Respiratory System呼吸 · 看不见的传送带

Place · Level 3

3 亿肺泡 · 70 m² 交换面 · 每天 20000 次呼吸 ~ 12000 L 空气 · O₂/CO₂ + 酸碱 + 高原 HIF + 氧感受诺贝尔奖通路

300 M alveoli · 70 m² exchange

Stop 1/6

3 亿肺泡 · 70 m² 交换面

The lung looks like a pair of fleshy blocks, but is actually a carefully folded tree — its total leaf-area is about 70 m² (roughly a tennis court), all packed inside your chest.

Lung geometry (Weibel 1963 classic measurement):

Airway branching: trachea → bronchus → 23 generations of bifurcation → 300 million alveoliAlveolus diameter ~0.2 mm, wall thickness ~0.5 μm (100× thinner than a hair)Total exchange surface ~70 m² (some estimates reach 100 m²; 70 is commonly cited)Total pulmonary capillary length ~1000 km — about 1/4 of all body capillaries are in the lungs
This design is up against physical limits: thin enough that any thinner would rupture, but any thicker would block diffusion (Fick's law has diffusion rate proportional to area / thickness); large enough that any larger wouldn't fit in the chest, but any smaller wouldn't keep up with cardiac output.

A day's workload: resting respiratory rate 12-16 breaths/min, ~20,000 breaths/day; resting tidal volume ~500 mL, ~12 m³ of air per day (about 1/5 of a bedroom); during exercise minute ventilation can spike 25× to 200 L/min.

The body has only two direct conduits to the outside world — the respiratory tract and the GI tract. The GI tract is relatively short, with mucus, gastric acid, and the liver layer-filtering it. The respiratory tract is directly exposed to ambient air — PM2.5, smoke, viruses, allergens — and is guarded by mucus + cilia + alveolar macrophages + IgA + antimicrobial peptides. This is why the vitamin-D / vitamin-A / zinc immune stories in the atlas land most directly here, in the respiratory tract.

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