what gas do we need to breathe in

You primarily breathe oxygen-containing air. The key gas required for life is oxygen (O2), which cells use to produce energy, and carbon dioxide (CO2) is the waste product that must be expelled. In ordinary air, you’re inhaling about 21% oxygen, with roughly 78% nitrogen as a major filler, and trace amounts of argon, carbon dioxide (about 0.04%), and other gases. Exhaled air has less oxygen and more carbon dioxide as the body's cells consume O2 and release CO2. If you’d like, I can break down how oxygen is taken up in the lungs, carried by blood, and used by tissues, or explain how atmospheric composition can vary with altitude or in certain environments.