Small sacks in the bottom of lungs where respiration takes place?

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

Answer: (A)

Gas exchange occurs in the alveoli, the tiny sacs at the ends of the airways. Their walls are extremely thin and are wrapped in a dense network of capillaries, which lets oxygen diffuse into the blood and carbon dioxide diffuse out into the air to be exhaled. The large total surface area of many alveoli makes this exchange efficient. A thin film and surfactant keep the surfaces moist and prevent collapse, helping keep gas exchange going smoothly. The other structures—trachea and bronchi are the main air passages, and bronchioles are smaller airways that funnel air toward the alveoli but do not perform gas exchange themselves. So the site where respiration really happens is in the alveoli.