Compare composition of air breathed out and in. 

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orchid101 | College Teacher | (Level 1) Associate Educator

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The air reaches the alveoli of the lungs during the inspiration. The atmospheric air contains oxygen 20.9%, carbon dioxide .04%, nitrogen and other inert gases 79% and water vapour variable. During the expiration we exhale roughly 78% nitrogen, 15 to 18% oxygen and 5% carbon dioxide. During respiration the lungs and the respiratory tract are never empty of air. Instead, there is a tidal volume of air(about 500ml). The respiratory gases such as oxygen and carbon dioxide move freely by the the process of diffusion. The process of diffusion is directly proportional to the pressure caused by the gas alone.

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llltkl | College Teacher | (Level 3) Valedictorian

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When we breathe air in, air is sucked into our lungs. The oxygen molecules pass from the air in the lungs into the blood by diffusion, as the concentration of oxygen is higher in the air and the alveoli than in the blood. The concentration of carbon dioxide is higher in the blood than in the alveoli and capillaries, so the two gases move across the walls of the alveoli and capillaries in the opposite direction. As a result, the percentage of oxygen dips while that of CO2, and water vapour rises in the exhaled air. This phenomenon of passage of gases between the air inside the alveoli, and the blood is called gas exchange. Approximate volume percentage values of major gases in these two streams of air are as follows:

                          O2         N2        CO2   inert gases  water vapour


Inhaled air          20.9      78       0.04     0.9         0.001 to 5

Exhaled air         17        78        4          0.9          saturated