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

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 for diffusion:

• 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

Animals have additional adaptations to ensure 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 alveolus in the lungs
• the network of blood capillaries in each villus in the small intestine

In the lungs, 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.