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How do the diaphragm and ribs support the lungs during breathing?

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Illustration showing different animals and their respiratory organs: a spider with book lungs, a chicken with lungs, a sponge with labeled inflow and outflow pores, a fish with gills, and a caterpillar with spiracles. — Respiratory system Different methods of respiration in animals.

The diaphragm and the rib cage work together as a mechanical pump to facilitate breathing by altering the volume and pressure within the thoracic cavity.1

Inspiration (Breathing In)

During inspiration, the diaphragm—the primary muscle of respiration—contracts and moves downward, which increases the vertical height of the thoracic cavity. Simultaneously, the intercostal muscles contract, causing the ribs to rotate upward and outward. This expansion of the semirigid rib cage increases the overall circumference of the chest. These combined movements lower the internal pressure within the lungs relative to the outside air, forcing air to flow into the lungs to restore equilibrium.234

Expiration (Breathing Out)

Expiration is primarily a passive process. It occurs when the inspiratory muscles relax, allowing the natural elastic recoil of the lung tissue and the thoracic cage to decrease the volume of the chest cavity. During labored breathing or exercise, the internal intercostal muscles and abdominal muscles may actively contract to further depress the rib cage and assist in expelling air.243