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How does exchange of gases take place in the alveoli?

Class 11 BiologyNCERT chapter: Breathing and Exchange of Gasesइसे हिंदी में पढ़ें

Exam-ready answer

In the alveoli, oxygen and carbon dioxide are exchanged by simple diffusion, driven by differences in their partial pressures. pO₂ is higher in alveolar air (104 mm Hg) than in deoxygenated blood (40 mm Hg), so O₂ enters the blood. pCO₂ is higher in deoxygenated blood (45 mm Hg) than in alveoli (40 mm Hg), so CO₂ diffuses out.

Explanation

Alveoli are the primary sites of gas exchange; exchange also occurs between blood and tissues. It takes place by simple diffusion, mainly based on pressure or concentration gradients. The solubility of the gases and the thickness of the membranes involved also affect the rate of diffusion. Partial pressure is the pressure contributed by an individual gas in a mixture of gases, written pO₂ for oxygen and pCO₂ for carbon dioxide.

Alveolar air has a pO₂ of 104 mm Hg, while the deoxygenated blood arriving in the alveolar capillaries has a pO₂ of only 40 mm Hg. Oxygen therefore diffuses from the alveoli into the blood. For CO₂ the gradient runs the other way: deoxygenated blood has a pCO₂ of 45 mm Hg and alveolar air 40 mm Hg, so CO₂ moves from the blood into the alveoli and is breathed out.

The diffusion membrane is very thin and has three layers: the thin squamous epithelium of the alveoli, the endothelium of the alveolar capillaries, and the basement substance between them. Its total thickness is much less than a millimetre. CO₂ is 20 to 25 times more soluble than O₂, so far more CO₂ can diffuse through the membrane per unit difference in partial pressure.

Points to remember

  1. Site: alveoli are the primary site; exchange occurs by simple diffusion.
  2. Factors: partial pressure gradient, solubility of the gases and thickness of the diffusion membrane.
  3. Alveoli: pO₂ 104, pCO₂ 40 mm Hg; deoxygenated blood: pO₂ 40, pCO₂ 45 mm Hg.
  4. The solubility of CO₂ is 20–25 times higher than that of O₂.
  5. Diffusion membrane: alveolar squamous epithelium + capillary endothelium + basement substance; much less than 1 mm thick.

Exam tip

Make a small table of pO₂ and pCO₂ values for atmospheric air, alveoli, deoxygenated blood, oxygenated blood and tissues, and draw the alveolus–capillary diagram with arrows.

Questions students also ask

What happens to the gases at the tissue level?
In tissues, pO₂ is about 40 mm Hg and pCO₂ about 45 mm Hg. Oxygenated blood arriving there has a pO₂ of 95 and a pCO₂ of 40 mm Hg, so O₂ diffuses from blood into the tissue cells and CO₂ diffuses from the tissues into blood.
Why does CO₂ diffuse easily despite a small pressure gradient?
The pCO₂ difference between deoxygenated blood and alveolar air is only about 5 mm Hg. However, CO₂ is 20 to 25 times more soluble than O₂, so the amount of CO₂ that can diffuse through the membrane per unit partial pressure difference is much higher.
What is partial pressure?
Partial pressure is the pressure contributed by one gas in a mixture of gases. It is written as pO₂ for oxygen and pCO₂ for carbon dioxide. Gases always diffuse from a region of higher partial pressure to a region of lower partial pressure.

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