Adaptations of exchange surfaces to maximise exchange

In multicellular organisms, surfaces and body organs are specialised for exchanging materials.

The ability to maximise exchange of substances in as short a time as possible in plants and animals is increased by having:

  • A large surface area to volume ratio
    • The flattened shape of structures such as leaves.
    • The alveoli in the respiratory system.
    • The villi in the digestive system.
  • A short distance required for diffusion to and from cells, when the cell membrane is very thin, as in
    • The flattened shape of structures such as leaves.
    • The walls of blood capillaries, which are one cell thick.
    • The epithelia of alveoli in the respiratory system, and the villi in the small intestine, which are one cell thick.
A taro leaf
Large, flat leaves like this green taro leaf have an effective exchange surface because they have a large surface area

Animals also have an efficient blood supply to transport molecules to and from the exchange surface which maximises the exchange. Examples of this include:

  • the network of blood capillaries that surrounds each alveolus in the lungs
  • the network of blood capillaries in each villus in the small intestine
Diagram illustrating how lipids pass through the gut wall

Villi are found in the small intestine of the digestive system. They increase the surface area of the intestines to allow digested food to be efficiently absorbed into the bloodstream and delivered to cells around the body.

Villi are filled with blood capillaries, and the blood constantly moving in them means that a steep concentration gradient is maintained. This increases the amount of dissolved, digested food that can be absorbed into the bloodstream from the small intestine.