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