Gas Exchange
Learn how oxygen and carbon dioxide move between the alveoli and the blood.
What happens at the alveoli
Gas exchange takes place in the alveoli, tiny air sacs surrounded by capillaries. Oxygen diffuses from the air in the alveoli into the blood, while carbon dioxide diffuses from the blood into the alveoli to be exhaled.
Oxygen and carbon dioxide crossing a thin membrane
A dense capillary network surrounds each alveolus, keeping the diffusion distance extremely short.
Exchange surface
Alveoli provide an enormous surface area with very thin walls, ideal for gas diffusion.
Blood supply
A dense network of capillaries surrounds each alveolus, bringing blood close enough for gases to diffuse quickly.
Passive movement
Gases move by diffusion, from an area of higher concentration to an area of lower concentration, without using energy.
Oxygen carrier
Oxygen binds to hemoglobin in red blood cells, which carries it to tissues throughout the body.
Gas exchange relies on a short diffusion distance.
The wall of an alveolus and the wall of a capillary are each extremely thin, allowing gases to diffuse quickly between air and blood.
Oxygen in, carbon dioxide out
As blood passes the alveoli, it picks up oxygen and releases carbon dioxide. This exchange keeps blood oxygenated for delivery to tissues and clears carbon dioxide produced by cellular metabolism.
Why this exchange matters
Every cell in the body depends on a steady oxygen supply and needs carbon dioxide removed. Gas exchange in the lungs is what makes this possible.
Exchange, not storage.
The lungs do not store oxygen in large amounts. Continuous breathing keeps gas exchange happening moment to moment.