Which gas is expelled from the body by the respiratory system?

The gas that is expelled from the body by the respiratory system is carbon dioxide. When we breathe in, our bodies take in oxygen, which is then transported to cells for use in cellular respiration, a process that produces energy. During this process, carbon dioxide is generated as a byproduct.

The primary function of the respiratory system is to facilitate the exchange of gases. As blood returns to the lungs after delivering oxygen to tissues, it carries carbon dioxide back from the body. In the lungs, oxygen from the air is absorbed into the bloodstream, and at the same time, carbon dioxide is released from the blood into the lungs to be breathed out. This exchange is crucial for maintaining the proper balance of gases in the body and for sustaining life.

Oxygen is the gas we inhale, carbon monoxide can be a harmful pollutant that may enter the body but is not typically expelled by normal respiratory function, and nitrogen constitutes a significant portion of the air we breathe but is largely inert and does not undergo gas exchange in the body. Therefore, carbon dioxide is the correct answer as it is the waste product from cellular metabolism that the respiratory system helps to expel.