What is Gas Exchange in the Alveoli?
Gas exchange in the alveoli is the process by which oxygen moves from inhaled air into the blood, and carbon dioxide moves from the blood into the air to be exhaled. It happens across the thin walls of millions of tiny air sacs called alveoli, each wrapped in capillaries. This exchange is essential for supplying every cell in the body with oxygen.
Gas exchange in the alveoli is diffusion of oxygen into the blood and carbon dioxide out of the blood, driven by differences in gas concentration across the thin alveolar-capillary membrane.
- 1↓Air Fills AlveoliInhaled air with high oxygen concentration reaches the alveoli
- 2↓O2 Diffuses into BloodOxygen moves from alveoli (high concentration) into capillary blood (low concentration)
- 3↓CO2 Diffuses into AlveoliCarbon dioxide moves from blood (high concentration) into alveoli (low concentration)
- 4CO2 ExhaledAir rich in carbon dioxide is breathed out
Step-by-step worked examples
The air in the alveoli has about 104 mmHg of oxygen pressure, while blood arriving has about 40 mmHg. Which way does oxygen diffuse?
Gases diffuse from high to low concentration/pressure 104 mmHg (alveoli) > 40 mmHg (blood) → O2 diffuses INTO the blood
Blood arriving at the alveoli carries CO2 at about 45 mmHg, while alveolar air has about 40 mmHg CO2. Which way does CO2 diffuse?
45 mmHg (blood) > 40 mmHg (alveoli) → CO2 diffuses INTO the alveoli to be exhaled
An adult has about 300 million alveoli with a total surface area of roughly 70 m². Why does this matter for gas exchange?
Larger surface area = more room for diffusion to happen at once 70 m² is roughly the size of a tennis court — packed into the chest for fast, efficient gas exchange
Flashcards
Quick quiz
Q1.Where does gas exchange happen?
Q2.Oxygen moves from alveoli to blood because…
Q3.What surrounds each alveolus to allow gas exchange?
Q4.Why is a large alveolar surface area important?
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Common mistakes
Thinking oxygen is 'pumped' into blood. — Correct: Oxygen moves by passive diffusion, not pumping.
Believing gas exchange occurs in the bronchi. — Correct: Gas exchange only happens in the alveoli, where walls are thin enough.
Assuming CO2 and O2 move in the same direction. — Correct: They diffuse in opposite directions based on their own concentration gradients.
Ignoring the role of capillaries. — Correct: Capillaries surrounding alveoli carry the blood that receives O2 and releases CO2.
FAQ
What is gas exchange in the alveoli?
It's the diffusion of oxygen into the blood and carbon dioxide out of the blood across the alveolar walls.
How does gas exchange in the alveoli work?
Gases diffuse from areas of high concentration to low concentration across the thin alveolar-capillary membrane.
What are examples of gas exchange in the alveoli?
Oxygen entering the blood during inhalation and carbon dioxide leaving the blood to be exhaled are both examples.
How is gas exchange efficiency determined?
It depends on surface area, membrane thickness, and the concentration gradient of the gases involved.