gas exchange

Gas exchange is the whole point of breathing: the swap, at the lungs, of oxygen into the blood and carbon dioxide out of it. Everything else the respiratory system does — drawing air in, branching it down to the alveoli, pumping blood past them — is there to make this one quiet trade possible.

It takes place across the alveolar–capillary membrane, an astonishingly thin barrier between alveolar air and the blood in the capillaries. Oxygen and carbon dioxide each move by passive diffusion, sliding from where their partial pressure is higher to where it is lower. The lungs offer a huge surface area and a very short distance, so the exchange is fast and efficient.

Good gas exchange needs three things to line up: enough fresh air reaching the alveoli (ventilation), enough blood flowing past them (perfusion), and a healthy membrane to cross. When ventilation and perfusion do not match well, or the membrane thickens, oxygen levels fall — a problem at the heart of many lung diseases.