Which of the following statements regarding normal gas exchange in the lungs is correct?

• A. The actual exchange of oxygen and carbon dioxide occurs in the capillaries.
• B. Oxygen and carbon dioxide diffuse across the alveolar walls and capillaries.
• C. The oxygen content in the alveoli is highest during the exhalation phase.
• D. Blood that returns to the lungs from the body has low levels of carbon dioxide.

Answer: (B)

The statement that oxygen and carbon dioxide diffuse across the alveolar walls and capillaries accurately describes the process of gas exchange in the lungs. During inhalation, oxygen enters the alveoli and is in a higher concentration compared to the surrounding blood in the capillaries. This concentration gradient facilitates the diffusion of oxygen from the alveoli into the blood. Conversely, carbon dioxide, which is present in higher concentrations in the blood returning from the body, diffuses from the blood through the alveolar walls into the alveoli to be exhaled. This dual process of diffusion is crucial for maintaining the necessary balance of oxygen and carbon dioxide in the body.

The other statements do not correctly reflect the physiological processes involved in gas exchange. For instance, while the capillaries play a role in transporting gases, the exchange itself occurs primarily at the alveolar level, where the gas exchange process is facilitated. The assertion regarding oxygen content being highest during exhalation is incorrect, as the oxygen concentration is actually higher during inhalation when fresh air enters the lungs. Additionally, blood returning from the body has high levels of carbon dioxide, since it has just delivered oxygen to the tissues and picked up carbon dioxide as a waste product.