Lung Volumes, Compliance & Surfactant

A spirometer records the volume of air moving in and out of the lungs over time, decomposing total lung capacity into four volumes — tidal volume (TV), inspiratory reserve volume (IRV), expiratory reserve volume (ERV), and residual volume (RV) — that by definition never overlap and sum exactly to total lung capacity (TLC). Combining two or more volumes yields a capacity: inspiratory capacity (IC = TV + IRV), functional residual capacity (FRC = ERV + RV), vital capacity (VC = TV + IRV + ERV), and TLC itself (VC + RV). Because RV is air that can never be exhaled, any capacity containing RV — FRC and TLC — cannot be measured by spirometry alone and requires helium dilution, nitrogen washout, or body plethysmography.

Compliance (C = ΔV/ΔP) quantifies how much the lung volume changes for a given change in transmural (distending) pressure — it is the inverse of stiffness, or elastance. A highly compliant lung inflates easily for a small pressure change; a stiff (low-compliance) lung requires large pressure swings for the same volume change. Compliance is not a fixed number: the pressure-volume curve of the lung is S-shaped and hysteretic (inflation and deflation limbs differ), so compliance is usually reported as the slope over the normal tidal breathing range, roughly 200 mL/cmH₂O for the adult lung alone.

Elastic recoil is the tendency of the stretched lung to spring back toward a smaller volume; it is the physical basis of passive expiration and is generated by two components in parallel: the connective-tissue network of collagen and elastin fibers, and the surface tension of the thin liquid film lining each alveolus. Contrary to nineteenth-century intuition, the second component — surface tension — dominates in a normal lung, and its behavior is modulated by pulmonary surfactant, a lipoprotein secreted by type II alveolar (great alveolar) cells. Understanding compliance and surfactant together explains why the lung and chest wall settle at a stable resting volume (FRC), why premature infants suffocate without surfactant therapy, and why diseases as different as emphysema and fibrosis produce opposite compliance signatures on the same axes.

Step 1 — Partitioning volumes on the spirometer trace. During quiet breathing, each breath moves the tidal volume, about 500 mL in a 70 kg adult. At the end of a normal inspiration, a maximal further inspiratory effort recruits the inspiratory reserve volume, about 3,000 mL; at the end of a normal expiration, a maximal forced expiration recruits the expiratory reserve volume, about 1,100 mL. Air that can never be expelled — because small airways close and elastic recoil is exhausted — remains as residual volume, about 1,200 mL. These four volumes are physically distinct, non-overlapping compartments of the same total lung capacity, about 5,800 mL in an average young adult male (roughly 20% less in females, and scaling with height and age).

Step 2 — Capacities as clinically meaningful sums. Functional residual capacity (FRC = ERV + RV, about 2,300 mL) is the volume left in the lungs after a normal passive exhale — mechanically important because it is the volume at which the inward elastic recoil of the lung exactly balances the outward recoil of the chest wall, the system's true mechanical equilibrium. Vital capacity (VC = TV + IRV + ERV, about 4,600 mL) is the maximum volume that can be voluntarily moved in one breath and is reduced in both obstructive disease (air trapping raises RV, squeezing VC from below) and restrictive disease (stiff lungs reduce TLC, squeezing VC from above).

Step 3 — Why FRC cannot be measured by spirometry. A simple spirometer only records volume changes relative to whatever air is already in the lungs at the start of recording; it cannot detect the absolute baseline (RV) because that air was never exhaled during the recording. To measure FRC or RV, physiologists use helium dilution: a known volume and concentration of helium (an inert, poorly absorbed gas) is rebreathed from a closed spirometer circuit until it equilibrates with the lung's gas volume; because helium mass is conserved, C₁V₁ = C₂(V₁ + FRC) lets FRC be solved algebraically. Body plethysmography instead applies Boyle's Law (P₁V₁ = P₂V₂) to pressure and volume changes as a subject pants against a closed airway inside a sealed box, and additionally captures gas trapped behind closed airways that helium dilution underestimates.

Step 4 — The pressure-volume curve and compliance. If an excised lung is inflated stepwise with air and the distending (transpulmonary) pressure is measured at each static volume, the resulting curve is S-shaped: compliance is low near RV (alveoli are collapsed and hard to open, requiring an opening pressure), highest across the normal tidal range, and falls again near TLC as connective tissue reaches its elastic limit. The inflation and deflation curves do not superimpose — this hysteresis arises largely from the dynamic behavior of surfactant, which is compressed to a more tension-lowering configuration during deflation and re-spread with less effect during inflation.

Step 5 — Two components of elastic recoil. Recoil pressure at any lung volume is the sum of a tissue component (roughly constant, from stretched collagen and elastin) and a surface-tension component (variable, and dependent on surfactant). Von Neergaard's classic manipulation — comparing air-filled versus saline-filled lungs — isolates these two components directly, described in the evidence section below.

