Which of the following best describes external respiration?

• A. Movement of gases in the digestive system
• B. Exchange of air in the trachea
• C. Gas exchange occurring in the lungs
• D. Transfer of oxygen between the heart and tissues

Answer: (C)

External respiration refers specifically to the process of gas exchange that occurs in the lungs. This process involves the intake of oxygen from the atmosphere into the alveoli, where it diffuses across the alveolar-capillary membrane into the bloodstream. Simultaneously, carbon dioxide, a waste product generated by metabolism in the tissues, diffuses from the blood into the alveoli to be exhaled.

This distinction is crucial in physiology, as external respiration is primarily concerned with how oxygen enters the body and carbon dioxide is expelled, exclusively focused on the lung's function. Other options, such as gas exchange in tissues or processes in the digestive system, pertain to other physiological mechanisms and do not accurately depict external respiration in the context of ventilation and gas exchange.