advanced practice nurse a registered nurse having education beyond the basic nursing education and certified by a nationally recognized professional organization in a nursing specialty, or meeting other criteria established by a Board of Nursing.
Quick Answer. The air that makes up Earth's atmosphere contains 20.9 percent oxygen. This is true up to about 70,000 feet above sea level. However, above 10,000 feet, the atmosphere becomes too thin to support human life, even though its proportions do not change much. Continue Reading.
But the crux of the matter is the oxygen-carbon dioxide tradeoff. Your lungs just aren't as good as you think they are at oxygen exchange. Every time you take a breath, which is 21 percent oxygen, your lungs are only able to extract about a fifth of that oxygen. The rest gets breathed right back out again. There wasn't much carbon dioxide in the air you breathed in: unless you are in a failing submarine or spaceship, it's around 0.04 percent. But when you breathe out, it's still pretty low, about 4 percent. There's not a huge difference between inhaled and exhaled air, chemically speaking.