Is it true that not all gas is exchanged with each breath?

• A. Yes, it is true.
• B. No, it is false.
• C. Only in certain conditions.
• D. It depends on the lung capacity.

Answer: (A)

The correct answer acknowledges that not all the gas in the lungs is exchanged with each breath. During normal breathing, only a portion of the air in the lungs participates in gas exchange. This phenomenon occurs because the lungs contain a mixture of fresh air and residual air that remains in the alveoli after exhaling. The residual volume is the air that cannot be expelled and is crucial for maintaining alveolar pressure and facilitating gas exchange over time.

In a typical breath, only the tidal volume, which is the amount of air inhaled or exhaled in one breath, is exchanged. The remaining volume in the lungs includes old air that has already undergone gas exchange and has a different composition than the inhaled air. This means that while fresh oxygen enters the lungs, not all of the contained air is replaced with new air during a single breathing cycle, ensuring that oxygen and carbon dioxide are continually exchanged through the respiratory process.

This fact is vital for understanding lung function, ventilation efficiency, and the overall mechanics of respiration.