What are alveoli?

• A. Blood vessels in the lung
• B. Small thin-walled air-containing compartments of the lung that are typically arranged in clusters that give the tissue a honeycomb appearance and expand its surface area for oxygen exchange
• C. Types of air filtration devices
• D. The air passageways in the throat

Answer: (B)

Alveoli are the tiny, thin-walled air-containing sacs at the ends of the airways where gas exchange happens. They’re clustered in a honeycomb-like arrangement, which dramatically increases the surface area available for oxygen to move into the blood and for carbon dioxide to leave it. The walls are extremely thin and are wrapped in a dense capillary network, so diffusion can occur efficiently. Surfactant produced by specialized cells reduces surface tension, helping keep the alveoli from collapsing and maintaining gas-exchange efficiency. So, the alveoli are the site of this crucial exchange, not blood vessels, filtration devices, or airway passages in the throat.