The reason inhaled air and exhaled air differ in oxygen and carbon dioxide is due to gas exchange occurring in the lungs and the ongoing cellular respiration in body tissues.
- Oxygen uptake: When air is inhaled, it contains about 21% oxygen. In the alveoli, oxygen diffuses across the thin respiratory membranes into the blood, where it binds to hemoglobin and is transported to cells. Only a portion of the inhaled oxygen is absorbed by tissues during each breath, so the oxygen content of exhaled air is lower than that of inhaled air.
- Carbon dioxide release: Carbon dioxide is produced as a waste product of cellular metabolism. It diffuses from the blood into the alveoli to be exhaled. Because this CO2 is continually being produced by tissues, exhaled air contains a higher concentration of CO2 than inhaled air.
- Water vapor and temperature: Exhaled air also carries more water vapor and is warmer than inhaled air, because the air is humidified and warmed as it travels through the respiratory tract.
Typical approximate compositions:
- Inhaled air: about 21% oxygen, 0.04% carbon dioxide, ~78% nitrogen, with small amounts of other gases.
- Exhaled air: lower oxygen content (roughly 16% O2) and higher carbon dioxide content (roughly 4% CO2), plus increased water vapor and a slightly higher temperature.
If you’re studying this for exams, the core idea is: oxygen is consumed by body tissues, carbon dioxide is produced by tissues, and the lungs exchange these gases so exhaled air has less O2 and more CO2 than inhaled air.
