The rate at which a gas diffuses across an exchange surface depends on three main factors. They are summarised by Fick's law:rate of diffusion ∝ (surface area × difference in concentration) ÷ thickness of the exchange surface So the rate is:
• directly proportional to the surface area: doubling the area doubles the rate;
• directly proportional to the difference in concentration across the surface: a steeper gradient gives a faster rate;
• inversely proportional to the thickness of the exchange surface (the diffusion distance): doubling the thickness halves the rate.Worked example 1: surface area is multiplied by 3 and the concentration difference by 2, with thickness unchanged. The rate is multiplied by 3 × 2 = 6.Worked example 2: surface area doubles but thickness increases 4 times. The rate is multiplied by 2 ÷ 4 = 0.5, so it halves.Worked example 3: in a lung disease, the surface area falls to one third and the barrier becomes twice as thick. The rate becomes (1/3) ÷ 2 = 1/6 of the original.Other factors also affect the rate:
• temperature: particles have more kinetic energy at higher temperature, so diffusion is faster;
• the size and nature of the molecule: small and lipid-soluble molecules diffuse faster;
• the medium: gases diffuse much faster in air than in water.These factors explain why gas exchange surfaces have the features they do, which is the subject of the next lesson.
• directly proportional to the surface area: doubling the area doubles the rate;
• directly proportional to the difference in concentration across the surface: a steeper gradient gives a faster rate;
• inversely proportional to the thickness of the exchange surface (the diffusion distance): doubling the thickness halves the rate.Worked example 1: surface area is multiplied by 3 and the concentration difference by 2, with thickness unchanged. The rate is multiplied by 3 × 2 = 6.Worked example 2: surface area doubles but thickness increases 4 times. The rate is multiplied by 2 ÷ 4 = 0.5, so it halves.Worked example 3: in a lung disease, the surface area falls to one third and the barrier becomes twice as thick. The rate becomes (1/3) ÷ 2 = 1/6 of the original.Other factors also affect the rate:
• temperature: particles have more kinetic energy at higher temperature, so diffusion is faster;
• the size and nature of the molecule: small and lipid-soluble molecules diffuse faster;
• the medium: gases diffuse much faster in air than in water.These factors explain why gas exchange surfaces have the features they do, which is the subject of the next lesson.