alveolus

An alveolus is a tiny air sac at the end of the airway, and it is where the respiratory system does its work. The route in is nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, and then clusters of alveoli, which together give the lungs an enormous surface area folded into a small space. Each sac is one cell thick, and each is wrapped in capillaries, so the distance between inhaled air and blood is two cells — the alveolar wall and the capillary wall. That thinness is what lets gases diffuse: oxygen moves from the air into the blood and binds hemoglobin, and carbon dioxide moves from the blood into the air. Surfactant, a mixture released by the alveolar cells, lowers the surface tension of the film of water lining each sac so that it does not collapse as it empties. In emphysema the walls between alveoli break down, which merges sacs, cuts the surface area and leaves less room for gas exchange.