Answers

2016-04-20T00:26:05+05:30
The rate of diffusion of oxygen is slow, whereas our demand for oxygen for respiration is very high. 

So, the process of diffusion is insufficient to meet the oxygen requirement of human beings.
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2016-04-20T14:31:55+05:30
This is because diffusion is very slow process takes lots of time to be accomplished.And if oxygen to be supplied by diffusion then it takes 8 years to diffuse from head to legs.
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