Gas Exchange & Diffusion

Gas exchange refers to the net movement of O2 from alveolar air into pulmonary capillary blood, and of CO2 from blood into alveolar air, across the thin tissue barrier separating them. This barrier — the respiratory membrane — consists of a thin film of surfactant-coated fluid, the alveolar epithelium (mostly flattened type I cells), a fused basement membrane, and the capillary endothelium. In healthy lung tissue this composite barrier is only 0.2 to 0.6 micrometers thick over most of its area, thinner than a single red blood cell is wide. The direction and rate of gas movement is dictated entirely by partial pressure, not concentration in the everyday sense. Partial pressure (Px) is the pressure a gas would exert if it alone occupied the total volume of a gas mixture, and it equals total pressure times the fractional concentration of that gas. Each gas diffuses independently down its own partial-pressure gradient — O2 moves from alveolar gas (high PO2) into capillary blood (low PO2), while CO2 simultaneously moves from blood (high PCO2) into alveolar gas (low PCO2), even though both processes occur across the same membrane in the same instant. The quantitative description of this movement is Fick's law of diffusion, adapted for a biological membrane. It states that the volume of gas diffusing per unit time (V̇gas) depends on the membrane's surface area, a diffusion coefficient reflecting the gas's solubility and molecular weight, the thickness of the membrane, and the partial-pressure difference across it. Every anatomical and pathological feature of the lung that affects gas exchange — emphysema, fibrosis, edema, exercise — can be understood as altering one or more terms in this single equation.

Fick's law, stated precisely. For diffusion across a thin sheet of tissue: V̇gas = (A × D × ΔP) / T where A is the membrane surface area, T is membrane thickness, ΔP is the partial-pressure difference across the membrane, and D is a diffusion coefficient for that specific gas in that specific tissue. D itself is proportional to the gas's solubility in tissue fluid and inversely proportional to the square root of its molecular weight (Graham's law folded into Fick's law for biological media): D ∝ Solubility / √(Molecular weight). This solubility term explains a fact that surprises many students: CO2 diffuses through tissue about 20 times faster than O2, even though the two molecules are similar in molecular weight (44 vs. 32 g/mol, giving nearly identical √MW terms). CO2 is roughly 24 times more soluble in plasma and tissue fluid than O2. The huge solubility advantage dominates the nearly-equal molecular-weight term, so CO2's overall diffusion coefficient is about 20-fold higher. This is clinically important: diseases that thicken the membrane impair O2 diffusion long before they measurably impair CO2 diffusion, because CO2 has so much more diffusive reserve.

The pressure gradients driving each gas. Atmospheric air at sea level has a total pressure of 760 mmHg. After humidification in the airways (adding water vapor pressure of 47 mmHg at body temperature) and mixing with residual alveolar gas, alveolar gas has approximately:

Note the striking asymmetry: the O2 gradient (60 mmHg) is ten times steeper than the CO2 gradient (6 mmHg), yet CO2 transfer proceeds just as effectively because its diffusion coefficient is roughly 20-fold higher. The product of gradient and diffusion coefficient — not the gradient alone — determines actual transfer rate, and the two gases end up transferring comparable volumes per minute (about 250 mL/min O2 uptake versus about 200 mL/min CO2 output, giving a respiratory exchange ratio of about 0.8 at rest).

Step-by-step passage of an O2 molecule. 1. O2 dissolves into the thin alveolar fluid film (this step requires adequate Compliance & Surfactant to keep alveoli open and their surface film intact). 2. It diffuses through the alveolar type I epithelial cell, the fused basement membrane, and the capillary endothelial cell — a total path of 0.2–0.6 μm. 3. It dissolves in plasma, raising plasma PO2. 4. It diffuses into the red blood cell across the erythrocyte membrane. 5. It binds hemoglobin cooperatively, described by the Oxygen-Hemoglobin Dissociation Curve — this binding is critical because it keeps free dissolved PO2 low inside the red cell, maintaining a steep diffusion gradient from plasma into the cell even as total O2 content rises enormously (hemoglobin binding lets blood carry about 70-fold more O2 than physical solution alone would allow).

