Which system is responsible for gas exchange between the body and the environment, including structures such as the lungs?

• A. Digestive System
• B. Nervous System
• C. Endocrine System
• D. Respiratory System

Answer: (D)

Gas exchange between the body and the environment is carried out by the respiratory system. Its main players—the lungs, airways, and alveoli—are specialized to bring in oxygen and remove carbon dioxide. Oxygen diffuses from the air in the alveoli into the blood because its partial pressure is higher there, while carbon dioxide diffuses from the blood into the alveolar air to be exhaled, driven by differences in partial pressures and facilitated by the large surface area and thin, moist membrane of the alveolar-capillary barrier. The respiratory system works in tandem with the circulatory system to deliver oxygen to tissues and remove carbon dioxide. Other systems, like the digestive, nervous, and endocrine systems, handle digestion, signaling, and hormonal regulation, respectively, but they do not perform the primary gas exchange with the environment.