Our atmosphere consists of three gases chiefly: Nitrogen, Oxygen and carbon dioxide. Human body needs (directly or indirectly) all three of these. Nitrogen is a key ingredient of proteins and carbon dioxide is utilized by the plants to make food (through the process of photosynthesis). However, it is the oxygen gas that we need directly and most critically. We breathe the gas in and exhale carbon dioxide. Oxygen is necessary for the process of cellular respiration, during which the (ingested) food is broken down into simpler molecules and energy molecules (ATP) are generated. It is interesting to note that we need oxygen all the time, even when we are sleeping or are at rest. The amount of oxygen required is a function of body activity, that is to say, sleeping requiring less energy (and hence oxygen) as compared to running. On an average, an adult human body may use 200-500 ml/min of oxygen, which will translate into about 300-700 lt/day of oxygen.
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