If two human characters have to wait underwater for as long as possible, would it be conceivable that they would last longer if they exchanged breaths in turns (ie: through a simple tube or mouth-to-mouth) than if they simply held their breaths alone?

Then as a second question, would it be possible that if this had become regular practice (culturally for example) for the past thousands of years, they would have evolved the ability to remain underwater for several hours with this technique and how could this work biologically? (no drawing oxygen through the surrounding water)

closed as off-topic by Mołot, dot_Sp0T, Hohmannfan, Thorsten S., slobodan.blazeski Jan 8 '17 at 17:36

This question appears to be off-topic. The users who voted to close gave this specific reason:

If this question can be reworded to fit the rules in the help center, please edit the question.

  • Welcome to Worldbuilding. You may have better luck asking the first half of this question in the Biology StackExchange. Then come back here once you have that. :-) biology.stackexchange.com – SRM Jan 8 '17 at 6:21
up vote 5 down vote accepted

Exchanging breaths would not help if the two are of similar skill level. The issue that causes us to feel the need to breath is the CO2 levels in the air. Alternating bodies would not change the rate at which CO2 gets taken out of the blood stream and into the air. Similarly, you would deplete the O2 at the same rate as if you just held it in your lungs.

If the two individuals were of different skill levels, the skilled practitioner might be able to help control the breathing patterns of the novice at a subconscious level. This would still result in the same chemical issues with CO2 and O2, but it could help with the panic of the novice if the expert could exert some control over their body by breathing.

Unfortunately, this wont help you for the long term evolution. Such evolutionary forces would be the same whether you're doing that odd buddy breathing process, or if you're all on your own. You're still limited by the capabilities of the human body. The best skin divers top out in the 10 minute region. You're not going to see evolutionary forces push that into hours over thousands of years. Maybe hundreds of thousands or millions.

Two people cannot last longer by breathing each other's air; two people using two lungfuls of air is effectively the same as one person using one lungful of air. The trick of keeping someone alive underwater by breathing another person's air requires the second person to come up to the surface between breaths, carrying fresh oxygen with each transfer.

This is a very impractical technique any way you look at it; the person transferring air from the surface must use up a lot of extra energy using very little oxygen, and it is hard to devise a situation where this would be more useful than a single person just going up for breath themselves. Of course, human culture being what it is, it is quite possible that a culture may develop with such a custom; perhaps as a marriage ceremony (symbolically indicating one person keeping another alive) or purification ritual (where a person must remain completely immersed for longer than would be physically possible).

  • It's not carrying fresh oxygen, it's getting rid of excess CO2 that limits time primarily. Once you have a rebreather that removes CO2 you can stay much longer under the water. Also in contrast to high CO2 levels which feels extremely uncomfortable, low O2 levels are not uncomfortable, you simply pass out once it passes a critical level. – Thorsten S. Jan 8 '17 at 14:54

Not the answer you're looking for? Browse other questions tagged or ask your own question.