Question
Describe the exchange of gases at the alveoli. How does oxygen from inhaled air get into the blood, and how does CO₂ move in the opposite direction?
Solution — Step by Step
The alveoli are tiny air sacs in the lungs, surrounded by a dense network of pulmonary capillaries. The walls of alveoli are just one cell thick, and so are the capillary walls — giving a total diffusion distance of about 0.2 µm. This ultra-thin barrier is critical for rapid gas exchange.
Each lung contains around 300 million alveoli, giving a total surface area of roughly 70 m² — the size of a singles tennis court.
Gas exchange works entirely by diffusion — passive movement of gases down a concentration gradient. No energy (ATP) is needed.
Fick’s Law tells us the rate of diffusion depends on:
- Surface area (large → faster)
- Concentration gradient (steeper → faster)
- Thickness of membrane (thinner → faster)
- Solubility of the gas (higher → faster)
All four factors are optimised in the alveoli.
The partial pressure of O₂ (pO₂) in fresh alveolar air is about 100 mmHg. Deoxygenated blood arriving at pulmonary capillaries has pO₂ of only 40 mmHg.
Because pO₂ is higher in the alveolus than in the blood, O₂ diffuses across the alveolar membrane → interstitial space → capillary wall → blood plasma → into red blood cells.
Inside RBCs, O₂ binds to haemoglobin to form oxyhaemoglobin: (simplified). This binding keeps pO₂ in the RBC low, maintaining the diffusion gradient and driving more O₂ in.
CO₂ is produced by cellular respiration in tissues. Blood arriving at the lungs has a partial pressure of CO₂ (pCO₂) of about 45 mmHg, while alveolar air has pCO₂ of only 40 mmHg.
CO₂ therefore diffuses from blood into the alveolus and is expelled during exhalation. CO₂ is about 20 times more soluble in water than O₂, so it crosses membranes even faster despite the smaller gradient.
| Gas | Alveolar air (mmHg) | Deoxygenated blood (mmHg) | Direction of net diffusion |
|---|---|---|---|
| O₂ | 100 | 40 | Blood → alveolus ✗ / Alveolus → blood ✓ |
| CO₂ | 40 | 45 | Blood → alveolus ✓ |
Why This Works
This is passive diffusion, not active transport. The lungs don’t “pull” O₂ in — the concentration gradient does all the work. The heart maintains this gradient by constantly delivering deoxygenated blood to the lungs and taking oxygenated blood away.
The fact that haemoglobin “traps” O₂ inside RBCs is crucial. Without it, blood would equilibrate with alveolar pO₂ very quickly and stop absorbing O₂. Haemoglobin gives blood a 70-fold higher capacity to carry O₂ compared to plasma alone.
NEET frequently tests partial pressure values. Memorise: alveolar pO₂ ≈ 100 mmHg, venous pO₂ ≈ 40 mmHg; alveolar pCO₂ ≈ 40 mmHg, venous pCO₂ ≈ 45 mmHg. The direction of diffusion always follows the gradient.
Alternative Method
You can also approach this using the oxyhaemoglobin dissociation curve. At high pO₂ (in lungs), haemoglobin has high affinity for O₂ and loads up (saturation ~98%). At low pO₂ (in tissues), affinity drops and O₂ is unloaded. This S-shaped curve is a favourite NEET graph question.
Common Mistake
Students often say “lungs pump oxygen into blood.” This is incorrect — no pumping occurs at the alveoli. O₂ diffuses passively. Also, do not confuse pulmonary exchange (lungs ↔ blood) with tissue exchange (blood ↔ body cells). The pressure gradients are reversed at tissue level: pO₂ in tissues is ~20 mmHg, lower than blood, so O₂ diffuses out of blood into cells.