The tiny air sacs in the lungs where gas exchange occurs are called __________.

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

Answer: (C)

Gas exchange happens in tiny air sacs called alveoli. They sit at the ends of the airway branches inside the lungs and are wrapped in an extensive network of capillaries. The walls are extremely thin and the air inside is moist, which lets oxygen diffuse from the alveolar air into the blood and carbon dioxide diffuse from the blood into the alveolar air to be exhaled. The vast number of alveoli gives a large surface area, making gas exchange very efficient. A substance called surfactant coats the inner surface to keep the alveoli from collapsing. The other structures don’t perform this gas-exchange function: the larynx is the voice box, the bronchi are the larger airways that lead to the lungs, and the diaphragm is the muscle that helps expand and contract the chest to breathe.