Factors affecting diffusion and Fick's law

Exchange surfaces such as the alveoli are built to make diffusion as fast as possible. Fick's law sums up what affects the rate.

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Three factors

The rate of diffusion is faster when there is:
1. a larger surface area;
2. a bigger concentration difference (a steeper gradient);
3. a thinner membrane (a shorter diffusion distance).
Temperature matters too: particles move faster when warmer.

Fick's law

rate of diffusion ∝ (surface area × concentration difference) ÷ thickness of membrane
So: doubling the surface area doubles the rate; doubling the concentration difference doubles the rate; doubling the thickness halves the rate.

Applying it to the lungs

Alveoli have a huge surface area, walls one cell thick and a rich blood supply. Breathing (ventilation) brings in fresh air, and blood flow carries oxygen away, so the concentration difference stays high.
Emphysema destroys alveolar walls, reducing the surface area, so less oxygen gets into the blood.
Worked example

A membrane's surface area triples, and its thickness doubles. The concentration difference stays the same. By what factor does the rate of diffusion change?

  1. Rate ∝ area × difference ÷ thickness
  2. New rate factor = 3 × 1 ÷ 2
  3. = 1.5

Answer: The rate is multiplied by 1.5.

Key idea

Diffusion is faster with a larger surface area, a bigger concentration difference and a thinner membrane: rate ∝ area × concentration difference ÷ thickness. Ventilation and blood flow keep the gradient steep in the lungs.

Key words
Diffusion
The net movement of particles from a higher to a lower concentration. It is passive and needs no energy from respiration.
Alveoli
Tiny air sacs in the lungs where gas exchange happens. They have a large surface area, thin walls and a rich blood supply.

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Extra practice

2 more questions

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  1. The surface area doubles and the concentration difference doubles. The thickness is unchanged. By what factor does the rate of diffusion change?

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    Answer: 4

    Rate ∝ surface area × concentration difference ÷ thickness: 2 × 2 = 4.

  2. Calculate a value for the rate of diffusion using rate = surface area × concentration difference ÷ thickness, when the surface area is 40, the concentration difference is 6 and the thickness is 0.2 (all in arbitrary units).

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    Answer: 1200

    40 × 6 ÷ 0.2 = 240 ÷ 0.2 = 1200

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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