How does the air we inhale compare to the air we exhale?
A. The air we inhale has the same percentage of oxygen as the air we exhale.
B. The air we inhale has a greater percentage of oxygen than the air we exhale.
C. The air we inhale has the same percentage of carbon dioxide as the air we exhale.
D. The air we inhale has a greater percentage of carbon dioxide than the air we exhale.

B. The air we inhale has a greater percentage of oxygen than the air we exhale.

Explanation:

First, inhaled air contains about 21% oxygen. Some proportion of this oxygen is used to oxygenate the blood in the alveoli of the lungs for eventual use in cellular respiration. Hence, the proportion of oxygen in exhaled air will be significantly lesser than the proportion of oxygen in inhaled air. Indeed, exhaled air contains about 16% oxygen.

On the contrary, the air we inhale has a lesser percentage of carbon dioxide (0.04%) than the air we exhale (4.4%) due to the additional carbon dioxide produced as a byproduct of cellular respiration—which must be excreted by the lungs.

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