Which body system supplies oxygen and removes carbon dioxide?

• A. Respiratory system
• B. Endocrine system
• C. Circulatory system
• D. Digestive system

Answer: (C)

Gas exchange—the process of taking in oxygen and expelling carbon dioxide—is carried out by the respiratory system. When you breathe in, air travels to the lungs where it reaches tiny air sacs called alveoli. The walls of the alveoli are thin and surrounded by capillaries, so oxygen passes into the blood and carbon dioxide passes out of the blood into the alveoli to be exhaled. This direct exchange of gases happens in the lungs and is the core function of the respiratory system. The circulatory system complements this by transporting the oxygen-rich blood to tissues and bringing carbon dioxide back to the lungs, but the actual gas exchange occurs in the respiratory system.