Which of the following gases exits the bloodstream during gas exchange?

During gas exchange in the lungs, carbon dioxide is the primary gas that exits the bloodstream. This process occurs in the alveoli, where oxygen and carbon dioxide are exchanged between the air and the blood. When blood reaches the lungs, it has a higher concentration of carbon dioxide compared to the air in the alveoli. As a result, carbon dioxide diffuses from the blood into the alveoli to be exhaled.

Conversely, oxygen in the alveoli, which is at a higher concentration compared to the blood, diffuses into the bloodstream, where it is then transported to various tissues throughout the body. This dynamic movement of gases—where carbon dioxide leaves the blood and oxygen enters—is a fundamental aspect of respiratory physiology ensuring that the blood has the necessary gases for cellular metabolism and maintaining homeostasis.

In contrast, nitrogen and helium are largely inert and do not participate in the gas exchange process in a physiological sense. They remain in the bloodstream without significant diffusion during normal respiratory function.