The diaphragm is the main muscle of breathing. When it contracts, it flattens and moves downward, and the external intercostal muscles lift the ribs outward — together this increases thoracic volume. Because the lungs are sealed inside the pleural cavity and follow the chest wall, lung volume increases too.
By Boyle's law (P·V = constant, for a fixed amount of gas at constant temperature), expanding the volume the air inside the alveoli occupies drops its pressure:
P_alv · V_lung = P_atm · V_FRC
→ P_alv drops below P_atm as V_lung rises above V_FRC
With alveolar pressure now below atmospheric pressure outside the body, air flows in down the pressure gradient — through the trachea, bronchi, bronchioles, and into the alveoli — until pressures equalize at peak inspiration.
The pressure inside the pleural space itself (intrapleural pressure, normally around −5 cmH₂O at rest) becomes even more negative during inspiration, because it must also balance the lung's elastic recoil pulling inward as it stretches:
P_pl ≈ P_alv − P_transpulmonary, P_transpulmonary = (V − V_FRC) / C_lung
When the diaphragm relaxes, it domes back upward, thoracic volume shrinks, alveolar pressure rises above atmospheric, and air flows out — quiet exhalation needs no muscular effort at all, just elastic recoil.
At the alveoli, O₂ diffuses down its concentration gradient into the surrounding capillary blood while CO₂ diffuses the opposite way, out into the alveolar air to be exhaled — the readout above scales this gas exchange with minute ventilation (tidal volume × rate), the real physiological driver of how much O₂ reaches the blood per minute.
- Rate — cycles more breaths per minute; raises minute ventilation and gas exchange.
- Depth (tidal volume) — how much extra air moves per breath; deeper breaths swing pressures further below/above atmospheric.
Simplification: real airway resistance, dead space, and non-linear compliance are omitted — this model isolates the volume ↔ pressure ↔ airflow relationship that Boyle's law predicts, not full pulmonary mechanics.