Adult human breath about 500 milliliter air with each breath, 14 times a minute makes it to 7 liters per min or 420 liters per hour and 10080 liters per day. This huge volume of air is exchanged between body and atmosphere through respiratory system which serves to provide oxygen and eliminate carbon dioxide from the body. This huge exchange of gas cannot be accomplished through skin surface in higher animals. Respiratory system provides a closer contact of air with blood across which gases can diffuse easily. Respiratory system is essential for providing oxygen continuously to the body cells and elimination of waste carbon dioxide. Without oxygen, the animal cannot survive more than few minutes.
At rest, most people breathe in and out 10-15 times each minute, or an average of 20,000 times each day. After any great activity, both breathing rate and breathing volume can increase four times or even more. More than 300 million alveoli in the lungs provide a huge surface area for taking in oxygen. Oxygen is so important that most people have trouble holding their breath for even one minute. Champion breath-holders can go without fresh oxygen for eight minutes or more.
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