Most internal gas exchanges in the human respiratory tract occur across the capillaries of which structures?

• A. Nose
• B. Bronchi
• C. Alveoli
• D. Trachea

Answer: (C)

Gas exchange happens by diffusion across a very thin barrier between the air in the lungs and the blood. The site where this occurs most extensively is the alveoli, tiny air sacs surrounded by a dense network of capillaries. The walls of the alveoli and the surrounding capillary endothelium form the alveolar-capillary membrane, which is extremely thin to allow gases to pass quickly. Oxygen moves from the alveolar air, where its partial pressure is higher, into the blood, while carbon dioxide moves from the blood, where its partial pressure is higher, into the alveolar air to be exhaled. The vast surface area of all the alveoli and the short diffusion distances make this the most efficient location for internal gas exchange.

The nose, trachea, and bronchi, in contrast, are part of the conducting airways. They warm, humidify, filter, and transport air but have thicker walls and less surface area, so they’re not suited for efficient gas diffusion.