Larger animals have a lower surface area to volume ratio. This means they need transport systems like our circulatory system and exchange surfaces like our lungs or fish gills. In single-celled organisms such as protists, and small multicellular organisms, essential molecules will move to where they're needed by diffusion.
As the size of an organism increases, its surface area to volume ratio decreases. ... Large multicellular organisms therefore cannot rely on diffusion alone to supply their cells with substances such as food and oxygen and to remove waste products. Large multicellular organisms require specialised transport systems.
