What gas enter the blood from the alveoli or air sacs?

Oxygen is the gas that enters the blood from the alveoli or air sacs in the lungs. During the process of respiration, oxygen diffuses across the thin walls of the alveoli into the bloodstream, where it binds to hemoglobin in red blood cells for transport to tissues throughout the body. At the same time, carbon dioxide, a waste product of metabolism, diffuses from the blood into the alveoli to be exhaled.