Questions: Lung Compliance and Elastic Recoil

Question 1

A patient with emphysema has extensive destruction of pulmonary elastic fibers. A medical student predicts this patient will have very low lung compliance and struggle to inhale. Is the student correct?

• A. Yes — elastic fiber destruction makes the lungs rigid and very difficult to inflate
• B. No — elastic fiber destruction actually increases compliance (easier to inflate) but eliminates elastic recoil, making expiration inefficient and trapping air
• C. Yes — but only during expiration, because elastic fibers are only engaged when the lung deflates
• D. No — elastic fibers have no role in compliance; only surfactant determines lung stiffness

Answer: (B)

This is the key clinical inversion: compliance and elastic recoil are opposite sides of the same mechanism. In emphysema, destruction of elastin fibers removes the elastic recoil that normally drives expiration. The lungs become highly compliant — paradoxically easy to inflate — but they cannot deflate efficiently on their own, trapping air and leading to hyperinflation. Reduced compliance (stiff lungs) is characteristic of restrictive diseases like pulmonary fibrosis, not emphysema. The student confused the two.

Question 2

Pulmonary surfactant is most critical for preventing alveolar collapse at which point in the breathing cycle?

• A. During peak inspiration, when alveoli are at maximum volume and surface tension is highest
• B. During expiration, when alveoli shrink, surfactant molecules compress together and reduce surface tension most — counteracting the Laplace pressure that would otherwise collapse the alveolus
• C. At high altitude, where reduced atmospheric pressure destabilizes small alveoli
• D. During strenuous exercise, when increased respiratory rate produces more turbulent airflow

Answer: (B)

Surfactant works by a concentration-dependent mechanism. As alveoli shrink during expiration, surfactant molecules are crowded together at the air-liquid interface, lowering surface tension to near zero — dramatically reducing the collapsing pressure (P = 2T/r). This prevents the catastrophic collapse that would otherwise occur in small alveoli. During inspiration, as alveoli expand, surfactant molecules spread apart and surface tension rises slightly, resisting overexpansion. This dynamic behavior is the key — surfactant stabilizes alveoli at both ends of the volume range.

Question 3

According to the Law of Laplace (P = 2T/r), a smaller alveolus generates higher collapsing pressure than a larger alveolus at the same surface tension.

• T. True
• F. False

Answer: True

The Law of Laplace states that the collapsing pressure of a spherical surface is P = 2T/r. With a smaller radius r, the same surface tension T generates a higher inward pressure P. Without surfactant, smaller alveoli would tend to collapse into larger ones because they experience greater collapsing pressure — an unstable situation. Surfactant counters this by reducing surface tension more in smaller (more compressed) alveoli, equalizing pressures across alveoli of different sizes.

Question 4

High lung compliance is generally beneficial because it means less work is required to expand the lungs during each breath.

• T. True
• F. False

Answer: False

High compliance is beneficial when it reflects normal elastic properties — easy inflation with intact elastic recoil to drive expiration. But pathologically high compliance, as in emphysema, is not beneficial: the loss of elastic fibers means the lungs cannot recoil to force air out, trapping stale air and reducing ventilation efficiency. Meanwhile, pathologically low compliance (stiff lungs, as in respiratory distress syndrome or pulmonary fibrosis) forces the respiratory muscles to work much harder for each breath. Optimal compliance is neither too high nor too low, balancing ease of inflation with adequate elastic recoil.

Question 5

Why does the absence of pulmonary surfactant — as in neonatal respiratory distress syndrome — make breathing so difficult?

Model answer: Without surfactant, the air-liquid interface of each alveolus is dominated by the full surface tension of water. By the Law of Laplace (P = 2T/r), this generates large collapsing pressures, especially in small alveoli. The total surface tension across 300 million alveoli makes the lungs extremely stiff (low compliance), requiring enormous inspiratory muscle effort to expand them. Worse, without surfactant's concentration-dependent tension reduction during expiration, alveoli collapse completely between breaths (atelectasis), so each breath must reinflate collapsed alveoli from scratch. Premature infants exhaust themselves within hours trying to breathe against this resistance.

The two-part answer covers both why each breath is hard (low compliance) and why the problem resets with every breath (atelectasis between breaths). Surfactant treatment (exogenous surfactant replacement) is the definitive therapy — within hours of administration, compliance improves dramatically and infants can breathe with normal effort.