The common gas that humans breathe in from the atmosphere is oxygen. Approximately 21% of the Earth's atmosphere consists of oxygen, which is crucial for the survival of most life forms. Humans inhale oxygen through the respiratory system, where it enters the lungs and is absorbed into the bloodstream. Oxygen is vital for cellular respiration, a process where cells use it to produce energy. The cycle is completed when we exhale carbon dioxide, a waste product of cellular metabolism. This exchange of gases is essential for maintaining life.