Which of the following is a component of the respiratory system?

• A. Alveoli
• B. Kidneys
• C. Stomach
• D. Skin

Answer: (A)

Gas exchange relies on specialized structures in the respiratory system, and alveoli are exactly where that exchange happens. Alveoli are tiny, thin-walled air sacs at the end of the airways in the lungs; oxygen diffuses from the air into the blood, and carbon dioxide diffuses out to be exhaled. Their large surface area and proximity to capillaries make them ideal for gas exchange, and surfactant helps keep them open so they don’t collapse.

The other options belong to different body systems: kidneys are part of the urinary system, the stomach is part of the digestive system, and the skin is part of the integumentary system.