Keep your leg at a 90-degree angle and your knee and foot at the same height. Avoid rotating your spine at any point during the exercise. Pause at the top for two seconds before lowering your knee to the floor. Repeat 15-25 times before performing the exercises with the other leg.
Inhaled air is about 20-percent oxygen. Exhaled air is about 15-percent oxygen. Therefore, about 5-percent of breathed air is consumed in each breath. That air is converted to carbon dioxide. So, as far as how much air is actually used, human beings take in about 550 liters of pure oxygen per day. A person who is exercising uses a lot more oxygen than that.
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.