Question

Why is diffusion insufficient to meet the oxygen requirements of multicellular organisms like humans?

Answer 1

In the process of diffusion insufficient to meet the oxygen requirement of multicellular organisms like humans because diffusion takes place very slowly i.e. when i take oxygen it takes two  years to reach oxygen to our foot's tissue as a result it is not fill up the need of all tissues of our body . 

Answer 2

In big multicellular organisms like human beings , the oxygen which we get from air will have to travel long distances inside the body of a human being or other large multicellular organisms to reach each and every cell of the body.

And we know very well that diffusion is a very slow process. It will take very long time to make oxygen available to all the cells inside the body.

So diffusion is insufficient to meet the oxygen required by the body of multicellular organisms.

This is because the volume of human body is so big that air can't diffuse into all the cells present in body at a high speed or rapidly.

And when the size of multicellular organism is large, then respiratory pigments like haemoglobin present inside the blood take up the oxygen from the air (in lungs) and carry it as quickly as possible to all the body cells .