What are alveoli?

• A. The membranous air sacs in the lungs
• B. The muscles that control breathing
• C. The membranes surrounding the heart
• D. The airways that conduct air to the lungs

Answer: (A)

Alveoli are the tiny membranous air sacs at the ends of the lung’s airways where gas exchange happens. Their walls are extremely thin and covered with a moist lining and a network of surrounding capillaries, which creates a large surface area for diffusion. Oxygen moves from the air inside the alveoli into the blood, while carbon dioxide moves from the blood into the alveolar air to be exhaled. The walls also contain specialized cells that produce surfactant, a substance that reduces surface tension to keep the tiny sacs from collapsing between breaths. This structure and placement in the lungs make alveoli the key site for turning inhaled air into usable blood oxygen. They are not muscles that drive breathing, nor membranes around the heart, nor the airways that conduct air to the lungs.