Tiny sacs of lung tissue specialized for the movement of gases between air and blood.

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Multiple Choice

Tiny sacs of lung tissue specialized for the movement of gases between air and blood.

Explanation:
Gas exchange happens where the air meets the blood, and that occurs in tiny sacs called alveoli. These are at the ends of the airways and are surrounded by a dense network of capillaries. The barrier between air and blood is extremely thin—made up of the alveolar epithelium, the capillary endothelium, and their fused basement membranes—so gases can diffuse quickly. The huge collective surface area of countless alveoli makes diffusion efficient, letting oxygen move from the air into the blood and carbon dioxide move from the blood into the air to be exhaled. Surfactant inside the alveoli lowers surface tension to keep them from collapsing and helps them stay open for gas exchange. In the bigger picture, the other structures are airways or the voice box and don’t provide this specialized thin barrier and vast surface area for gas exchange, which is why alveoli are the correct focus.

Gas exchange happens where the air meets the blood, and that occurs in tiny sacs called alveoli. These are at the ends of the airways and are surrounded by a dense network of capillaries. The barrier between air and blood is extremely thin—made up of the alveolar epithelium, the capillary endothelium, and their fused basement membranes—so gases can diffuse quickly. The huge collective surface area of countless alveoli makes diffusion efficient, letting oxygen move from the air into the blood and carbon dioxide move from the blood into the air to be exhaled. Surfactant inside the alveoli lowers surface tension to keep them from collapsing and helps them stay open for gas exchange. In the bigger picture, the other structures are airways or the voice box and don’t provide this specialized thin barrier and vast surface area for gas exchange, which is why alveoli are the correct focus.

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