I would think the exact opposite would occur - you’d need lung capacity in order to extract the oxygen you need. You’d still be inhaling and exhaling. If anything, I’d expect the adaption would be towards greater efficiency and the opposite of atrophy.
In real life, people who live at lower air pressures (on top of mountains like the Himalayas and Andes) have lungs that operate just fine at higher pressures. Saturation divers live for extended periods in environments with much, much higher pressures than normal, although not years at a time, and go from sea level to higher pressures without a lot of trouble - although living at increased pressure for long periods might be associated with reduced lung function, this is a possible side effect and not extensively researched.
I’m not a pulmonologist but I see it this way; under normal air pressure, an average person inhales 500 ml of air in each breath. This breath of air contains approximately 100 ml of oxygen and 400 ml of nitrogen (I’m ignoring trace elements).
Now put them in a pure oxygen atmosphere. They only need to inhale 100 ml of air to get that 100 ml of oxygen. So their lungs will get accustomed to only inhaling and exhaling 100 ml breaths.
Take a person whose lungs have become accustomed to working with 100 ml breaths and switch them back to a normal atmosphere. If they only inhale and exhale 100 ml breaths, they’re only getting 20 ml of oxygen per breath.
It was supposed to fly with a low-pressure O2 cabin (about 5 psi absolute), but it was supposed to launch with a high-pressure O2 cabin atmosphere (about 2 psi above atmospheric, i.e. 16.7 psi absolute) . 5 psi of pure O2 may have presented some hazard during spaceflight, but 16.7 psi of pure O2 during the plugs-out test was crazy-hazardous, as the fire demonstrated.
Just to be clear, you don’t need that CO2 in the air you’re inhaling; your body will add the CO2 to whatever’s in your lungs, eventually compelling you to breathe. So as long as there’s enough partial pressure of O2 in your lungs to sustain you until the CO2 buildup compels you to breathe, you’ll be fine.
I was curious about a space habitat or alien planet situation. How much of what’s in Earth-normal sea-level atmosphere can be dispensed with and still keep people and maybe their animals healthy over the long haul, for years or lifetimes?
I definitely count having one’s lungs maladapted to returning to Earth-normal atmosphere as something bad, but still interesting if someone were able to live out a normal life in permanent exile that way.
What you’re missing is that humans don’t need a particular volume of oxygen. What they need is a particular number of oxygen molecules. How much volume those oxygen molecules take up will depend on the pressure (and on the temperature, but we don’t have much room for variation on that, for human health and comfort). In an atmosphere as described, you have the same amount of oxygen molecules per breath as normal; those molecules are just taking up the full 500 mL instead of 100 mL (and actually, they’re taking up 500 mL here on Earth, too: They’re just sharing that space with nitrogen that’s also taking up that space).
Nor is the different pressure likely to cause significant issues. Mostly what matters with pressure is differences in pressure, not the absolute pressure. There might be some secondary effect on blood pressure as a result of the lower air pressure, but living on the Moon (in 1/6 normal gravity) will have a far greater effect, there.
Okay, now I have to pretend to be a chemist as well as a doctor.
Let’s say there are sixty quintillion O2 molecules in 100 ml of pure oxygen. Or sixty quintillion O2 molecules and two hundred and forty quintillion N2 molecules in 500 ml of normal air.
So if you’re breathing in a normal atmosphere, your lungs need to inhale 500 ml of air (or three hundred quintillion molecules) to get the sixty quintillion O2 molecules your body needs. But if you’re in a pure oxygen atmosphere, your lungs only need to inhale 100 ml (sixty quintillion molecules) to get those sixty quintillion O2 molecules. So your lungs need to inhale a smaller volume of gas (or fewer molecules) in a pure oxygen atmosphere than they do in atmosphere of normal air.
Then throw out volume altogether. Let’s just compare the work your lungs will do inhaling sixty quintillion molecules to the work they will do inhaling three hundred quintillion molecules. It seems to me the latter will involve five times as much effort. Is that not correct?
At most, you have about 6 liters of air capacity, you generally only breathe in and out about half a liter.
Sea level air is 1.225 grams per liter, so your average breath is going to involve moving about .6 grams. In a 20% pressure environment, you are instead moving .1225 grams of air with each breath.
It might be noticable, but it wouldn’t be nearly enough to actually cause issue with atrophy of the diaphragm. It seems that the movement of all the muscles and tissues involved is going to overwhelm the difference.
Your lungs will exchange oxygen more quickly, I think, if you get rid of whatever other species (e.g. nitrogen) are mixed with it, and keep the mass and number of oxygen molecules the same. The nitrogen is in the way and the oxygen has to diffuse through it. This would be a reason it’d be easier to breathe without the nitrogen.
That said, there are a bunch of other things also going on. Which is easier all things taken together is maybe a more complicated story and different answer. But there’s reason to think it wouldn’t be the same.
If your space habitat has lower atmospheric pressure, even though the oxygen level remains the same, one consequence would be that water would boil at a different temperature, making it impossible to make a decent cup of tea.
Obviously adding atmospheric nitrogen to the environment is vital for tea-drinkers.
The alveoli, the sacs in your lungs in which gas exchange between air and blood takes place, are really small, a few thousands of an inch across. The rate at which O2 is removed at the alveolar surface is low compared to the rate at which the constituent gases can diffuse across such a short distance, with the net result being that concentration gradients within the alveoli are probably not meaningfully different for pure O2 of a given pressure versus a blend with O2 at the same partial pressure.
Until your post, we’ve been discussing what kind of muscular/mechanical effort is needed to inflate/deflate the lungs, with the general understanding that denser gases (whether through higher pressure, higher molecular mass, or both) require more effort. Did you mean something different when you said “easier to breathe?”
At 20% of atmospheric, you can barely serve coffee or tea at a decent temperature. The boiling point of water is only ~60 C at 20 kPa. That’s really at the bottom range of what you’d ever serve coffee at. Might be just barely ok if you had mugs that maintained the temperature (as opposed to serving at a higher temperature so that it stays in a reasonable range as it naturally cools).
So assuming dry air inhaled at 70F, each mole of air will come in contact with 17 moles of water in the lung. The 17 moles are necessary to raise exhale temperature to 94.5 F !!!
In terms of mass, the lungs will wash every pound of air we breathe with 11 lbs of water.
Where does all this water come from in the lungs ? Am I understanding this correctly ?
I was taking “breathe” to mean inflating and deflating the lungs as required to oxygenate the blood, and not just going through the motions without regard to its underlying motivation.
I’m thinking that normal Earth air is far from dry.
Part of the discomfort of long-haul air travel is that the atmosphere at altitude is so very low in humidity. Once that’s compressed to a decent pressure it has the same gas mix as ground-level air, but is still real low in water vapor. People exhale a LOT of water in that situation.
Based on math I did elsewhere, I’d estimate water loss due to complete humidification of 0%RH inhaled air as about 21 grams per hour, less than an ounce. This assumes you’re at rest (e.g. stuck in an economy class seat). If you’re exerting yourself (e.g. you’re in a struggle with the Federal Air Marshal after having angrily demanded yet another glass of water)), then your tidal volume and breathing rate will be considerably higher, and so will your water loss rate.
Not if you install a Nutri-Matic on the spacecraft
Douglas Adams - Hitchhikers Guide to the Galaxy
He had found a Nutri-Matic machine which had provided him with a plastic cup filled with a liquid that was almost, but not quite, entirely unlike tea .