B3 1.4 Exchange materials - the lungs

As living organisms get bigger and more complex, their surface area: volume ratio gets smaller. This makes it increasingly difficult to exchange materials quickly enough with the outside world. Gases and food molecules can no longer reach every cell inside the organism by simple diffusion. So in many larger organisms there are special surgaces where gas and solute exchange take place. They are adapted to be as effective as possible. You can find them in people, in other animals and in plants.

Adaptations for exchanging materials

There are various adaptations to make the process of exchange more efficient. The effectiveness of an exchange surface can be increased by:

• having a large surface area
• being thin, which provides a short diffusion path
• having an efficient blood supply, in animals. This moves the diffusing substances away and maintains a concentration (diffusion) gradient
• being ventilated, in animals, to make gaseous more efficient by maintaining steep concentration gradients.

Different organisms have very different adaptations for the exchange of materials, such as the leaves of a plant, the gills of a fish…