A Thin Boundary Between Air and Blood
An alveolus is a microscopic pocket at the end of an airway. Each is wrapped in tiny blood vessels called capillaries. The wall between the air inside and the blood outside is extremely thin, so gases can cross it by diffusion: molecules move from where they are more concentrated to where they are less concentrated.
After a breath, oxygen is more concentrated in the alveolar air than in the blood arriving from the body. It crosses the wall and enters red blood cells, which carry it onward. Carbon dioxide moves in the opposite direction, from blood into the alveolus, and is removed when you breathe out. The lungs refresh this concentration difference with every breath.
Keeping the Exchange Surface Open
Alveoli must stay open for air to reach their inner surfaces. A substance called surfactant coats them and reduces the pull of surface tension—the force that would otherwise make the moist walls cling together. This helps prevent the sacs from collapsing, especially during exhalation.
Their many branching pockets spread the exchange surface through the lungs while keeping each air-to-blood crossing short. That combination matters: oxygen and carbon dioxide have only a tiny distance to travel, and blood flows past the sacs continuously. In effect, each breath brings fresh air close to a moving stream of blood, letting the lungs exchange gases without mixing the two.
