How to make a human that doesn't need to breathe

Question

I want to make a creature very similar to a human but with one major difference: they breathe like any human in their everyday life, but if they are suffocating, drowning, or there isn't O₂ in the air they can survive for several hours (or maybe more) without air, obviously with reduced physical activity and metabolism (if it's necessary, the creature can be unconscious).

My question is: how they would be able to survive without oxygen. Basically I'm asking how to make a facultative anaerobic animal...

I know that humans are able to make lactic acid fermentation in their muscles when they are doing a lot of exercise and they can't breathe enough oxygen to feed all the cells. But that mechanism, called Cori cycle has a very limited use, the cells aren't able to get rid of all the lactic acid produced by the fermentation, and when the acid reaches a high concentration in the muscle it causes acute pain until you stop exercising. Also, it's very inefficient way of producing energy.

I need a mechanism that can be used by extended periods of time without collapsing due high levels of something (like acid in the above example).

The answer doesn't need to exactly provide a new mechanism to get energy without air, if you know how to improve the actual, you are free to post it here.

P.S: I found that Narwhal have some capabilities to reduce their air consumption while they are under water. In the english wikipedia there aren't described but they have:

• Reduce flow of blood to non-vital organs.
• Increased amount of myoglobin in their muscles (to store oxygen).
• Increased amount of slow twitch muscle fiber ("red" muscle) that contain high amount of myoglobin and mitochondria for large periods of constant use instead of fast twitch muscle fiber ("white" muscle) for small periods of strong use.

Answer 1

Nitrate Reduction Cycle. Bacteria used to breath nitrates when they haven’t developed the necessary complex IV for oxygen reduction. So they used nitrates. The primary drive away from non-water respiratory metabolism was primarily the high cost and low availability of the hetereoatom substrates, not because of the lower efficiency.

If the mitochondria within your people somehow gained a bacterial complex VI gene from horizontal gene transfer......

Then, we can conceive that your people accumulates the nitrates when there is enough oxygen (probably by respiratory oxidization of urea), accumulating a very high concentration within a buffer within their vital organs(probably because their need for frequent diving, or having to live extensively underwater, as an example), using it when the membrane potential of their mitochondria is lowered below a certain threshold, converting the nitrates into ammonia and releasing a lot of energy(up to 8 hours normal metabolism without oxygen, if not for consuming slightly more sugars per unit of energy), (the complex normally for reducing oxygen can use nitrates when oxygen is not available), effectively granting your people a built-in rechargeable bioelectrochemical battery.

You can add in a cooldown for the skill as well, since the nitrate synthesis process is exoenergetic, it will require time and about as much metabolism as it is used to charge up the "battery", therefore once the 8 hours have expired, just a few breaths won’t be sufficient for another long term diving immediately.

Answer 2

Simple answer: increase the amount of myoglobin in these people's tissues. Myoglobin stores oxygen and is what allows whales to hold their breath for more than an hour even while exerting themselves. May not work for "several hours", but perhaps an hour or slightly more.

Answer 3

In addtion to the increased myoglobin, have the creature able to drop it's metabolic rate without harm. A number of animals can do that including some hummingbirds so they don't starve to death while sleeping.

This more useful if the creature goes unconsious as the brain uses huge amount of energy and thus oxygen. Think of that rare occurance where a human recovers from an extended period of asphyxia because they fell into icy water and hypothermia was induced at the same time.

Answer 4

In Hal Clement’s The Nitrogen Fix, intelligent beings that don’t live in an oxygen atmosphere are described using nitrogen chemistry. He explains he has to eat a balance of oxidizing and reducing forms to produce energy from his food.

In Ocean on Top by the same author, the human doesn't understand why they don’t breathe, and speculates (incorrectly) that some special food is supplying oxygen.