0.21
Explanation
Breathing involves the process of inhaling atmospheric air and exhaling air modified by gas exchange in the lungs. The composition of inhaled air corresponds to the standard composition of the atmosphere.Analysis of Options:
- 0.16 is incorrect: 16% represents the approximate percentage of oxygen in exhaled air. During respiration, oxygen is absorbed by the blood in the alveoli, reducing its concentration in the air leaving the lungs.
- 0.21 is correct: Inhaled air consists of approximately 21% Oxygen, 78% Nitrogen, and small amounts of other gases (including Argon and Carbon Dioxide). This matches the composition of the atmosphere.
- 0.0004 is incorrect: 0.04% represents the approximate percentage of Carbon Dioxide in inhaled air.
- 0.04 is incorrect: 4% represents the approximate percentage of Carbon Dioxide in exhaled air, which increases from 0.04% as a result of metabolic waste being expelled from the body.
Key Takeaway: Inhaled air contains approximately 21% Oxygen and 0.04% Carbon Dioxide, whereas exhaled air contains approximately 16% Oxygen and 4% Carbon Dioxide.