Mammalian gas exchange surfaces, the link to surface-area-to-volume ratio, and oxygen transport by haemoglobin including the dissociation curve.
Large active organisms cannot rely on diffusion across the body surface — they have a small surface-area-to-volume (SA:V) ratio — so they need specialised gas exchange surfaces and a transport system.
Features of an efficient gas exchange surface
Large surface area (many alveoli) — more diffusion.
Thin (one-cell-thick alveolar and capillary walls) — short diffusion distance.
Moist — gases dissolve.
Good blood supply / ventilation — maintains a steep concentration gradient.
These all follow Fick's law: rate ∝ (surface area × concentration difference) ÷ diffusion distance.
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