How is external respiration best defined?

• A. Gas exchange between the blood and tissues
• B. Gas exchange between the alveoli and blood
• C. Exchange of gases in the airway
• D. Gas transfer in the lungs only

Answer: (B)

External respiration is best defined as the process of gas exchange between the alveoli and blood. This physiological function occurs in the lungs, where oxygen from the inhaled air diffuses across the alveolar membrane into the bloodstream, while carbon dioxide diffuses from the blood into the alveoli to be exhaled. This exchange is crucial for maintaining proper oxygen levels in the blood and for the removal of carbon dioxide, which is a waste product of metabolism.

The other options do not capture this specific aspect of external respiration. For instance, gas exchange between the blood and tissues refers to internal respiration, which occurs at the cellular level and involves the delivery of oxygen and the uptake of carbon dioxide. The term "exchange of gases in the airway" is overly broad and does not accurately describe the location or the process involved in external respiration. Lastly, stating that gas transfer occurs in the lungs only leaves out the vital step that happens at the alveolar level, which is central to the definition of external respiration. Thus, the chosen definition emphasizes the correct site and mechanism of gas exchange that characterizes external respiration.