Compliance: how easily the lung fills
Compliance answers a single question: for a given push of pressure, how much does the lung volume change? A high-compliance lung is floppy and fills easily, like a worn-out balloon; a low-compliance lung is stiff and resists, like a brand-new one. The formula is simply the change in volume divided by the change in transpulmonary pressure. Elastance is just the flip side of the same coin — how strongly the lung springs back — so a stiff lung has low compliance and high elastance.
Compliance C = ΔVolume / ΔPressure (units: mL per cmH2O)
Elastance E = 1 / C (stiff lung -> high E)
Healthy lung: add 5 cmH2O of transpulmonary pressure -> +1000 mL
C = 1000 / 5 = 200 mL/cmH2O (easy to fill)
Fibrotic lung: same 5 cmH2O of pressure -> +250 mL
C = 250 / 5 = 50 mL/cmH2O (stiff, hard to fill)The hidden enemy: surface tension
Here is something surprising: most of the lung's recoil does not come from elastic fibres at all. It comes from water. Every alveolus is lined with a wet film, and water molecules cling to each other, creating surface tension that pulls the curved surface inward — exactly the force that makes a soap bubble try to shrink. With hundreds of millions of tiny wet spheres, this inward pull is enormous, and it fights every breath you take.
There is a deeper danger too. The law of physics governing bubbles says that a smaller sphere generates a higher pressure than a larger one at the same tension. So a small alveolus would tend to empty itself into a neighbouring larger one, deflating and collapsing. Left unchecked, the lung would become a chaos of a few giant sacs and many collapsed ones — useless for gas exchange.
Surfactant to the rescue
The lung's answer is surfactant, a soapy mix of fats and proteins made by the type II pneumocytes that dot the alveolar lining. Surfactant slips between the water molecules and breaks up their grip, dramatically lowering surface tension. The effect is twofold: it makes the lung far easier to inflate (raising compliance), and it cleverly self-adjusts. When an alveolus shrinks, the surfactant crowds together and lowers tension even more in the small sac, so small and large alveoli end up balanced rather than collapsing into one another.