So,
SWIM has heard multiple things about re-breathing
nitrous. He's heard that after a while, a SWIMmer is just breathing CO2, that the nitrous is depleted, that it expends the O2 levels more rapidly, ect. Now, SWIM has some background in biology in chemistry, so this got him to thinking: What actually happens, and what are the mechanisms behind it?
The first thought SWIM had was "Well, it makes sense.
N2O is depleted in the balloon and replaced by CO2, just like in normal respiration. But, a problem arises. To SWIM's knowledge, N2O isn't capable of
donating it's oxygen to biological processes (becoming N2 and the oxygen going on to become CO2 later.) Otherwise, a SWIMmer wouldn't become hypoxic. (He could be wrong on this and there could be an internal metabolism for N2O, but he couldn't find any references.) So, at least to some degree, N2O must be maintained during re-breathing. But what about CO2 levels, or more importantly the concentration of CO2 (Shortened to [CO2]).
Well, gas exchange in the lungs is regulated by relative concentration. Under normal respiratory conditions, intake air has ~.1% CO2, and exhaled air has ~4% CO2. So, one would think that as the balloon is re-breathed, the concentration of CO2 would raise 4% each breath, right? But the problem is this: Once the [CO2] in the balloon is greater than the [CO2] in the lungs, the CO2 in the balloon would actually move back into the blood, until the two levels reach an equilibrium. So, the levels of CO2 in the balloon can't actually get significantly above 4%.
So far, SWIM and friends have established that the total N2O is (relatively) conserved, and [CO2] can't rise above 4%. So, theoretically, this would mean that no matter how many times one re-breathed the same nitrous, it would have the effects of a 96% mixture. If that were the case, one could stay elevated off of the same balloon for an entire night, in theory. There is clearly something going on here that prevents this, as any SWIMmer will say that eventually, re-breathing reduces the effectiveness (which must be measured in % N2O in the mixture) of the balloon.
So, after much deliberation, SWIM realized that what must cause this effect is the residual air remaining in the lungs before the first inhalation of nitrous takes place. SWIM looked this number up to be ~1.2L of gas in an average adult male. So, this is the dilution factor each time the balloon comes up for another "hit."
Why this post, what value does it add? Well, partly, SWIM was bored and wanted to think through why things acted the way they did. But the practical value is this: If a SWIMmer were to clear their lungs as completely as possible before each round of inhalations, and exhale fully at the end, it is very reasonable for one to get multiple balloons worth of effect off of one balloon.
Additionally, the number of "in and out" breaths taken per cycle (A cycle being considered the time when a SWIMmer is breathing only balloon air, and no room air) would have no effect on the total levels of nitrous in the balloon, and therefore would not reduce efficiency of the nitrous process.
Finally, larger balloons would have more "buffer capacity" against diluting past the critical point where nitrous levels in the blood would fall (SWIM believes there is a certain percentage where not as much will enter the bloodstream, but according to SWIM's understanding of gasses and diffusion, until that level is reached, the total effect would be the same, regardless if it were 90% N2O or 70% N2O).
This leads to the most efficient (or, most bang for SWIyour buck) way to enjoy nitrous: Use large balloons, exhale fully before taking the nitrous in, and take as many re-breaths per cycle as possible without passing out (because then, SWIY will let go of the balloon and lose all of the still-valuable product).
SWIM is curious of other opinions on the scientific theory behind this, or arguments for why things would work differently.
