The gas that is essential for human survival is oxygen. Oxygen is necessary for cellular respiration, a biochemical process wherein cells convert nutrients into energy. This process is vital for sustaining human life, as it enables cells to function properly. Humans inhale oxygen from the atmosphere through their lungs, which then enters the bloodstream and is transported to cells throughout the body. In addition to its importance in respiration, oxygen is also crucial for processes such as combustion and oxidation, making it a fundamental element in both biological and chemical functions.