external respiration

External respiration is the moment the air you breathe finally meets your blood. It happens deep in the lungs, where a thin film of air sits right beside a thin film of blood, and oxygen and carbon dioxide quietly swap places across the wall between them.

Technically, it is the exchange of gases between the air in the alveoli and the blood in the surrounding pulmonary capillaries, across the alveolar–capillary membrane. Oxygen moves from the alveolar air, where its partial pressure is higher, into the blood; carbon dioxide moves the other way, out of the blood into the air to be exhaled. The whole transfer happens passively, by diffusion down each gas's pressure gradient.

External respiration is contrasted with internal respiration, which is the equivalent exchange out in the tissues. It is called external because it is the body's gas exchange with the outside environment. When this step fails — because the membrane is thickened, alveoli are flooded, or blood flow and air do not match up — blood oxygen falls and the person becomes short of breath.