Compliance
Go to the experiment ↓The last lesson showed that a pressure difference across the lung wall holds the lung open. This one asks how much. Two lungs can sit at the same transpulmonary pressure and hold very different volumes, because their walls stretch differently.
Compliance is the volume you gain for each unit of pressure: the slope of the pressure-volume curve. A stiff lung has low compliance and needs more pressure for the same breath. A floppy lung has high compliance and gives a bigger breath for the same pressure.
C = ΔV / ΔPL
Volume change per unit change in transpulmonary pressure. A healthy adult lung is about 200 mL per cmH₂O. The chest wall has its own compliance, and the two add in series, which is why the effort dial buys less than the lung alone would.
Three lungs in a jar
The same jar as before, with a choice of lung: stiff, normal, or floppy. The muscle effort dial is unchanged. The chart now plots lung volume against transpulmonary pressure, with the live point riding on the line for the lung you chose.
Before you start: You hold the same 5 cmH₂O effort with a stiff lung and with a floppy one. Which breath is bigger, and by roughly how much?
- Measure normal
- With the normal lung, hold an effort of 5 and wait for flow to stop. Read the volume above FRC and the transpulmonary pressure. Their ratio is the compliance.
- Swap the lung
- Choose Stiff and hold 5 again. Then Floppy. Watch the point slide onto a different line and the breath change size at the same effort.
- Match the breath
- With the stiff lung, raise the effort until you get 500 mL back. Read pleural pressure. That extra pull is the cost of a stiff lung, every breath.
About this model. Compliance is linear here, so the pressure-volume curve is a straight line. Real lungs stiffen as they fill, which bends the curve over at high volumes, and the curve during deflation does not retrace the one during inflation. Chest wall compliance is held fixed at 200 mL per cmH₂O.
What happened?
A held effort of 5 cmH₂O gives about 500 mL with the normal lung, roughly 290 mL with the stiff lung, and about 600 mL with the floppy one. The transpulmonary pressure needed for 500 mL is 2.5 cmH₂O above rest in the normal lung and over 6 in the stiff one, so the stiff lung pulls pleural pressure much further below zero for the same breath. The chest wall halves what the effort buys in every case, because its compliance sits in series with the lung's.
Compliance decides how much breath a pressure buys. Stiff lungs cost more pull.
Check your understanding
The floppy lung takes a bigger breath for the same effort. So why is emphysema a disease rather than an advantage?
Reveal explanation
Because compliance is only half of the picture. A floppy lung has lost its elastic recoil, and recoil is what pushes air out again and what holds small airways open during expiration. The floppy lung fills easily but empties poorly, traps air, and sits at a larger resting volume where the muscles work at a disadvantage. The next lesson, on resistance and time constants, shows the emptying side.
Further reading: OpenStax, The Process of Breathing.