The gas used in the respiration process of living organisms is oxygen. Oxygen is critical for aerobic respiration, a metabolic process that produces energy by converting glucose and oxygen into carbon dioxide and water. This energy is essential for sustaining cellular functions in most multicellular organisms. Organisms obtain oxygen through various methods, such as breathing in animals or through photosynthesis in plants, which release oxygen as a byproduct. Understanding oxygen's role in respiration is fundamental in biology and ecology.