Gas Exchange
Elementary School
Definition
The movement of oxygen into the blood and carbon dioxide out of it, which in humans takes place in the lungs. Oxygen passes into the blood for the body's cells, while carbon dioxide waste is breathed out.
Worked examples
In the alveoli, \(O_2\) diffuses from high concentration in air (21%) to low concentration in blood, while \(CO_2\) diffuses from blood to air.
Gas exchange relies on concentration gradients — each gas moves down its own gradient across the thin alveolar membrane.
A runner breathes faster during exercise to supply more \(O_2\) for cellular respiration and remove the extra \(CO_2\) produced by active muscles.
Increased gas exchange meets the higher metabolic demands of working cells.
Common mistakes
- Gas exchange happens in the trachea or bronchi → Gas exchange occurs in the alveoli (tiny air sacs in the lungs) Airways only transport air; the alveoli have thin walls and capillaries where \(O_2\) and \(CO_2\) actually cross into and out of blood.
- Only oxygen is exchanged during gas exchange → Both \(O_2\) (into blood) and \(CO_2\) (out of blood) are exchanged Gas exchange is a two-way process — carbon dioxide waste must be removed just as oxygen is delivered.
- Blood carries oxygen as dissolved gas only → Most \(O_2\) binds to hemoglobin in red blood cells; only a small amount dissolves in plasma Hemoglobin dramatically increases the blood's oxygen-carrying capacity beyond simple dissolving.
Where you'll use it next
You'll revisit gas exchange when studying cellular respiration, the circulatory system, respiratory diseases like asthma and emphysema, and adaptations to high altitude or exercise in biology and health science.
See also
Reviewed by Pat Cheng, M.Ed. — StudyPug Curriculum Lead · Last updated June 6, 2026