Why is diffusion not sufficient to obtain enough oxygen in multicellular animals?

Diffusion is not sufficient to obtain enough oxygen in multicellular animals because the oxygen demand of these organisms is much higher than that of single-celled organisms. In multicellular animals, there are many layers of cells that need to be supplied with oxygen, and the distance from the surface of the organism to the deepest cells can be significant. Diffusion is a slow process, and it becomes even slower as the distance increases. As a result, diffusion alone would not be able to provide enough oxygen to all of the cells in a multicellular animal.

To overcome this problem, multicellular animals have evolved specialized respiratory systems that use active transport to move oxygen from the environment to the cells. These systems can be very complex, but they all share the basic principle of using energy to move oxygen against a concentration gradient. This allows multicellular animals to obtain the oxygen they need to survive, even though diffusion alone would not be sufficient.