Which body system's primary function includes gas exchange during respiration?

• A. Excretory
• B. Muscular
• C. Circulatory
• D. Respiratory

Answer: (D)

The respiratory system's primary function is gas exchange during respiration. This system is specifically designed to facilitate the intake of oxygen from the atmosphere and the expulsion of carbon dioxide from the body. When air is inhaled, oxygen travels through the respiratory passages—including the trachea and bronchi—down to the alveoli, which are tiny air sacs in the lungs. Here, oxygen diffuses into the bloodstream, where it binds to hemoglobin in red blood cells to be transported throughout the body. Concurrently, carbon dioxide, a waste product of cellular metabolism, diffuses from the blood into the alveoli to be exhaled.

Other body systems, such as the circulatory system, play supportive roles in this process by transporting gases between the lungs and tissues, but they do not perform gas exchange themselves. Similarly, the muscular system is involved in the mechanics of breathing but does not handle gas exchange directly. The excretory system focuses on waste elimination and regulation of body fluids, which is unrelated to the primary function of gas exchange inherent to the respiratory system. Thus, the respiratory system is correctly identified as the key system responsible for this vital function.