Exchange surfaces

Many organ systems are specialised so that they can maximise the amount of materials exchanged over their membranes. There are a number of ways in which the effectiveness of an exchange surface can be increased.

• Large surface area: a larger surface area means that more diffusion can occur at once.
• Thin membrane: this speeds up the exchange because the diffusion path is shorter so the molecules have less distance to travel. For example, most green plants have thin leaves so that the exchange of gases (oxygen and carbon dioxide) is sped up.
• Efficient blood supply: in animals the blood system is very efficient for transporting a variety of substance around the body, for instance oxygen and nutrients. It also transports waste products, like carbon dioxide, so that they can leave the body.
• Ventilation: gaseous exchange is also a very important for exchanging materials. Oxygen and carbon dioxide are able to move easily between the lungs and the blood by diffusion.

Gas and solute exchange surfaces in humans and other organisms have been adapted to maximise their effectiveness. However, the larger an organism gets and the more complicated it is, the harder for materials to be exchanged.