Pulmonary alveoli are the tiny sacs inside the lungs that fill with air with each breath and facilitate the exchange of gas with the blood. According to HowStuffWorks, the oxygen concentration in the alveoli is high, relative to that of the surrounding tissues, which permits easy passage to the blood.
With each breath, air from outside the body is pulled down the trachea and into the lungs through a branching network of pipes called bronchioles. These bronchioles get progressively smaller until they terminate in the alveolar sacs. The sacs have very thin walls, and they are heavily vascularized, according to HowStuffWorks. Their thin walls permit oxygen to cross over into the blood that is delivered by the pulmonary arteries and permits waste gasses like carbon dioxide to escape into the air in the sac.
Once the exchange takes place, a process that lasts just fractions of a second per cycle, the waste air is expelled with exhalation and the oxygen-rich blood is free to pass back to the heart via the pulmonary veins. This gas-exchange process in the lungs takes place exclusively within the alveoli and is known as external respiration, as opposed to the internal respiration that takes place across the membrane of individual cells, notes HowStuffWorks.