Absorption of Air into the Blood

Air consists of approx. 21% oxygen, nitrogen comprising most of the rest. During respiration, oxygen is absorbed into our blood thru the alveoli, but does the entire air molecule get absorbed? I’m thinking that nitrogen also gets absorbed, and under normal air pressure it does not present a problem, presumably because the body gets rid of it thru some mechanism, but under increased pressure (underwater) the nitrogen forms bubbles, resulting in the “bends.” If it is just atoms of oxygen that are absorbed, nitrogen would accumulate in the lungs. Is this correct?

I’ll first note two nitpicks: first, there’s no such thing as an “air molecule”. Air consists of oxygen molecules (molecules of O[sub]2[/sub]) and nitrogen molecules (N[sub]2[/sub]). They’re separate molecules and can take part in chemical reactions separately from each other. Second, the bends is caused by decreased, not increased, pressure, which causes nitrogen dissolved in the blood to come out of solution, forming bubbles of nitrogen gas. This also answers your main question: nitrogen is indeed dissolved in the blood under normal conditions. Also note that oxygen is not just dissolved in the blood, it is bound to hemoglobin, which allows the blood to carry much more oxygen than it would if the oxygen were just dissolved in the blood. I think this means that indeed more oxygen than nitrogen is removed from the air in the lungs, but nitrogen doesn’t “accumulate” there, since you exhale it out on each breath.

Excellent question!

I don’t think markn+ mentioned that the percentage of nitrogen in blood is generally the same as the percentage in the surrounding atmosphere, “air”.

Here is a fairly simple explanation wit diagrams.

Now a hijack. Looking for a webpage to illustrate nitrogen respiration I came across this paper from 1912 titled


Reading about how basic (to us now) science was created is fascinating to me.

Isn’t the pressure on your body increased under water? If so, I assume then as you ascend in the water, the pressure is then decreased which causes the bends. But the pressure is just returning to what it was before you dived. Because the bends is caused when you rise too fast, I assume it is the sudden decrease from the increased pressure that is the cause.

Yes, under water the pressure is increased. That causes more nitrogen to dissolve into the blood than at sea level pressure. At that point, everything is fine. You experience no adverse effects from this. (Actually at very high pressure you will experience “nitrogen narcosis”, which is why deep divers use a gas mix that doesn’t include nitrogen. But that’s a separate issue from the bends.) The bends occurs when you ascend from that high pressure depth. The decreased pressure causes your blood to be unable to hold the amount of nitrogen that it held at higher pressure, so the nitrogen comes out of solution as gas bubbles. The bubbles are the cause of the bends. So ultimately the bends are caused by the fact that you spent some time at depth, but the proximate cause is the decrease in pressure as you ascend.

At the surface you are breathing air. Some nitrogen does dissolve into your tissues, and of course oxygen too. Your body’s metabolism uses the oxygen. The internal pressure of nitrogen in your body comes to an equilibrium with the external pressure of nitrogen in the air.

As you scuba dive you are still breathing a mixture of oxygen and nitrogen, just compressed and delivered to you at about the same pressure as the water surrounding you.

As you start your dive there is relatively little nitrogen in your body compared to how much it could hold at the high pressure you now find yourself. Your body starts to have more nitrogen dissolved into your tissues, again until it reaches an equilibrium with the external pressure of nitrogen around you.

If you stay on a dive long enough, you will reach some maximum amount of saturation, at which point you will not have a net increase of nitrogen in your body. Staying longer does not result in more nitrogen being dissolved in your tissues. Usually dives are nowhere near so long.

Once you start to ascend, the nitrogen in your body starts to leave once you reach a depth where the pressure of nitrogen in your body exceeds the pressure of nitrogen in the gas your are breathing.

If you keep the pressure difference between the pressure of dissolved nitrogen in your body and the pressure of nitrogen in your breathing gas to a low enough level then your body can handle it just fine. But if the pressure difference between the pressure of dissolved nitrogen in your body and the pressure of nitrogen in your breathing gas is too great, nitrogen will leave your tissues so rapidly that it forms gas phase bubbles which can cause symptoms. These symptoms are “the bends”.

In Steven Gould’s novel Exo, the protagonist has to pre-breathe oxygen for about 45 minutes before each teleport to high altitude or orbit, in order to make sure nitrogen dissolved in her blood doesn’t cause the bends under the low pressure her space suit provides.

Sci-Fi is reflecting reality there. Astronauts do have to concern themselves with pressure changes and do pre-breathe oxygen before a space walk.

A key concept to remember is that the pressure in question when considering the bends is the individual pressure of each individual gas. Each gas exerts pressure in proportion to its concentration in a gas mixture.

So air at the surface at 1atm pressure consists of roughly 0.21atm partial pressure of O[sub]2[/sub] and 0.79atm partial pressure of N[sub]2[/sub].* If you then stepped through an airlock into a container that holds pure O[sub]2[/sub] at atmospheric pressure, the total pressure would still be 1atm, but composed of 1atm partial pressure of O[sub]2[/sub] and 0atm partial pressure of N[sub]2[/sub]. Nitrogen would start pouring out of your body, perhaps so rapidly as to form bubbles and give you the bends, even though the total pressure of the gas outside your body remained the same.

The International Space Station maintains sea level pressure (1atm = 14.7psi=101.3kPa) for the comfort and safety of the astronauts. But an astronaut’s space suit maintains a much lower pressure (0.32atm=4.7psi=32.4kPa). At a pressure that low, the partial pressure of O[sub]2[/sub] would be only about .07atm if the breathing gas inside the suit was air. That is not enough to support human life. So the space suit uses 100% O[sub]2[/sub]. That means an even greater partial pressure of N[sub]2[/sub] difference, thus the need to pre-breathe O[sub]2[/sub]

*rounded and ignoring minor gasses.

Two things missing from this discussion:

  1. Oxygen is absorbed by chemical means into blood and is therefore not limited by Henry’s law (I.e. A lot of it can be absorbed by the red blood cells). Nitrogen absorption is purely based on Henry’s law (I.e. Vapor pressure which changes as the total pressure changes).

  2. The volume of Nitrogen when released from a dissolved state goes up approximately 500 times or more. A teaspoon of Nitrogen in blood under pressure will become a gallon when the pressure is released.

I mentioned this in reply #2.

What do you mean by “volume” of nitrogen in a dissolved state? Are you referring to the volume of the liquid in which the nitrogen is dissolved? I’m not sure that’s terribly relevant; the symptoms of the bends is caused by the fact that there are bubbles in the blood, not by an increase in volume.