Step by Step
1
Dalton's Law — the core principle
In a mixture of gases, each individual gas exerts its own partial pressure independent of the others, and every gas diffuses from an area of high partial pressure to an area of low partial pressure.
2
Gas exchange at the lungs
Alveolar air has a PO₂ of 104 mmHg and PCO₂ of 40 mmHg. Blood arriving at the lungs (from the body) has a PO₂ of only 40 mmHg and PCO₂ of 45 mmHg. Since O₂ is higher in the alveoli than the blood, it diffuses INTO the blood; since CO₂ is higher in the blood than the alveoli, it diffuses OUT into the alveoli to be exhaled.
3
Gas exchange at the tissues — the reverse pattern
Body tissues have low PO₂ (~40 mmHg, since cells are constantly consuming oxygen) and high PCO₂ (~45 mmHg, since cells are constantly producing carbon dioxide). This means O₂ diffuses OUT of the blood into tissues, while CO₂ diffuses INTO the blood from tissues — the exact opposite pattern from what happens at the lungs.
4
Fick's Law — what determines the rate of diffusion
The rate of gas diffusion is proportional to surface area and the concentration difference between the two sides, and inversely proportional to membrane thickness — a larger surface area or bigger concentration gradient speeds up diffusion, while a thicker membrane slows it down.
Applied Walkthrough
1
Blood arrives at the lungs with a low PO₂ (40 mmHg) and relatively high PCO₂ (45 mmHg), having just come from oxygen-depleted tissues throughout the body.
2
Since alveolar air has a much higher PO₂ (104 mmHg) than the arriving blood, oxygen diffuses from the alveoli into the blood, following Dalton's Law's high-to-low diffusion principle.
3
Simultaneously, since the arriving blood has a higher PCO₂ (45) than the alveolar air (40), carbon dioxide diffuses out of the blood into the alveoli, to be exhaled.
4
Once this oxygen-rich, carbon-dioxide-depleted blood reaches body tissues (which have low PO₂ and high PCO₂ due to ongoing cellular metabolism), the entire pattern reverses: oxygen now diffuses OUT of the blood into the tissues, and carbon dioxide diffuses INTO the blood from the tissues.
Exam Application
Exams test whether you can correctly apply Dalton's Law (diffusion from high to low partial pressure) to predict the direction of O₂ and CO₂ movement at both the lungs and the tissues, and whether you understand the factors from Fick's Law that affect diffusion rate.
⚠ Common Trap
The most common trap is forgetting that the direction of gas exchange reverses between the lungs and the tissues — oxygen moves INTO blood at the lungs but OUT of blood at the tissues, while carbon dioxide does the exact opposite in each location.
✓ Quick Self-Check
1. What does Dalton's Law state about gas diffusion?
Each gas exerts its own partial pressure, and diffuses from an area of high partial pressure to an area of low partial pressure.
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2. At the lungs, does oxygen diffuse into or out of the blood?
Into the blood, since alveolar PO₂ is higher than the PO₂ of arriving blood.
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3. At the tissues, does oxygen diffuse into or out of the blood?
Out of the blood, since tissue PO₂ is lower than blood PO₂.
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4. What factors does Fick's Law say affect the rate of gas diffusion?
Surface area, concentration difference, and membrane thickness (rate is proportional to the first two, inversely proportional to the third).
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5. What is the approximate PO₂ of blood leaving the lungs?
About 100 mmHg.
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