The pleural space
Go to the experiment ↓The open cylinder had a piston you could move. A lung has no piston. It is a soft elastic bag that would collapse on its own, hanging inside a chest that would spring outward on its own. Between them is a thin sealed space, the pleural space, and that is where the pull comes from.
Breathing muscles enlarge the chest, which lowers the pressure in the pleural space. A lower pressure around the lung, with the airway open to the room, means a bigger pressure difference across the lung wall. The lung expands, alveolar pressure dips, and air flows in.
PL = Palv − Ppl
Transpulmonary pressure, the difference across the lung wall, is what holds the lung open. Ppl is the pressure in the pleural space and sits below zero even at rest.
Lung in a jar
A lung hangs inside a rigid glass jar. Its trachea passes through the bung at the top and opens to the room. The floor of the jar is a rubber diaphragm. The muscle effort dial pulls that diaphragm down, and the model reports the pressure at the airway opening, inside the lung, inside the jar, and in the room.
Before you start: Hold a breath in with your mouth open. What is the pressure inside your lungs? What is the pressure around them?
- Hold an effort
- Leave Breathe off and raise Muscle effort to 5. Watch the diaphragm bow, the lung grow, and pleural pressure fall. Wait for the flow to stop, then read alveolar pressure and pleural pressure.
- Breathe
- Press Breathe. Follow one cycle on the chart: pleural pressure swings, alveolar pressure dips below zero and then rises above it, and air moves in and then out.
- Read the four gradients
- Below the dial are four differences between the four pressures. Find which one is zero whenever flow stops, and which one never reaches zero.
About this model. A rigid jar with a diaphragm floor stands in for the chest wall, with linear compliance for both lung and wall, one airway resistance, and no gas exchange. Lung expansion and diaphragm travel are drawn larger than life for a quiet breath and compressed for big efforts, so the lung stays inside the jar. The lungs model is illustrative anatomy, not patient imaging.
What happened?
At rest, pleural pressure is about −5 cmH₂O while the lung sits at zero, so the lung wall already carries 5 cmH₂O of transpulmonary pressure holding it open. A held effort of 5 cmH₂O pulls pleural pressure to about −7.5. Alveolar pressure dips only while air is moving and returns to zero once the lung has filled, because the airway is open to the room. The lung grows by about 500 mL, the same breath as the open cylinder, but this time nothing touched the lung. Only the pressure around it changed.
The lung is pulled open from outside, never pushed from inside.
Check your understanding
Flow has stopped and the lung is holding 500 mL above rest. Alveolar pressure reads zero. If there is no pressure difference between the lung and the room, what is keeping the extra air in?
Reveal explanation
The transpulmonary pressure. Alveolar pressure equals the room, so the transairway gradient is zero and no air moves. But pleural pressure is still −7.5 cmH₂O, so the difference across the lung wall is 7.5 cmH₂O, and that is what holds the lung at its larger volume. Let the muscles relax, pleural pressure rises back toward −5, the lung recoils, alveolar pressure goes briefly positive, and the air leaves.
Further reading: OpenStax, The Process of Breathing.