Every living cell requires oxygen for performing cellular respiration. In unicellular organisms (e.g., Amoeba), the single cell is in direct contact with environment. Oxygen passes into it through diffusion. In simple multicellular organisms (e.g., Hydra), every cell may also get oxygen through diffusion from environment. This is not possible in complex multicellular organisms like humans. The body is covered by dead cells. The living cells are not in contact with external environment. Air containing intercellular spaces are absent. Therefore, quick diffusion cannot occur. Cell to cell diffusion is a very slow process. Passage of oxygen from lungs to toes through cell to cell diffusion will take about three years. Therefore, diffusion cannot meet the oxygen requirement of multicellular organisms like humans.