Tiny sacs in the lungs that exchange oxygen and carbon dioxide with the bloodstream are called what?

Tiny sacs in the lungs that exchange oxygen and carbon dioxide with the bloodstream are called what?

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

Answer: (D)

Gas exchange happens in the tiny sacs called alveoli. Their walls are incredibly thin and surrounded by a dense network of capillaries, creating a large surface area for diffusion. Because the oxygen concentration is higher in the alveolar air than in the blood, O2 diffuses into the blood and binds to hemoglobin. Conversely, carbon dioxide is more concentrated in the blood, so it diffuses into the alveolar air to be exhaled. The thin barrier and moist surface maximize the rate of diffusion, making the exchange efficient. Bronchi and trachea are just conducting airways, not gas-exchange sites, and the diaphragm is a breathing muscle that helps move air in and out but does not participate in the exchange itself.

Gas exchange happens in the tiny sacs called alveoli. Their walls are incredibly thin and surrounded by a dense network of capillaries, creating a large surface area for diffusion. Because the oxygen concentration is higher in the alveolar air than in the blood, O2 diffuses into the blood and binds to hemoglobin. Conversely, carbon dioxide is more concentrated in the blood, so it diffuses into the alveolar air to be exhaled. The thin barrier and moist surface maximize the rate of diffusion, making the exchange efficient.

Bronchi and trachea are just conducting airways, not gas-exchange sites, and the diaphragm is a breathing muscle that helps move air in and out but does not participate in the exchange itself.