Which of the following are tiny air sacs in the lungs where gas exchange occurs?

• A. Air Sacs That Store Air
• B. Blood Vessels Supplying The Lungs
• C. Tiny Air Sacs In The Lungs Where Gas Exchange Occurs
• D. Muscles Used For Coughing

Answer: (C)

Gas exchange in the lungs happens in the alveoli, the tiny air sacs at the ends of the airways. Each alveolus is surrounded by a dense network of capillaries, and its walls are extremely thin, which lets oxygen diffuse from the air into the blood and carbon dioxide diffuse from the blood into the air to be breathed out. The surface area is enormous because there are millions of these sacs, and a film of fluid with surfactant lines the walls to keep them open and support efficient gas transfer.

The other options don’t match the site of gas exchange: air sacs that store air are part of the airway system but not where diffusion occurs; the blood vessels supplying the lungs are essential for transporting gases, but the actual exchange happens across the alveolar walls; muscles used for coughing move air but do not participate in the exchange of gases.