Question

why is diffusion insufficient to meet the oxygen requirements cellular organisms like human​

Answer 1

multicellular organism such as humans possesses Complex body designs they have specialised cells and tissues for performing where is necessary functions all the body such as intake of food and oxygen.

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Answer 2

in multicellular organism only the cells of the skin are in direct contact with the environment thus if all the raw materials required to sustain life are transported by the process of diffusion it will take very long time to reach all parts of the body to overcome this problem multicellular organism have specific organ or organ system for the transport of various substances like digest food,oxygen,carbon dioxide,water and other waste product in the body