The whole point of a lung
Every breath you take serves a single purpose: to bring fresh air close enough to your blood that oxygen can move in and carbon dioxide can move out. This swapping of gases is called gas exchange, and the part of it that happens in the lungs — air to blood — is external respiration. The airways that carry air down toward the lungs are just plumbing; the real work happens at the very end of the line.
At the end of the line sits the alveolus — a tiny, rounded air sac. You have roughly 300 million of them. Clustered together they look like bunches of grapes, and the whole functional cluster fed by one small airway is called an acinus. If you could spread all your alveoli out flat, they would cover an area about the size of a tennis court, all packed inside your chest.
The thinnest wall in the body
Wrapped tightly around each alveolus is a mesh of tiny blood vessels. The barrier separating air from blood is the alveolar–capillary membrane, and it is extraordinarily thin — often less than a thousandth of a millimetre, thinner than a sheet of tissue paper. It is built from just three layers: the flat alveolar lining cell, a fused shared basement membrane, and the capillary wall.
Thinness matters because gases move across by diffusion — they drift from where they are crowded to where they are sparse, with no pumping required. The thinner the wall, the faster oxygen and carbon dioxide can cross. Many lung diseases work by thickening, scarring, or flooding this membrane, which is why understanding it explains so much of pulmonology.
Keeping the sacs open
A wet, rounded sac wants to collapse, because the water lining it pulls inward (surface tension). To stop this, scattered among the lining cells are type II pneumocytes, which manufacture surfactant — a soapy substance that lowers surface tension and keeps alveoli from caving in at the end of each breath. Without it, breathing would be exhausting and tiny sacs would snap shut.