Living things must be capable of transporting nutrients, wastes, and gases to and from cells. Single-celled organisms use their cell surface as a point of exchange with the outside environment. Multicellular organisms have developed transport and circulatory systems to deliver oxygen and food to cells and remove carbon dioxide and metabolic wastes. Simple multicellular organisms such as sponges, multicellular fungi, and algae have a transport system. Sea water is the medium of transport and is propelled in and out of the sponge by ciliary action. Simple animals, such as hydra and planaria, lack specialized organs such as hearts and blood vessels, and instead use their skin as an exchange point for materials. This, however, limits the size an animal can attain. To become larger, they need specialized organs and organ systems. In lower plants such as algae and fungi, transport of material takes place through the body surface and cytoplasmic streaming movements.
Any system of moving fluids which reduces the functional diffusion distance that nutrients, wastes, and gases must traverse may be referred to as an internal transport or circulatory system.
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