Inhalation

Inhalation is the inward movement of respiratory gas into the lungs. During ordinary spontaneous breathing, contraction of the respiratory diaphragm and contribution from external intercostal muscles expand the thoracic space; the resulting pressure relationships permit inward flow.^[ref] Positive-pressure ventilation can instead assist inward flow by raising pressure at the airway opening.

The flow-driving difference is between airway-opening pressure and alveolar pressure. In a simple resistive model,where $R$ is airway resistance. For constructed gauge pressures $p_{\mathrm{opening}}=0$ and $p_{\mathrm{alv}}=-2$ cm H${}_2$O, with $R=4$ cm H${}_2$O·s/L, inward flow is 0.5 L/s. These are illustrative pressures, not a measurement or a prescribed breathing pattern.

Transpulmonary pressure answers a separate question:with $p_{\mathrm{pl}}$ the pleural pressure. If the same example has $p_{\mathrm{pl}}=-7$ cm H${}_2$O, the distending difference is 5 cm H${}_2$O. At a later no-flow instant, $p_{\mathrm{alv}}$ can equal atmospheric pressure while $p_{\mathrm{pl}}$ remains negative. The expanded lung can then have positive transpulmonary pressure without continued inward airflow.

It is consequently misleading to use any single negative intrathoracic pressure as a synonym for inspiration. Pleural pressure, alveolar pressure, the applied airway pressure and volumetric flow rate refer to different parts of the mechanical system. Inhalation names the flow phase; the relevant pressure differences explain how it is produced.