How do lungs exchange gases?

Each breath draws air inward because the diaphragm contracts downward and flattens, expanding the chest and lungs until internal air pressure falls below outside pressure. Branching airways carry that air toward tiny exchange spaces. Moving air is ventilation; oxygen and carbon dioxide crossing between air and blood is gas exchange. These connected processes do different jobs.

Air passes through the windpipe and branching airways to tiny air spaces called alveoli. Very thin walls separate them from capillaries, the smallest blood vessels. This provides a large exchange surface with a short crossing distance. Oxygen and carbon dioxide diffuse across it through the random motion of molecules, rather than being pumped through a tiny open hole.

Each gas has a partial pressure, a measure of its tendency to diffuse that allows comparison between alveolar air and gas dissolved in blood. Oxygen's partial pressure is normally higher in alveolar air than in arriving blood, giving net transfer into blood. Carbon dioxide's gradient runs the other way. Breathing refreshes the air while blood flow brings more blood to exchange, helping maintain these differences.

During quiet exhalation, breathing muscles relax and elastic recoil reduces lung volume, pushing air outward. The repeating expansion and recoil refresh air near circulating blood, supporting the gas-specific differences that sustain diffusion. Ventilation moves air in bulk, while gas exchange can continue between breaths as long as air and blood remain in contact.