The substance that humans breathe in to survive is oxygen, a vital component of the air we inhale. Oxygen is crucial for the process of cellular respiration, where cells convert nutrients into energy by utilizing oxygen to metabolize glucose. The atmosphere consists of approximately 21% oxygen, which is essential for sustaining life. Humans inhale oxygen through their lungs, where it is absorbed into the bloodstream and transported to cells throughout the body. Adequate oxygen levels are essential for maintaining metabolic processes and overall health.