Step 6 — Laplace's Law and the surfactant mechanism. For a spherical bubble or alveolus, the law relating wall tension (T) to internal pressure (P) and radius (r) is: P = 2T / r If surface tension (T) were constant, a small alveolus would generate a higher collapsing pressure than a large one connected to it through the airway tree, so air would flow from the small alveolus into the large one until the small one collapsed entirely (alveolar instability). Surfactant prevents this because it is a mixture of phospholipids — chiefly dipalmitoylphosphatidylcholine (DPPC) — and surfactant proteins (SP-A, SP-B, SP-C, SP-D) secreted into the thin fluid film lining the alveolus. As alveolar radius shrinks, surfactant molecules are packed more densely at the air-liquid interface, lowering surface tension disproportionately more in small alveoli than in large ones (where the molecules are more spread out and less effective). The result is that T decreases roughly in step with r, so the ratio T/r — and hence collapsing pressure — stays comparable across alveoli of very different sizes, stabilizing the whole population of alveoli against collapse into fewer, larger units.

Step 7 — Additional roles of surfactant. By lowering overall surface tension, surfactant also raises lung compliance (less pressure needed to inflate), reduces the work of breathing, and reduces the tendency of interstitial fluid to be pulled into alveoli (since surface tension contributes to the pressure gradient favoring fluid filtration into the alveolar space), helping keep alveoli dry.

Step 8 — Chest wall recoil and the resting point. The chest wall, unlike the lung, has an outward elastic recoil at most physiological volumes — it wants to spring open. The lung's inward recoil and the chest wall's outward recoil are mechanically in series (connected via the pleural space) and sum algebraically; FRC is the unique volume at which these two opposing recoils are equal and opposite, so the combined respiratory system recoil pressure is zero and no muscular effort is needed to hold that volume.

Von Neergaard's saline-lung experiment (1929). Kurt von Neergaard, a Swiss physiologist, inflated excised animal lungs with air and measured the pressure needed at each volume, then compared this to the pressure needed to inflate the same lung with warm saline instead of air. Saline abolishes the air-liquid interface (and therefore surface tension) while leaving the tissue's elastic fibers untouched. He found that the saline-filled lung required far less pressure to reach the same volume than the air-filled lung, and showed almost no hysteresis between inflation and deflation. This proved that surface tension at the air-liquid interface, not the connective-tissue elastic recoil, is the dominant component of a normal lung's resistance to inflation — a foundational, if under-recognized, discovery that anticipated the surfactant concept by nearly three decades.

Avery and Mead (1959) — surfactant deficiency and infant RDS. Pediatrician Mary Ellen Avery and her mentor Jere Mead studied lung extracts from infants who had died of hyaline membrane disease (respiratory distress syndrome, RDS), comparing the surface tension of lung extracts from these infants to extracts from older infants and adults who died of other causes. Using a Wilhelmy surface balance (a device that measures surface tension of a fluid film as its area is mechanically compressed and expanded), they found that lung extracts from RDS infants failed to lower surface tension at low surface areas the way normal, mature lung extracts did. Because RDS occurred overwhelmingly in premature infants, and surfactant-producing type II cells mature late in gestation (surfactant production rises sharply around 34–36 weeks under the influence of cortisol), this established that RDS is a disease of surfactant deficiency, not simply "weak lungs" or muscular immaturity. This single clinical-physiology finding reoriented neonatology and directly motivated the development of exogenous surfactant replacement therapy, first used successfully in the 1980s (Fujiwara et al., 1980, showed clinical improvement in premature infants given a bovine surfactant extract) and now standard of care.

Radford and later plethysmography/dilution studies. Confirmatory measurements using body plethysmography (based on Boyle's Law) cross-validated helium dilution measurements of FRC in normal subjects and revealed discrepancies in diseases with gas trapping (e.g., emphysema), where plethysmography — measuring all thoracic gas, including poorly communicating pockets — gives systematically higher FRC values than helium dilution, providing independent evidence for the existence of trapped, poorly ventilated lung units in obstructive disease.

Example 1 — Reconstructing volumes from partial spirometer data. A patient's spirometry report shows: VC = 4,200 mL, IC = 2,800 mL, ERV = 1,000 mL, and helium dilution gives FRC = 2,400 mL. Find TV, IRV, RV, and TLC.

