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I'm building a some hypothetical habitable planets for a book, and one of these planets has a thick atmosphere (around 10 bar). I'm not sure if it's even possible, but I want to figure out how the animals on this planet could avoid nitrogen narcosis (the atmosphere is mainly nitrogen, as on Earth). Could the problem be avoided simply through the mechanics of breathing? For example, could a high efficiency with oxygen and an ability to simply not breathe/close the respiratory tubes between breaths prevent N2 from building up? Maybe the breathing pattern could be - inhale, absorb oxygen, quickly exhale nitrogen and CO2, then hold? In this scenario, I'm imagining they only experience brief periods of narcosis, every time they breath in? Or could there be some kind of biological barrier to nitrogen in their membranes? This seems potentially impossible, since N2 is such a small, simple, and chemically inactive molecule.

I'm trying to keep these planets bound by hard science, but I can't find much info on biological response to high nitrogen partial pressures that doesn't pertain to deep sea diving.

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In Earth, complex organisms with high metabolic rates (like most non-microscopic land animals) don't tolerate high levels of nitrogen not because it is impossible or very hard, it is just because there was no evolutionary pressure in this way when they evolved. If air pressure and nitrogen content raises slowly and gradually over a course of some millions of years, they will adapt for that. If the atmosphere were like this since the Neoproterozoic or Mesoproterozoic, they would eventually evolve in that way.

The difference between those organisms and current ones don't need to be great. The simplest answer is to make them simply tolerate high levels of nitrogen build-up in their organisms.

Sharks, for example, have high levels of urea in their bodies that would be lethal to most of other organisms, because they evolved in a way to tolerate that.

In the Archean, there was almost no oxygen in the atmosphere nor dissolved in the seas and it was poisonous to 99.9% of the life from that era. When oxygen started to accumulate in the oceans and in the air, it triggered a mass extinction event. However, eventually, organisms that tolerate oxygen evolved. From those, organisms that not only tolerate, but actually require oxygen for living, also evolved.

Moreover, most present-day organisms could not live in the Archean atmosphere or Archean oceans not only because they lacked oxygen, but even because that old atmosphere and oceans would be actually poisonous for them. Evolution frequently make changes that are very advantageous for an organism in a specific time that would lead it to a quick extinction if it happened in some other time.

There are a lot of variables that works as handles and knobs for this that you could fine-tune. Here are a few examples, and they aren't mutually exclusive, so a combination of many of them is possible:

  1. Make an intracellular organelle that absorb, concentrate or expel excess nitrogen.

  2. Overload some existing organelle like the Golgi complex or peroxyssomes with the purpose of expelling excess nitrogen.

  3. Decrease the metabolic rate in order to allow oxygen to be consumed more slowly.

  4. Develop larger lungs, so more oxygen can be consumed even if more nitrogen is also consumed.

  5. Make a more complex lung with an intermediate layer with cells covered in specific proteins that are responsible for selectively push O2 to the inside of the body and bad gases to the outside while trying to reject the entrance of bad gases and the exit of O2. Bad gases include at least N2 and CO2.

  6. Create a new organ which is partly a kidney and partly a lung that is responsible for taking N2 from the blood and expelling it in the air. It could also expel other gases like CO2 or whatever else could build up and should be expelled.

  7. Make a more complex circulatory system that expel the nitrogen entering from the lungs before the blood is used by the rest of the body.

  8. Build up neurons and muscles that are chemically more resilient in tolerating nitrogen microbubbles.

  9. Develop proteins that are abundant in the cells which have the purpose of expelling nitrogen.

  10. Create some specific cell in the blood similar to a red blood cell (let's call it blue blood cell) that is responsible for collecting and concentrating nitrogen in order to avoid bubble formation and then disposing it in some organ.

  11. There are nitrogen-fixing bacteria that actually use the N2 for their metabolic processes. Now, imagine a large organism that could also do that.

There should be thousand of ways of doing this.

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  • $\begingroup$ 12. Tweak the structure of neuronal cell membranes to adjust the solubility of nitrogen. $\endgroup$ Sep 10, 2023 at 16:31
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The detailed mechanisms of nitrogen narcosis, which is just a particular special case of inert gas narcosis, are not known, so nobody right now can tell you the exact biochemical adaptations that would be needed to eliminate it.

However, the best current theoretical explanations are that the increased solubility of inert gasses in lipid membranes at high pressures somehow interferes interferes with nerve function. (Solubility is increased in all cell membranes, but it's the neurological effects that we care about.) So... you just need to tweak the structure of the neural cell membranes, or the ion channels embedded in them, to function in the environment of higher dissolved nitrogen. Or design your creatures to use an entirety different nerve signalling system that happens not to be sensitive to gasses dissolved in the lipid membrane--say, optical signalling along silica fibers, or direct electron transport, etc.

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