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I am a human. I take a deep breath. I swim underwater... After some time, I need to blow out some air. I blow out some air... By doing so, I can swim underwater longer. Blowing out some air gives me more time underwater. Why?

Apparently, dolphins encounter this phenomenon too.

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    $\begingroup$ "I am a human" congrats! As for the question, most probably because blowing out air decreases your buoyancy, I've never heard of such a biological response which helps us to swim longer by blowing out air. Dolphins should encounter it too as laws of physics apply on everyone. $\endgroup$ – another 'Homo sapien' Feb 27 '16 at 10:39
  • $\begingroup$ Please add a citation regarding your dolphin statement. It would also be nice to see some primary literature where this was tested in a vigorous way, not just your personal experience. $\endgroup$ – MattDMo Feb 27 '16 at 20:14
  • $\begingroup$ Whales blow out when they surface. Hence the fountain of air/water known as the 'blow'. $\endgroup$ – RHA Feb 28 '16 at 7:40
  • $\begingroup$ @another — Blowing out some air gives me more time underwater. $\endgroup$ – Nicolas Barbulesco Mar 19 '16 at 22:21
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This is purely a psychological response, and it does not help in decreasing the speed of CO2 level rise. As CO2 level in your blood rises, you instinctively want to breath out the 'bad' air and breath in fresh one. The latter is impossible under water, but the former eases you psychologically a little; you have "started doing something" with your predicament of high CO2 level.

Most freediving courses (as well as my own experience - humble as it is) suggest not to have the last breath substantially larger than your normal one; this comes from people who train to achieve 6-7 minutes dives, so I would assume they meticulously tried various methods.

The key here is not to have a victory over physiology; the key is psychology. Most people fail at diving because they unnecessarily get into a positive feedback loop: they get excited, their brain and muscles become even more active, they produce more CO2, brain receives information about rising CO2 in blood, and it becomes excited even more, produces even more CO2, etc, etc, until a panicked diver surfaces or inhales water. Without such unnecessary excitation, each moderately healthy human would be easily physiologically capable of 3-4 minute dives.

My take is the amount of air you have in your lungs should be primarily the amount that eases your psyche the most and helps to stop needless stress.

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  • $\begingroup$ The problem is with the high carbon dioxide, not with the low oxygen. Humans don't have oxygen detectors, so we can't respond to that. Humans have carbon dioxide detectors, so we do respond to that. $\endgroup$ – kubanczyk Jan 31 '17 at 9:52
  • $\begingroup$ Very interesting answer! Before apnea diving, taking a last BIIIG breath seems the logical thing to do, in order to have as much oxygen as possible. Why doing otherwise? $\endgroup$ – Nicolas Barbulesco Feb 1 '17 at 11:07
  • $\begingroup$ Can you please explain "the problem" after taking more air than "the optimum"? Thank you! $\endgroup$ – Nicolas Barbulesco Feb 1 '17 at 11:16
  • $\begingroup$ I didn't say that taking more air causes a problem. The problem is the instinctive need to breathe. It is triggered to get carbon dioxide OUT (you produce it), not because you need more oxygen IN. The focus is usually on decreasing the carbon dioxide production. $\endgroup$ – kubanczyk Feb 1 '17 at 11:49
  • $\begingroup$ But, in order to avoid having too much CO2, we can blow CO2 out, so the CO2 detectors won't ring too soon? $\endgroup$ – Nicolas Barbulesco Feb 1 '17 at 12:34

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