Final answer:
The gas in the air that keeps us alive is oxygen. It is essential for cellular respiration, which our bodies use to extract energy from food. Without oxygen, which makes up approximately 21% of Earth's atmosphere, humans cannot survive.
Step-by-step explanation:
The gas in the air that keeps us alive is oxygen. Our atmosphere contains a mixture of different gases, with nitrogen making up approximately 78% of this mixture. Despite nitrogen being the most abundant, it is actually oxygen that is crucial for human survival.
We inhale air containing approximately 21% oxygen with each breath; this oxygen is essential for cellular respiration, a process that allows our bodies to convert the nutrients from food into energy. Hence, oxygen is the gas that supports life processes, distinguishes Earth as habitable for humans, and allows us to extract energy from the food we consume.
Breathing, or respiration, is the process by which we draw this life-sustaining air into our lungs. It is critical because without regular intake of oxygen through respiration, our cells would be unable to perform the vital function of cellular respiration, and we would not be able to survive.