The need for quantity of oxygen depends on amount of labor the person does. When taken a deep breath lungs can accommodate maximum 3.3 liters of air, which comprises about 21% of oxygen. But in a breath taken while relaxation the quantity of air is not more than an average of 0.5 liter. While relaxing we normally breathe once in three-four seconds. Hence, the rate of breathing comes to 15-20 breaths per minute. Multiply it with 0.5 liter, 21% of which should be oxygen.
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