Exchange surfaces

Single-celled organisms have relative large surface area to volume ratios. Larger multicellular organisms have smaller surface area to volume ratios. So, they have evolved exchange surfaces to exchange molecules with their surroundings.

The effectiveness of exchange surfaces in plants and animals is increased by having:

A large surface area:

  • the flattened shape of structures such as leaves
  • the alveoli in the respiratory system
  • the villi in the digestive system

A short distance required fordiffusion:

  • the membranes of cells
  • the flattened shape of structures such as leaves
  • the walls of blood capillaries are one cell thick
  • the epithelia of alveoli in the respiratory system and the villi in the small intestine are only one cell thick
A large taro leaf
Large, flat leaves like this green taro leaf have an effective exchange surface

Animals have additional adaptations for effective exchange surfaces.

An efficient blood supply to transport molecules to and from the exchange surface increases effective exchange. Examples of this include:

  • the network of blood capillaries that surrounds each of the alveoli in the lungs
  • the network of blood capillaries in each of the villi in the small intestine
Diagram of a villus in the small intestineCross section of one of the villi

The process of breathing, or ventilation, brings air to, and removes air from the exchange surface – the alveoli.

The moving blood and ventilated surfaces mean that a steep concentration gradient can be maintained. This increases effective exchange.