If the air is only made up of 21% oxygen,why can we breathe?

Question

If the air is only made up of 21% oxygen,why can we breathe?

Answer 1

We don't use very much oxygen, even the air you exhale is still has about 16% oxygen.  In fact, breathing 100% oxygen can actually cause harm over long periods of time.  Our bodies evolved over a long period of time to thrive on exactly the levels of oxygen present in Earth's atmosphere.

Answer 2

because your lungs dont use all the oxygen you inhaile.

Answer 3

Oxygen is highly reactive.  We don't need very much.

Also, we have evolved to live in an environment with about this much oxygen.

The Earth had very little oxygen in the atmosphere originally.  Then plants evolved to use sunlight to create sugars, with a byproduct of filling the atmosphere with oxygen.  It nearly killed everything.  Then organisms figured out how to use it to advantage.

Answer 4

lungs act as a filter

Answer 5

Lungs filter the air. And as said, Oxygen is very subtle, not much is needed. We have adapted to use less and less over time.

Long time ago, plants began to fill the Earth's Atmosphere with Oxygen. This is how it got here.

Answer 6

It's enough oxygen for us. If the amount of oxygen in the air were 50%, things would catch fire *much* easier - the whole world would burn...

Answer 7

Evolution is the answer. If 20%, or thereabouts, oxygen was not enough for complex animals to utilise, we wouldn’t be here to talk about it.  

We, and all animals with lungs, have a massive oxygen absorption surface – the lungs have millions of little “sacs” making the surface area of your lungs about the same as a tennis court.  

Insects breathe through their skins, so only have a tiny oxygen collector system.  That is why they are limited in size.

It is not rocket science.  If it wasn’t so, none of us would be here. Life has developed based on available resources.  

Answer 8

because you dont need 100% oxygen.  

Answer 9

Plant's breath in Carbon dioxide and Exhale Oxygen.

And therer is oxygen lmost everywhere there are plants.

Answer 10

It works by a phenomenon called "partial pressure". Although air is only 21% oxygen, the blood in our lungs has much less oxygen (since it's already been used up by our bodies). Thus, within our lungs, there exists a low partial pressure of oxygen, compared to the partial pressure of oxygen in the air that we breath. The oxygen moves across a gas permeable membrane into our bloodstream in an attempt to equalize the partial pressure of oxygen on both sides.  

This effect is dependant on the air pressure around us.  For instance, astronauts in a space station breath much higher concentrations of oxygen than is present in the air on Earth. The higher concentrations are necessary because of the low air pressure that they live in.  

Answer 11

Great Q that demonstrates how scientific inquiry works!  You assume that we need 100% oxygen to live, discover that air only has about 1/5 oxygen by volume, and wonder how do we do that?  Clearly, your assumption was wrong.

btw, just for the record, several answers stated that the lungs "filter" the air...to the best of my knowledge, the lungs do not filter the air.  The mucus membranes and hair in the nose filter the air, but that has nothing to do with this.

Answer 12

Because we don't need to breathe pure oxygen. We evolved on a planet with a nitrogen/oxygen atmosphere, so that's what we are able to breathe. Had it been something else, we'd have evolved to breathe that. Pure oxygen at full atmospheric pressure is toxic to humans and most other life as well, in addition to being extremely hazardous in terms of the risk of fire.