Why is diffusion insufficient to meet the oxygen requirements of multi-cellular organisms like humans?
The random motion of particles to attain equilibrium of concentration is called diffusion. As the size of an organism increases, it's surface area to volume ratio decreases. Consequently, large multicellular organisms cannot rely on diffusion alone to supply their cells with substances such as food and oxygen and to remove waste products. The surface area limits the diffusion to the volume ratio of the organism.