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.

• Diffusion is unable to meet the oxygen requirements of multi-cellular organisms such as humans because diffusion is one of the slow processes.

• That is a lot of time is required for oxygen to circulate to all the cells of the body.

• Besides, in humans oxygen requirement is more at cellular and tissue level. Therefore, through diffusion, it becomes difficult for oxygen to reach all the cells.