely a passive process as the lungs and chest return to their resting shape, while abdominal muscles become important during forced expiration to push air out more forcefully. The pectoralis minor can assist during heavy breathing, but it does not drive normal inspiration. The diaphragm’s central role is underscored by its nerve supply from the phrenic nerve (C3–C5), which coordinates most of the inspiratory effort.