How Does Diaphragm Contraction Create Inhalation?

The diaphragm is a dome-shaped sheet of skeletal muscle located below the lungs and separating the thoracic cavity from the abdominal cavity. It plays a major role in breathing, especially during inhalation.

When the diaphragm contracts, it moves downward and flattens, increasing the vertical space of the thoracic cavity. This expansion of the chest cavity reduces the pressure inside the lungs (intrapulmonary pressure) compared to the outside atmospheric pressure.

Because air moves from a region of higher pressure to lower pressure, air flows into the lungs through the nose or mouth to equalize the pressure difference — this process is called inhalation (inspiration).

Simultaneously, the external intercostal muscles between the ribs contract, lifting the rib cage upward and outward, further expanding the chest cavity. This coordinated action ensures that enough air enters the lungs for gas exchange.

When the diaphragm relaxes, it moves upward, decreasing thoracic volume and increasing pressure, forcing air out — called exhalation (expiration). Thus, diaphragm contraction creates negative pressure in the thoracic cavity, allowing air to enter the lungs — a key mechanism of normal breathing.