Factors affecting the rate of diffusion

In a nutshell

Diffusion is the movement of a substance from an area of high concentration to an area of low concentration. There are several factors that can affect the rate of diffusion across an exchange surface. The diffusion rate can be calculated using Fick’s law.

Factors affecting the rate of diffusion

There are many factors that can affect the rate of diffusion of substances across an exchange surface. Some factors that make diffusion more effective include:

Factor	Example(S)
An increase in surface area	Alveoli in the lungs Villi in the small intestines
A shorter diffusion distance	The surface of capillaries is only one cell thick
The maintenance of a steep concentration gradient	Lungs are ventilated Animals have an efficient blood supply

Fick’s law

The rate of diffusion of substances can be calculated using Fick’s law.

$rate\ of\ diffusion\propto \frac{surface\ area \times concentration\ difference}{diffusion\ distance}$

The ' $\propto$ ' symbol in this equation means ‘proportional to’. Two variables are considered to be proportional to each other if one causes a change in the other that is related to a constant factor. The greater the surface area and the concentration difference (top row) are, the greater the rate of diffusion will also be. The greater the thickness of the membrane (bottom row), the lower the rate of diffusion will be.

The rate of diffusion will double if the surface area/concentration difference is doubled or the diffusion distance is halved.