Which gas is exchanged for oxygen in the lungs?

The correct choice, carbon dioxide, plays a critical role in the respiratory process. In the lungs, oxygen is inhaled and transported into the bloodstream, where it binds to hemoglobin in red blood cells. Simultaneously, carbon dioxide, which is a waste product of metabolism generated by the body's cells, is carried back to the lungs from the bloodstream. Here, it is expelled from the body when we exhale.

This gas exchange occurs in the alveoli, the tiny air sacs in the lungs where oxygen and carbon dioxide diffusion happen. Carbon dioxide has a higher concentration in the blood than in the air within the alveoli, leading to its transfer out of the blood and into the alveolar air. This process ensures that the body maintains a proper balance of gases, facilitating respiration and maintaining overall homeostasis.

Other gases presented have different roles: nitrogen is largely inert and does not undergo significant exchange in the lungs, carbon monoxide is a toxic gas that can bind to hemoglobin, inhibiting oxygen transport, and ozone is a pollutant that can be harmful to respiratory health but is not part of the normal gas exchange process in the lungs. Thus, carbon dioxide is distinctly recognized as the gas exchanged for oxygen in the lungs during respiration.