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The Pressures That Move Air

Why don't the lungs collapse to a tiny ball? Why does a punctured chest stop a lung working? The answers live in two pressures — intrapleural and transpulmonary — and the thin film of fluid that glues lung to chest wall.

Two surfaces, one thin film

Each lung is wrapped in a slippery double membrane called the pleura: one layer hugs the lung, the other lines the inside of the chest wall, and between them is a microscopically thin film of fluid. That fluid does the same thing two wet glass slides do when you try to pull them apart — it lets them slide freely but resists being separated. So the lung is free to glide as you breathe, yet it is firmly coupled to the chest wall. Wherever the chest wall goes, the lung is dragged along.

Two opposite springs meet at this film. The lung, full of stretched elastic tissue, constantly wants to shrink inward; the chest wall, like a bent bow, wants to spring outward. Each tugs on the fluid between them. Because liquid cannot be stretched, that tug-of-war pulls the pressure in the pleural space slightly below atmospheric. We call that the intrapleural pressure, and at rest it sits around −5 cm H₂O.

Transpulmonary pressure: what keeps the lung open

The single most important number for whether a lung is open or collapsed is the transpulmonary pressure: the pressure inside the air sacs minus the pressure just outside the lung, in the pleural space. As long as this difference is positive — pushing out a little harder than the pleura pulls in — the lung stays inflated. At the resting point, FRC, the gentle positive transpulmonary pressure exactly offsets the lung's elastic recoil, so nothing moves and you sit there comfortably between breaths.

Breathing in is simply a way of making this number bigger. When the diaphragm drops and the chest expands, the pleural pressure becomes even more negative (say −8 instead of −5). The air sacs are now pulled open more strongly, the transpulmonary pressure rises, the lung stretches, and air flows in. Breathing out reverses it: pleural pressure climbs back toward −5, the transpulmonary pressure falls, and the elastic lung shrinks again.

Transpulmonary pressure  =  Alveolar pressure  −  Intrapleural pressure

Resting (FRC), no airflow:
    P_alveolar = 0   (open to atmosphere, nothing flowing)
    P_pleural  = -5
    P_transpulmonary = 0 - (-5) = +5  cmH2O   -> lung held open

Mid-inspiration, chest expanding:
    P_alveolar = -1  (briefly below atmosphere, air rushing in)
    P_pleural  = -7
    P_transpulmonary = -1 - (-7) = +6  cmH2O  -> lung stretched larger
A positive transpulmonary pressure keeps the lung inflated; breathing in raises it, breathing out lowers it.

When the seal breaks

This whole system relies on the pleural space being sealed. If air gets in — through a hole in the chest wall or a tear in the lung — the intrapleural pressure jumps up to atmospheric, the negative pull that was holding the lung open vanishes, and the transpulmonary pressure collapses toward zero. The lung's own elastic recoil then crumples it inward. That is a pneumothorax: a lung that has lost its grip on the chest wall.