Gas exchange surfaces and Fick's law

Fick's law

Rate of diffusion is proportional to:

(surface area x difference in concentration) / thickness of the exchange surface

So an efficient exchange surface has a large surface area, a thin barrier (short diffusion distance) and a steep concentration gradient.

Adaptations of the lungs

• Millions of alveoli give a very large surface area.
• Alveolar walls are one layer of thin, flattened (squamous) epithelial cells, and each is wrapped in capillaries whose walls are also one cell thick: a short diffusion distance.
• Ventilation brings in fresh air and removes stale air, and blood flow carries oxygen away and brings CO2: both keep the concentration gradients steep.
• A moist lining lets gases dissolve before diffusing.

Using the law

Because rate is proportional to area and to gradient, and inversely proportional to thickness, you can predict changes. For example, if thickness doubles, rate halves. Diseases that destroy alveoli (lower area) or thicken the walls reduce gas exchange.

Worked example

The surface area doubles and the thickness of the barrier halves. By what factor does the rate of diffusion change?

1. Rate is proportional to area / thickness.
2. New rate factor = 2 / 0.5 = 4.

Answer: The rate increases by a factor of 4.

Check yourself

1. The thickness of an exchange surface doubles; nothing else changes. By what factor does the rate of diffusion change?
   Show answer

   0.5. Rate is inversely proportional to thickness: it halves (factor 0.5).

2. Surface area is tripled and the concentration difference is doubled. By what factor does the rate of diffusion change?
   Show answer

   6. 3 x 2 = 6.

3. How are alveolar walls adapted for gas exchange?
   • They are thick and muscular
   • They contain cartilage rings
   • They are one cell thick, giving a short diffusion distance
   • They have a small surface area
   Show answer

   They are one cell thick, giving a short diffusion distance. Thin squamous epithelium shortens the diffusion path.

4. How is a steep oxygen concentration gradient kept between alveoli and blood?
   • Thick alveolar walls
   • Ventilation and continuous blood flow
   • Reducing the number of capillaries
   • Holding the breath
   Show answer

   Ventilation and continuous blood flow. Fresh air in and oxygenated blood carried away maintain the gradient.

5. Emphysema destroys alveolar walls. Why does this reduce gas exchange?
   • The concentration gradient becomes steeper
   • The diffusion distance becomes shorter
   • More capillaries form
   • The surface area for diffusion is reduced
   Show answer

   The surface area for diffusion is reduced. Fewer, larger air spaces have less total surface area.

6. According to Fick's law, which change would increase the rate of diffusion?
   • Increasing the thickness of the membrane
   • Increasing the concentration difference
   • Decreasing the surface area
   • Reducing blood flow
   Show answer

   Increasing the concentration difference. Rate is proportional to concentration difference.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.