Lung Mechanics: Compliance, Surface Tension and the Work of Breathing
Filling the lungs means overcoming both the stretch of the tissue and the surface tension of the film lining each alveolus. Surfactant lowers that tension more in small alveoli than in large ones, which stops the small ones emptying into the big ones.
What a learner can do afterwards
- Explains why surface tension would collapse small alveoli without surfactant.
- Reads a pressure-volume loop as compliance and as work done.
- Predicts the effect of stiffer lung tissue on the effort of breathing.
1 · Read
As with blood, air flows from high pressure to low. Muscles widen the cavity, pressure falls below the outside air, and air flows in. Relaxing reverses the change and pushes air out, so muscles pay for the in breath while recoil refunds the out breath.
The lungs pack millions of tiny sacs, each wrapped in capillaries with only a thin wall between air and blood. A soapy film called surfactant coats the sacs and stops the smallest from collapsing, since small bubbles face the highest collapsing pressure. Without that film, every breath would have to pry shut sacs back open.
Plotting pressure against volume shows compliance and work together. Compliance is how easily the lung stretches per push, read as the slope: steep means stretchy, flat means stiff. The area inside the loop is the work of the breath, so stiffer tissue needs larger pressure swings and tires patients faster.
For any breathing question, ask which load grew: stretch of tissue or tension of the film, then pick the fix that lightens it.
Muscles set the pressure, surfactant keeps small sacs open, and stiffness sets the price.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.