Carbon dioxide increases in expired air from respiration

Steps:

• Inspired air has 0.04% CO2; expired air has ~4% due to exhalation of metabolic CO2.
• Oxygen decreases from ~21% to ~16% in expired air as it's consumed.
• Nitrogen remains constant at ~78% as it's not involved in gas exchange.
• Identify the gas with low inspired percentage (0.04%) rising sharply to 4.14% as CO2.

Why A is correct:

• CO2 is produced by cellular respiration and exhaled, raising its concentration from 0.04% in inspired air to 4.14% in expired air, per respiratory physiology.

Why the others are wrong:

• B: No gas increases from 0.50% to 2.00%; values don't match standard air composition.
• C: Oxygen decreases from 20.80% (inspired) to ~16% (expired), not 12.20%.
• D: Nitrogen is inert and unchanged at 78.50% in both.

Final answer: A