Oxygen

Oxygen is the third-most abundant element in the universe, after hydrogen and helium. Naturally, oxygen is continuously replenished by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide. Since oxygen is inhaled during the process of breathing, it commonly used medically, often through a mask, to increase oxygen in the blood. An application of O2 as a low-pressure breathing gas is also in modern space suits.