= Inhalation
\b[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.<How the Lungs Work: What Breathing Does for the Body by the National Heart, Lung, and Blood Institute>[Breathing in and breathing out]{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,
$$
Q=\frac{p_{\rm opening}-p_{\rm alv}}R,
$$
where $R$ is <airway resistance>. For constructed gauge pressures $p_{\rm opening}=0$ and $p_{\rm alv}=-2$ 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:
$$
p_L=p_{\rm alv}-p_{\rm pl},
$$
with $p_{\rm pl}$ the <pleural pressure>. If the same example has $p_{\rm pl}=-7$ cm H$_2$O, the distending difference is 5 cm H$_2$O. At a later no-flow instant, $p_{\rm alv}$ can equal atmospheric pressure while $p_{\rm pl}$ 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.
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