transpulmonary pressure
Transpulmonary pressure is the net pressure pushing outward on the lung wall — the difference between the pressure inside the alveoli and the pressure just outside the lung in the pleural space. It is the true distending force that decides how inflated the lung is.
Think of the lung as a balloon inside a sealed box. What stretches the balloon is not the inside pressure alone but the difference between inside and outside. Likewise, the lung inflates when alveolar pressure exceeds pleural pressure; that gap, the transpulmonary pressure, is always positive in normal breathing, which is why the lung stays open.
It rises during inspiration (the chest enlarges, pleural pressure drops more negative, the gap widens, the lung expands) and falls during expiration. Its relationship to the resulting volume change is exactly what compliance measures.
The concept matters greatly in mechanical ventilation. The harm of high airway pressures depends on the transpulmonary pressure, not the airway pressure alone — a stiff chest wall can absorb much of the pressure, sparing the lung. Estimating it (via an oesophageal balloon that approximates pleural pressure) can guide safer ventilator settings.
In a very obese patient on a ventilator, high airway pressures may look alarming, yet a measured oesophageal pressure can show the transpulmonary pressure is modest because the heavy chest wall absorbs much of it.
Chest-wall stiffness separates airway from transpulmonary pressure.
If transpulmonary pressure ever falls to zero or below — as when air enters the pleural space in a pneumothorax and abolishes the negative pleural pressure — the lung loses its distending force and collapses.