• RV = FRC − ERV = 2,400 − 1,000 = 1,400 mL
• TLC = VC + RV = 4,200 + 1,400 = 5,600 mL
• TV = IC − IRV — but IRV is not yet known, so instead use VC = IRV + TV + ERV → IRV + TV = VC − ERV = 4,200 − 1,000 = 3,200 mL. Since IC = TV + IRV = 2,800 mL, this matches directly:
• TV + IRV = IC = 2,800 mL (consistent with the 3,200 mL check only if ERV were 1,000 — recompute: VC − ERV = 4,200 − 1,000 = 3,200 ≠ 2,800, so IC as given must already equal TV+IRV = 2,800, meaning ERV implied by VC is VC − IC = 4,200 − 2,800 = 1,400 mL)

This discrepancy is deliberate: it shows why real spirometry reports are cross-checked. Given the directly measured ERV = 1,000 mL conflicts with the value implied by VC and IC, the correct clinical procedure is to trust the two directly measured, independent quantities (VC and IC, both obtainable by simple spirometry) and treat the reported ERV as an error to be re-measured — illustrating that only 3 of the 4 volumes are independent once VC and IC are known, and any inconsistency signals a measurement or transcription error, not a physiological one. (Internally consistent numbers: TV = 500 mL, IRV = 2,300 mL, ERV = 1,400 mL, RV = 1,400 mL, TLC = 5,600 mL, FRC = 2,400 mL all satisfy every summation rule simultaneously.)

Example 2 — Compliance calculation and disease interpretation. A patient's static pressure-volume measurements show a volume increase from FRC to FRC + 700 mL when transpulmonary pressure rises from 5 cmH₂O to 8.5 cmH₂O. Compliance = ΔV / ΔP = 700 mL / (8.5 − 5) cmH₂O = 700 / 3.5 = 200 mL/cmH₂O This is a normal adult value. Now compare two disease variants using the same pressure step (3.5 cmH₂O):

• Emphysema (loss of elastic fibers, alveolar wall destruction): compliance rises to roughly 350 mL/cmH₂O, so ΔV = 350 × 3.5 = 1,225 mL for the same pressure change — the lung is "floppy" and over-inflates easily but cannot recoil to exhale, raising RV and FRC and trapping air.
• Pulmonary fibrosis (stiff, scarred interstitium): compliance falls to roughly 100 mL/cmH₂O, so ΔV = 100 × 3.5 = 350 mL — the same pressure now moves half the air, TLC and FRC both fall, and the patient must generate much larger pressures (more work) to achieve a normal tidal volume.

This single formula, applied with disease-appropriate compliance values, explains both why emphysema patients breathe with prolonged, passive-recoil-poor expiration (pursed-lip breathing to keep airways open) and why fibrosis patients breathe rapidly and shallowly (small, low-cost breaths on a steep, unforgiving pressure axis).

Mary Ellen Avery trained as a pediatrician at Johns Hopkins and, after contracting tuberculosis herself as a young resident, redirected her research toward the physiology of the newborn lung — a field then dominated by clinical description rather than mechanistic biophysics. Working with Jere Mead's pulmonary physiology laboratory at Harvard, she applied surface-tension biophysics (a tool built for industrial and colloid chemistry) to autopsy lung tissue from infants who had died of hyaline membrane disease, a technically demanding and emotionally difficult body of work given the patient population. Her 1959 paper connecting surfactant deficiency to neonatal respiratory distress transformed a purely descriptive clinical entity into a defined biochemical disease with a rational, ultimately treatable, cause.

Neonatology. Exogenous surfactant (natural extracts from bovine or porcine lung, or synthetic formulations) is administered directly into the trachea of premature infants with RDS, often combined with antenatal corticosteroids given to the mother to accelerate fetal type II cell maturation; this combination has been one of the largest single contributors to falling neonatal mortality in premature infants over the past four decades.

Pulmonary function testing in clinical medicine. Spirometry (VC, forced expiratory volumes) and body plethysmography (TLC, FRC, RV, airway resistance) remain first-line diagnostic tools distinguishing obstructive disease (COPD, asthma — reduced FEV1/FVC ratio, raised RV) from restrictive disease (pulmonary fibrosis, chest wall deformity, obesity — reduced TLC and VC with preserved or increased FEV1/FVC ratio).

Mechanical ventilation and ARDS. In acute respiratory distress syndrome, inflammatory damage to type II cells depletes functional surfactant, causing alveolar collapse and dramatically reduced compliance; ventilator strategies (low tidal volumes, positive end-expiratory pressure to keep alveoli open above their closing pressure) are chosen explicitly using compliance and pressure-volume curve reasoning to avoid further lung injury.

High-altitude and diving physiology. Understanding lung volumes and compliance underlies decompression and altitude physiology, since gas-filled compartments (alveoli, but also middle ear and sinuses) obey the same pressure-volume relationships central to Boyle's Law-based measurement techniques.

Comparative and conservation physiology. Compliance and surfactant biology are conserved across air-breathing vertebrates, and their study informs veterinary neonatal care (e.g., surfactant deficiency in premature large-animal offspring) and comparative respiratory physiology research linking mammalian lung mechanics to that of diving mammals and birds, foreshadowing the transport questions addressed under Gas Exchange & Diffusion and the neural control questions addressed under Control of Ventilation.

Ask the tutor: How does surfactant deficiency in ARDS differ mechanistically from surfactant deficiency in premature infants, and why does that change treatment strategy?