While you are breathing into the plastic bag, what happens to the levels of oxygen in the bag?

Well, I’m not so sure about a plastic bag; but my boyfriend and I got bored one time don’t ask… and we started just breating the air back and forth between eachother without taking in outside air. It felt really weird! And I got light-headed like WOAH hehe :)

The oxygen levels drop inside the bag drop and your body starts to breath in the Co2 that you already exhaled. Eventually it builds up inside your body and you pass out.

thats why when some when they freak out, or hyperventilate they breathe into a paper bag. you breathe in the carbon dioxide which will make you tired, and call you down.

The carbon dioxide level in the bag builds up of course, as you are not letting fresh air in, and you are absorbing any oxygen that was in there in the first place into your bloodstream and giving out C02, so the C02 level in the bag builds up.
This is the reason that people breathe in and out of paper bags when hyperventilating as the C02 has an effect on reducing their heart rate and calming them (I can’t remember the exact science as to why, but it does).

@nimarka1

The breathing into the paper bag thing isn’t for making someone tired, it’s to make sure their oxygen levels don’t go too high while hyperventilating. I guess getting tired is a desired side effect most of the time, though…

Two of you said this, am I wrong? I thought hyperventilating was dangerous because of the oxygen buildup. Breathing into a paper bag isn’t supposed to be used as a sedative anymore than holding your breath is, right? That doesn’t sound healthy.

@zophu right on. When people hyperventilate, the O2 levels go too high and can cause spasms that will increase the level of anxiety and in turn worsen the hyperventilating.

When you hyperventilate your carbon dioxide level goes down, which significantly affects blood pH and other body chemistry (respiratory alkalosis), which might cause muscle spasms, as well as decreasing blood flow to the brain, which makes you feel lightheaded.

Re-breathing in a paper bag restores CO2 levels. While it’s true that very high CO2 levels can cause sleepiness, that doesn’t come into play here.

As the lungs lose some of their carbon dioxide through hyperventilation, the volume difference is made up by slightly higher oxygen levels. Remember that most inhaled and exhaled gas is (inert) nitrogen.

With continued rebreathing you will suffer from both hypoxia, as oxygen is extracted by the lungs & not replenished, and hypercarbia, as CO2 builds up because it has nowhere to go.