Each adaptation unlocks once you've improved that control. Real alveoli have all four.
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1 Large surface area
More room to diffuse
A bigger surface area means more oxygen molecules can cross at the same time, so the overall rate of diffusion is higher.
Your lungs have ~300–500 million alveoli, giving a total surface area of about 70 m² — roughly half a tennis court.
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2 Thin walls
A short diffusion distance
Thinner walls mean oxygen travels a shorter distance to reach the blood, so it crosses faster.
The alveolar wall and the capillary wall are each just one cell thick — the whole barrier is around 1 µm.
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3 Good blood supply
Keeps the gradient steep
Flowing blood carries oxygenated blood away and brings deoxygenated blood in. This keeps oxygen low in the blood, so the concentration gradient between air and blood stays steep — and diffusion stays fast.
Breathing in and out (ventilation) does the same job on the air side, keeping alveolar oxygen high.
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4 Moist lining
Lets gases dissolve
Oxygen must dissolve into a thin film of moisture before it can diffuse across the membrane. A dry surface means almost no exchange.
This is a requirement for diffusion, not a dial you turn up — that's why it's a switch, not a slider.
Higher tier — putting it together
Fick's Law
The rate of diffusion depends on three things — and three of your four controls map straight onto them:
rate of diffusion ∝
surface area × concentration difference
diffusion distance
Surface area (slider 1) and concentration difference — kept steep by blood flow (slider 3) — are on top, so increasing them increases the rate. Diffusion distance — the wall thickness (slider 2) — is on the bottom, so making it smaller increases the rate.
Where's moisture? It isn't in Fick's law at all. It's an enabling condition — gases have to dissolve before any of this can happen.