Anatomy PhysiologyOxygen Crosses Thin Alveoli Walls by Diffusion
Medi stands at the entrance of a grape-cluster-shaped air sac deep inside a lung, holding a glowing oxygen molecule and pointing to the paper-thin wall where tiny capillaries wrap around it like a net.
- Identify the alveoli as the site of gas exchange in the lungs.
- Explain how diffusion moves oxygen and carbon dioxide across the alveolar wall.
- Compare the concentration of oxygen and carbon dioxide on each side of the alveolar membrane.
- Predict the direction gases will move when concentration gradients change.
- Describe two structural features of alveoli that make gas exchange efficient.
Key terms
- Alveolus
- A tiny air sac in the lung where gas exchange with the blood occurs.
- Diffusion
- The passive movement of particles from high to low concentration.
- Concentration gradient
- A difference in the amount of a substance between two regions.
- Capillary
- A microscopic blood vessel with walls thin enough for gases to cross.
- Surface area
- The total exposed area available for gas exchange to take place.
Diffusion Is Passive, Not Pumped
The most important idea in gas exchange is that no energy or pumping moves the gases across the alveolar wall. Diffusion happens because randomly moving particles naturally spread out, so over time they drift from a crowded region into an emptier one until both sides even out. Oxygen is crowded in freshly breathed air and scarce in returning blood, so it diffuses into the blood. Carbon dioxide is crowded in the blood and scarce in the alveolus, so it diffuses outward. The heart only moves the blood; it never pushes individual gas molecules across the wall.
Structure Built for Speed
Two structural features make alveolar diffusion incredibly fast. First, the wall between air and blood is only a few cell layers thick, so each gas molecule travels a tiny distance, and diffusion speeds up sharply when the distance is short. Second, the lungs pack roughly 480 million alveoli into a surface area near 70 square metres, so millions of exchanges happen at the same instant. Together, a short distance and a vast surface area let your lungs load enough oxygen and dump enough carbon dioxide to keep every cell supplied even during hard exercise.
Worked examples
Predict the direction each gas diffuses at the alveolus
- Note that freshly inhaled alveolar air is high in oxygen, while returning blood is low in oxygen.
- Diffusion always moves a gas from high to low concentration, so oxygen moves from alveolus into blood.
- Returning blood is high in carbon dioxide, while the alveolus is low in carbon dioxide.
- Therefore carbon dioxide diffuses the opposite way, from blood into the alveolus to be exhaled.
Answer: Oxygen diffuses into the blood and carbon dioxide diffuses out of the blood, each down its own gradient.
Activity
Drag each gas molecule to the correct side of the alveolar wall to show the direction it diffuses during gas exchange. The arrow is a reference tool showing the direction gases diffuse — from high to low concentration.
Practice
Predict what happens to oxygen diffusion if the alveolar wall becomes thicker due to disease.
Explain why having millions of tiny alveoli works better than having two large lung balloons.
Common mistakes to avoid
- The heart pumps gases across the wallGases cross by passive diffusion down their gradients; the heart only moves the blood.
- Oxygen and carbon dioxide move the same directionThey diffuse in opposite directions, since each follows its own concentration gradient.
Check your understanding
During gas exchange in the lungs, which direction does oxygen move?
Why does carbon dioxide move from the blood into the alveolus during gas exchange?
Which structural feature of alveoli MOST directly speeds up the rate of diffusion across the alveolar wall?
Recap
In the alveoli, oxygen and carbon dioxide cross a paper-thin wall by passive diffusion down their concentration gradients, and a huge surface area plus a short distance make the exchange fast enough to supply every cell.
Reflect
How would gas exchange change if your alveolar walls were as thick as the wall of an artery?