Question

When we realese our breath after holding it for sometimes we breath heavily

Answer 1

This is because when we hold our breath our body cant get sufficient oxygen to carry out the process of respiration, so when we start breathing after holding it, we breathe heavily to fulfil the oxygen scarcity in the body

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Answer 2

This might be due to the fact when we hold our breath the blood will not be able to release Carbon dioxide or take in oxygen. So suddenly as we start breathing the blood starts taking more oxygen.
And another fact might be that without oxygen there takes place anerobic respiration and lactic acid will be formed so to remove that more oxygen will be utilised as we start breathing.