Transit time versus equilibration time. At rest, a red blood cell spends about 0.75 seconds transiting a pulmonary capillary (capillary blood volume ~70–100 mL divided by pulmonary blood flow ~5 L/min). Under normal conditions, PO2 in the capillary blood equilibrates with alveolar PO2 within about the first 0.25 seconds of transit — meaning there is roughly a three-fold diffusion reserve. This reserve is why healthy lungs tolerate substantial thickening of the membrane, moderate anemia, or the shortened transit time of vigorous exercise (transit time can fall to ~0.25 s at maximal cardiac output) without arterial hypoxemia. Disease erodes this reserve; severe fibrosis or edema can push equilibration time past the available transit time, producing hypoxemia that is worst during exercise, before it appears at rest.

Diffusing capacity (DL). Clinically, the lung's overall diffusion performance is summarized as the diffusing capacity, DL = V̇gas / ΔP, essentially a rearrangement of Fick's law that folds A, D, and T into one empirically measurable constant for the whole lung. DLCO (using carbon monoxide as the test gas) is the standard clinical measurement, because CO binds hemoglobin so avidly that its capillary partial pressure stays near zero, making the test purely membrane-limited rather than perfusion-limited.

Establishing that gas exchange is passive diffusion, not active secretion. In the early twentieth century, physiologists genuinely debated whether the lung actively secreted oxygen into blood — an idea championed by the Danish physiologist August Krogh's contemporaries and, notably, by John Scott Haldane, who found that in some fish (like the eel) with a swim bladder, O2 could be pumped in against a pressure gradient by a countercurrent "gas gland" mechanism, and mistakenly generalized this to mammalian lungs. August Krogh and his wife Marie Krogh settled the question for mammals with careful measurements published in 1910. Using a specially designed apparatus, they measured the partial pressures of O2 and CO2 in alveolar gas and compared them to the partial pressures in blood entering and leaving the pulmonary capillaries (inferred from blood gas sampling and tonometry). They found that alveolar PO2 was always equal to or higher than capillary PO2, consistent with passive diffusion down a gradient, and never found evidence of blood O2 partial pressure exceeding alveolar partial pressure, which active secretion would require. Marie Krogh also developed a single-breath method for measuring the lung's diffusing capacity for carbon monoxide, the direct ancestor of the modern clinical DLCO test still used today. This work — later reinforced by more precise measurements in the mid-20th century using mass spectrometry and rapid blood-gas analysis — established diffusion, quantified by Fick's law, as the correct and sufficient mechanism, without requiring any active transport step. August Krogh received the Nobel Prize in Physiology or Medicine in 1920, primarily for his related discovery of capillary regulation in muscle, but his pulmonary diffusion work is what directly underlies this chapter's mechanism.

Measuring diffusion versus perfusion limitation. A second classic line of evidence, developed substantially by physiologists including Johannes Piiper and Peter Scheid in the mid-20th century using isolated, perfused animal lungs with precisely controlled blood flow and inspired gas mixtures, distinguished diffusion-limited from perfusion-limited gas exchange. Using inert, low-solubility gases (analogous to CO) versus highly soluble gases, they showed experimentally that:

• CO never reaches equilibrium during normal capillary transit because it is bound so tightly and rapidly by hemoglobin that plasma PCO stays near zero throughout transit — so its uptake is limited purely by the membrane's diffusing capacity, regardless of how much blood flows through (diffusion-limited).
• O2 (and N2O, a gas used experimentally because it does not bind hemoglobin) equilibrates with alveolar gas within the first fraction of the capillary's length under normal resting conditions — so further transfer of that gas is limited only by how much new, unequilibrated blood arrives, i.e., by blood flow (perfusion-limited).

