Pulmonary gas exchange
Also known as: external respiration, diffusion, Fick's law, Dalton's law, Henry's law, partial pressure, alveolar gas equation, A–a gradient, DLCO
Passive diffusion of O2 from alveolar gas into pulmonary capillary blood and of CO2 in the opposite direction, each down its partial-pressure gradient across the respiratory membrane.
Overview
Dalton's law: each gas in a mixture exerts a partial pressure proportional to its fraction. Henry's law: the amount of gas dissolving in a liquid is proportional to its partial pressure and solubility. Fick's law: rate of diffusion ∝ area × diffusion constant × ΔP / thickness, where the diffusion constant ∝ solubility/√(molecular weight). CO2 is ≈24 times more soluble than O2 and so diffuses ≈20 times faster despite its smaller gradient.
Partial pressures (mmHg, sea level): inspired dry air PO2 ≈160; humidified tracheal air ≈150 (PH2O 47); alveolar PAO2 ≈100–104, PACO2 ≈40; mixed venous PvO2 ≈40, PvCO2 ≈45–46; arterial PaO2 ≈95–100 (lower with age), PaCO2 ≈40. The alveolar gas equation PAO2 = FIO2 × (PB − 47) − PaCO2/0.8 predicts PAO2; the A–a gradient (PAO2 − PaO2) is normally ≈5–15 mmHg (≈ age/4 + 4), owing to a small physiological shunt (bronchial and Thebesian veins) and V/Q scatter.
Red cells take ≈0.75 s to traverse a pulmonary capillary at rest but reach equilibrium within ≈0.25 s, so O2 uptake is normally perfusion-limited (more blood flow, more uptake). It becomes diffusion-limited when the membrane is thickened, at altitude (smaller gradient) and in heavy exercise (shorter transit). Internal respiration — the reverse exchange in systemic capillaries — follows the same laws.
- Location
Alveolar–capillary interface (external respiration); systemic capillaries (internal respiration).
Function
- Oxygenates blood (≈250 mL O2/min at rest).
- Eliminates CO2 (≈200 mL/min at rest).
Clinical significance
Five causes of hypoxaemia
(1) Low inspired PO2 (altitude) — normal A–a gradient; (2) hypoventilation (opioids, neuromuscular disease) — ↑PaCO2, normal A–a gradient; (3) diffusion impairment (fibrosis) — worse on exercise; (4) V/Q mismatch (asthma, COPD, PE) — commonest, corrects with O2; (5) shunt (pneumonia, atelectasis, ARDS, intracardiac) — corrects poorly with 100% O2. Causes 3–5 widen the A–a gradient.
Diffusing capacity (DLCO)
Measured with a single breath of dilute CO (uptake limited only by diffusion). ↓ in emphysema, interstitial lung disease, pulmonary vascular disease and anaemia; ↑ in pulmonary haemorrhage, polycythaemia and asthma.
Compiled from standard anatomy and physiology references — see the Respiratory System sources. For education only; not medical advice.