Mechanism of Breathing — Diaphragm, Intercostals & Lung Volume 3D

Breathing works by changing the volume of the thoracic (chest) cavity, which changes the pressure inside the lungs. During inspiration the diaphragm contracts and flattens and the external intercostal muscles raise the ribs and sternum, increasing thoracic volume; the intra-pulmonary pressure falls below atmospheric pressure and air moves into the lungs. During expiration these muscles relax, the volume decreases, the pressure rises above atmospheric, and air is pushed out.

The diaphragm is a dome-shaped muscle that forms the floor of the thoracic cavity and is the principal muscle of inspiration. When it contracts it flattens and moves downward, increasing the vertical (length) dimension of the chest and thus the thoracic volume. When it relaxes it returns to its dome shape, decreasing the volume and helping expiration.

Both lie between the ribs. The external intercostals contract during inspiration and pull the ribs and sternum upward and outward, increasing the antero-posterior and transverse diameters of the chest. The internal intercostals contract during forceful expiration, pulling the ribs downward and inward to reduce thoracic volume and force extra air out.

During quiet (normal) expiration no muscle actively contracts to push air out. The diaphragm and external intercostals simply relax, and the stretched, elastic lung and thoracic tissues recoil to their original size. This recoil decreases thoracic volume and raises the pressure, so air leaves the lungs without muscular effort — hence 'passive.' Forceful expiration, by contrast, is active and uses the internal intercostal and abdominal muscles.

Forceful breathing is active in both directions. Forceful inspiration recruits accessory muscles such as the sternocleidomastoid and scalenes (in addition to the diaphragm and external intercostals) to lift the rib cage further. Forceful expiration uses the internal intercostal muscles and the abdominal muscles, which push the diaphragm upward and pull the ribs down, expelling the expiratory reserve volume.

Air moves down a pressure gradient. When thoracic volume increases, the pressure inside the alveoli (intra-pulmonary pressure) drops just below atmospheric pressure, so outside air flows in through the nose, trachea and bronchi into the alveoli. When thoracic volume decreases, the intra-pulmonary pressure rises just above atmospheric pressure and air flows back out along the same path. The lungs themselves have no muscles — they follow the chest wall because of pressure in the pleural cavity.