|MadSci Network: General Biology|
The heart and respiratory system work actually separately. However when one holds his breath there is a negative suction pressure in the thorax that tends to pull open the organs or inflate them. Due to this the heart tends to take more time in filling and so it fills more and pumps more blood but its rate of pumping decreases due to longer filling periods. Now about the brain, if your brain does not get oxygen for about 3-4 min it will start dying. But you cant hold your breath for long by yourself because when the brain comes in high stress it pushes you automatically to breathe.
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