External respiration primarily occurs where?

• A. In the alveoli of the lungs
• B. In the airways
• C. In the mitochondria
• D. In the bloodstream

Answer: (A)

External respiration is the gas exchange between the air in the lungs and the blood, and it happens across the thin walls of the alveoli where they interface with pulmonary capillaries. Here, oxygen moves from the inhaled air into the blood, and carbon dioxide moves from the blood into the alveolar air to be exhaled. This diffusion is driven by the partial pressure gradients and the large surface area of the alveolar membrane, making the alveoli the primary site of external respiration.

Airways conduct air but do not exchange gases; mitochondria use oxygen inside cells for energy, not for exchanging gases with the air; and the bloodstream transports gases after diffusion, not the place where diffusion with air occurs.