Why is diffusion insufficient to meet the oxygen requirements of large multicellular organisms like humans?
In small unicellular animals, such as Amoeba, the volume of their body is so small that oxygen can be introduced quickly into the whole body by the process of diffusion. This is because the cell of Amoeba is so small that the oxygen does not have to go far. But in case of large multicellular animals like humans, the volume of the body is so big that oxygen cannot quickly diffuse into all the cells of the body. This is because in these cases, the oxygen has to travel a very large distance to reach each and every cell of the body.