What are alveoli?

• A. Air Sacs Where Gas Exchange Occurs
• B. Lining of the Airways
• C. Muscle Groups Supporting Respiration
• D. Blood Vessels That Supply the Lungs

Answer: (A)

Alveoli are the tiny air sacs in the lungs where gas exchange occurs. They sit at the ends of the airways and are surrounded by a dense network of capillaries. The walls of the alveoli and the capillaries form an extremely thin barrier, and the moist surface inside allows oxygen from inhaled air to diffuse into the blood while carbon dioxide moves from the blood into the alveolar air to be exhaled. The vast total surface area of countless alveoli makes gas exchange very efficient. A film of surfactant reduces surface tension, helping keep the alveoli open between breaths. Alveoli are distinct from the lining of the airways (the mucous membranes of the trachea and bronchi), the muscles involved in breathing, and the blood vessels that supply the lungs; they are the actual sites where the exchange of gases happens.