intrapulmonic pressure

Intrapulmonic pressure refers to the air pressure within the lungs' alveolar spaces. This pressure fluctuates relative to atmospheric pressure, driving the mechanics of breathing.

Air enters the lungs (inspiration) when intrapulmonic pressure falls below atmospheric pressure. Conversely, air is expelled (expiration) when intrapulmonic pressure rises above atmospheric pressure. These pressure changes are caused by the expansion and contraction of the chest and abdominal muscles, which alter lung volume.

The lung-chest system acts like two opposing springs. The lungs tend to collapse, while the chest wall tends to expand. A negative pressure in the pleural space (between the lung and chest wall) is necessary to keep the lungs inflated. This pressure becomes more negative as lung volume increases during inspiration. The energy for breathing is primarily used to stretch this system and facilitate airflow.12