Where blood meets air
Gas exchange happens across the alveolar-capillary membrane, an extraordinarily thin barrier roughly half a micron thick where the air inside an alveolus is separated from blood in a capillary by little more than two flattened cell layers and their shared basement membrane. Oxygen diffuses from the higher concentration in the alveolus into the blood, while carbon dioxide moves the other way, each following its own pressure gradient.
The lungs hold hundreds of millions of alveoli, giving a combined surface area large enough to make this passive diffusion fast and efficient.
Why thinness and surface area matter
Diffusion is quicker across short distances and larger areas, so the membrane's thinness and the huge alveolar surface are what let a red blood cell pick up nearly a full load of oxygen during its brief passage past an alveolus. Diseases that thicken the membrane, flood the air spaces, or destroy alveolar surface impair this exchange and reduce oxygen uptake.
This is general educational information, not medical advice. Breathing difficulty or low oxygen symptoms should be evaluated by a qualified clinician.