CorpusHuman Anatomy

Alveolus & Gas Exchange

Alveolar sac with capillary network and a magnified respiratory membrane

Left: the end of the airway — a terminal bronchiole leads into a respiratory bronchiole, an alveolar duct and a cluster of alveoli (an alveolar sac), each wrapped in a mesh of pulmonary capillaries. Blood arrives O₂-poor from a pulmonary arteriole (blue) and leaves O₂-rich via a pulmonary venule (red); the capillary colour gradient shows oxygenation along the way, and the cluster gently inflates with each breath.

Right: the boxed area of alveolar wall magnified to show the respiratory membrane (blood–air barrier): surfactant film, type I pneumocyte, fused basement membranes and capillary endothelium, with a surfactant-secreting type II pneumocyte, an alveolar macrophage, the interstitium and erythrocytes flowing through the capillary. O₂ (blue) diffuses from alveolar air into the blood while CO₂ (orange) diffuses out; red cells turn from O₂-poor to O₂-rich within the first third of their transit. Partial pressures are typical resting adult values. Layer thicknesses are exaggerated for clarity.

Step-by-step

  1. Air reaches the alveoli

    Inspired air travels down the conducting zone, through a terminal bronchiole into the respiratory zone: respiratory bronchioles (with occasional alveoli in their walls), alveolar ducts (whose walls are almost entirely alveolar openings, ringed by smooth-muscle knobs) and alveolar sacs. Fresh air mixes with the functional residual capacity, so alveolar gas is very stable: PAO₂ ≈ 100–104 mmHg, PACO₂ ≈ 40 mmHg.

  2. O₂-poor blood arrives

    A pulmonary arteriole delivers mixed venous blood (PvO₂ ≈ 40 mmHg, PvCO₂ ≈ 45–46 mmHg, Hb ≈ 75 % saturated) into a dense capillary network that wraps each alveolus like a mesh, giving a blood–air interface of ≈ 70 m² across ≈ 300–500 million alveoli.

  3. The respiratory membrane

    Gases cross the blood–air barrier: surfactant-containing fluid lining → type I pneumocyte cytoplasm → fused basement membranes → capillary endothelium. On the thin side it is only ≈ 0.2–0.6 µm thick — ideal for diffusion (Fick's law: rate ∝ area × solubility × ΔP / thickness).

  4. O₂ diffuses into the blood

    Driven by a partial-pressure gradient of ≈ 60 mmHg (104 → 40), O₂ (blue particles) dissolves in plasma and then enters erythrocytes, where it binds haemoglobin. Because haemoglobin soaks up O₂, plasma PO₂ rises only gradually, keeping the gradient steep.

  5. CO₂ diffuses out

    CO₂ (orange particles) moves the other way down a much smaller gradient (≈ 46 → 40 mmHg, ≈ 6 mmHg). It still equilibrates as fast as O₂ because CO₂ is ≈ 20 times more soluble. Most CO₂ arrives as plasma HCO₃⁻, which re-enters the RBC (reverse chloride shift) and is converted back to CO₂ by carbonic anhydrase.

  6. Rapid equilibration

    At rest an erythrocyte spends ≈ 0.75 s in the capillary but reaches equilibrium with alveolar gas within ≈ 0.25 s — the first third of the capillary (watch the red cells change colour). Gas exchange is therefore normally perfusion-limited; the reserve allows full oxygenation even when transit time falls to ≈ 0.25 s in heavy exercise. A thickened membrane (fibrosis) makes it diffusion-limited, so hypoxaemia appears first on exertion.

  7. O₂-rich blood leaves

    Blood leaving the capillaries has PO₂ ≈ 100–104 and PCO₂ ≈ 40 mmHg (Hb ≈ 97–98 % saturated). It drains via pulmonary venules and veins to the left atrium; systemic arterial PO₂ is slightly lower (≈ 95–100 mmHg) because of the small physiological shunt.

  8. Surfactant and defence

    Cuboidal type II pneumocytes (≈ 5 % of the surface but ≈ 60 % of alveolar epithelial cells) store surfactant in lamellar bodies and secrete it onto the alveolar surface, where it lowers surface tension, preventing collapse of small alveoli (Laplace) and keeping alveoli dry; they also regenerate type I cells. Alveolar macrophages ("dust cells") patrol the air space, phagocytosing particles and microbes.

Labelled parts