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How do the ribs and thoracic muscles contribute to breathing?

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Medical illustration of the human chest showing the lungs, diaphragm, ribs, trachea, bronchus, and bronchioles, with one lung cut away to reveal internal structures. — human respiratory system The lungs serve as the gas-exchanging organ for the process of respiration.

The ribs and thoracic muscles are essential components of the respiratory system, working together to facilitate the movement of air into and out of the lungs.

The rib cage, a semirigid but expansile structure, protects the lungs and provides a framework for the muscles of respiration. During inspiration, the intercostal muscles and other accessory muscles—such as the scalene and sternocleidomastoid—contract to expand the internal space of the thorax. This expansion reduces the internal pressure within the lungs below that of the atmosphere, causing air to flow inward.123

While the diaphragm is the primary muscle of respiration, these thoracic muscles assist and augment its action. During ordinary breathing, expiration is typically a passive process driven by the elastic recoil of the lungs and the thoracic cage. However, during forceful expiration, the contraction of abdominal muscles and expiratory muscles of the chest can further reduce lung volume by increasing pressure and displacing the diaphragm upward.342