What is external respiration?

• A. Exchange of gases between lungs and blood
• B. Diffusion of carbon dioxide into surrounding air
• C. Oxygen diffusion into mitochondria
• D. External respiration occurs in lungs

Answer: (A)

External respiration is the diffusion of gases between the air in the lungs and the blood in the pulmonary capillaries. In the alveoli, oxygen moves from the air into the blood because its partial pressure is higher there, while carbon dioxide moves from the blood into the alveolar air because its partial pressure is higher in the blood. This gas exchange across the thin alveolar membrane is driven by diffusion and is the first step in delivering oxygen to tissues. It’s different from internal respiration, which is the gas exchange between blood and body tissues, and from cellular respiration, which is the use of oxygen inside cells to produce energy. The description that states the exchange of gases between lungs and blood best captures what external respiration is. The other options are incomplete or refer to different processes: CO2 diffusion into surrounding air alone is only part of the exchange, oxygen diffusion into mitochondria describes cellular respiration, and saying external respiration occurs in the lungs points to location rather than the exchange process.