Which gas has a lower partial pressure in the alveoli compared to its level in the blood arriving at the lungs?

In the respiratory system, the partial pressures of gases play a crucial role in the exchange of oxygen and carbon dioxide between the blood and the alveoli. Carbon dioxide is transported through the blood from the tissues to the lungs, where it is then expelled during exhalation. The partial pressure of carbon dioxide in the blood arriving at the lungs is typically higher than that in the alveoli due to the metabolic activity of the tissues, which produce carbon dioxide as a waste product.

During gas exchange, carbon dioxide diffuses from an area of higher partial pressure in the blood to an area of lower partial pressure in the alveoli. This diffusion continues until equilibrium is reached, allowing for effective removal of carbon dioxide from the bloodstream. In contrast, oxygen has a higher partial pressure in the alveoli than in the blood, enabling it to diffuse into the blood. Nitrogen and helium do not play significant roles in respiration and are not actively exchanged in the lungs.

Thus, carbon dioxide is the gas that has a lower partial pressure in the alveoli compared to its level in the arriving blood, making it the correct answer.