Which statement accurately contrasts external and internal respiration?

• A. External respiration is gas exchange between blood and tissues; internal respiration is gas exchange between air and blood in the lungs
• B. External respiration is gas exchange between air and blood in the lungs; internal respiration is gas exchange between blood and tissues
• C. External respiration is cellular respiration in mitochondria; internal respiration is gas exchange in blood
• D. External respiration is gas exchange between air and blood in the lungs; internal respiration is gas exchange between blood and tissues

Answer: (D)

The key idea is where gas exchange occurs. External respiration happens at the lungs, where air and blood exchange gases in the alveolar region—oxygen moves from the air into the blood and carbon dioxide moves from the blood into the air to be exhaled. Internal respiration happens at the tissues, where blood and tissue cells exchange gases—oxygen moves from blood to tissues to support cellular processes, and carbon dioxide moves from tissues into the blood to be carried back to the lungs.

This aligns with the statement that external respiration is gas exchange between air and blood in the lungs, and internal respiration is gas exchange between blood and tissues. It’s important to keep in mind that cellular respiration is a separate metabolic process inside cells that uses oxygen to produce energy and carbon dioxide as a byproduct, not the gas exchange between blood, tissues, and air.