In gas exchange, what leaves the bloodstream and enters the alveoli?

In the process of gas exchange in the lungs, carbon dioxide leaves the bloodstream and enters the alveoli. This occurs during the process of respiration. Blood that flows through the pulmonary capillaries has a higher concentration of carbon dioxide compared to the air in the alveoli, allowing for diffusion. As a result, carbon dioxide moves from the blood, where there is a higher partial pressure, to the alveoli, where the partial pressure of carbon dioxide is lower, thus facilitating its removal from the body.

Oxygen is absorbed from the alveoli into the bloodstream during this process, while nitrogen is generally inert and does not participate in gas exchange. Water vapor is present in the air we breathe but is not a primary focus in the exchange of gases involved in respiration. Therefore, the correct choice is that carbon dioxide leaves the bloodstream and enters the alveoli during gas exchange.