Why do humans need a complex circulatory system while an amoeba does not?
Humans have many cells, *all* of which need to be fed and oxygenation, and need to have waste products taken away. A circulatory system is an effective way to reach every cell on a regular, cyclic basis. An amoeba is one-celled. All of the contact with medium for nutrition and waste removal happens by diffusion and transport directly through the cell membrane.