This experimental distinction, confirmed repeatedly with varying inspired O2 tensions, exercise workloads, and artificially thickened membranes in animal and human studies, explains why O2 exchange is normally perfusion-limited (with large diffusion reserve) but becomes diffusion-limited in disease or extreme exercise — exactly the reserve margin depicted in the equilibration figure above.

Example 1 — Quantifying diffusing capacity in disease. A patient with idiopathic pulmonary fibrosis has membrane thickening. Diffusing capacity for CO (DLCO) is measured at rest:

• Healthy reference DLCO ≈ 25 mL CO/min/mmHg
• Patient's measured DLCO = 10 mL CO/min/mmHg

Using DL = V̇gas/ΔP, and Fick's law logic (DL ∝ A·D/T), a DLCO reduced to 40% of predicted indicates a proportional loss of effective gas transfer capacity — from fibrotic thickening (↑T) and destruction of alveolar-capillary surface area (↓A) as fibrotic tissue replaces normal septa. Applying this to O2: at rest, transit time is long enough (0.75 s) that even a 60% reduction in DL may still allow near-complete equilibration, so resting PaO2 might look only mildly low. But if we halve the available transit time during exercise (to ~0.35 s, from increased cardiac output), the reduced DL now cannot achieve full equilibration in the time available — capillary blood leaves before reaching alveolar PO2. This is precisely why patients with early interstitial lung disease often have a normal resting PaO2 but a striking, symptomatic drop in oxygen saturation during exercise — a clinical pattern directly predicted by Fick's law and the transit-time/equilibration relationship, and it is the classic diagnostic clue that prompts exercise oximetry testing.

Example 2 — CO2 retention versus O2 desaturation in emphysema. A patient with severe emphysema has lost alveolar surface area (A) from destroyed septa, reducing DL for both gases roughly proportionally. Because DL(CO2) at baseline is about 20-fold higher than DL(O2) (from the solubility term in Fick's law), even after a 5-fold reduction in overall DL from emphysema:

• Effective DL(O2) falls from a comfortable reserve to a value close to the minimum needed for equilibration → hypoxemia develops readily.
• Effective DL(CO2), starting 20-fold higher, still remains far above the minimum needed for equilibration → CO2 elimination across the membrane itself stays adequate.

This is why, mechanistically, pure diffusion impairment causes hypoxemia far more readily than hypercapnia — clinically, CO2 retention in advanced COPD arises mainly from a different problem (alveolar hypoventilation and ventilation-perfusion mismatch, not diffusion failure per se), a distinction that becomes important when this topic connects forward to Ventilation-Perfusion Matching and Control of Ventilation.

Measuring DLCO is now a routine pulmonary function test used to diagnose and stage interstitial lung diseases (pulmonary fibrosis, sarcoidosis), emphysema, pulmonary vascular disease, and to monitor drug toxicity (certain chemotherapy agents and amiodarone characteristically reduce DLCO before other tests change). In critical care, understanding diffusion versus perfusion limitation guides the interpretation of arterial blood gases in ARDS and pulmonary edema, where flooded alveoli both thicken the effective diffusion path and destroy usable surface area. In high-altitude and exercise physiology, the diffusion-reserve concept explains why elite endurance athletes — who achieve very high cardiac outputs and correspondingly short capillary transit times — can develop exercise-induced arterial hypoxemia even with structurally normal lungs, motivating research on pulmonary capillary blood volume and recruitment. In neonatology, immature, thick fetal-type alveolar septa and insufficient surfactant (linking directly back to Compliance & Surfactant) both reduce effective DL and increase membrane thickness, contributing to neonatal respiratory distress syndrome. Aerospace and diving medicine apply the same Fick's-law reasoning to predict gas uptake and elimination under altered ambient pressures, informing decompression and hyperbaric oxygen protocols.

Ask the tutor: How does ventilation-perfusion mismatching change the effective partial-pressure gradients that drive diffusion at the alveolar-capillary membrane?