Medicneet Logo

Factors Affecting Diffusion of Gases in the Lungs

Biology · Breathing and Exchange of Gases · NEET

Gases move across the alveoli by simple diffusion, and NCERT says three things control the rate: the partial pressure (concentration) gradient, the solubility of the gas, and the thickness of the diffusion membrane. O2 and CO2 diffuse because each moves from high partial pressure to low partial pressure. Memory hook: "Pressure pushes, Solubility speeds, Thin membrane helps" (P-S-T).
Factors Affecting Diffusion of GasesALVEOLUSpO2 high (104)pCO2 low (40)DIFFUSIONMEMBRANE3 layers,< 1 mm thinBLOODpO2 low (40)pCO2 high (45)O2CO2Rate depends on: pressure gradient + solubility + membrane thickness
O2 and CO2 cross the thin diffusion membrane each along its own partial pressure gradient; the rate is set by the gradient, the gas solubility, and the membrane thickness.

Your doubts, answered

What are the three factors that affect the rate of diffusion of gases?

NCERT lists exactly three: (1) partial pressure / concentration gradient of the gas, (2) solubility of the gas, and (3) thickness of the membranes involved in diffusion. A steeper gradient, higher solubility, and a thinner membrane all make diffusion faster. NEET questions usually ask you to name these three, so learn them as P-S-T.

Why does CO2 diffuse much faster than O2 even though its pressure gradient is smaller?

Because solubility wins here. The pressure gradient for O2 (alveoli to blood) is large, but the gradient for CO2 (blood to alveoli) is small. Yet the solubility of CO2 is 20 to 25 times higher than O2, so the amount of CO2 that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher. This is why a small CO2 gradient still moves plenty of CO2.

Is diffusion of gases in the lungs an active or passive process?

It is passive. Gases are exchanged by simple diffusion, mainly based on the pressure/concentration gradient. No ATP or carrier pump is used. This is a common NEET trap: diffusion of O2 and CO2 needs NO energy, unlike active transport.

How does membrane thickness affect diffusion, and why is the alveolar membrane so thin?

The thinner the membrane, the faster the diffusion. The diffusion membrane has three layers (thin squamous epithelium of alveoli, endothelium of alveolar capillaries, and a basement substance in between), but its total thickness is much less than a millimetre. This tiny thickness keeps the diffusion path short so gases cross quickly.

Which direction do O2 and CO2 diffuse and why?

O2 diffuses from alveoli to blood and blood to tissues, because pO2 is highest in alveoli and lowest in tissues. CO2 diffuses in the opposite direction, from tissues to blood and blood to alveoli. Each gas simply follows its own partial pressure gradient from high to low.

⚠️ The NEET trap
✗ O2 diffuses faster than CO2 because O2 has a larger partial pressure gradient at the alveoli.
✓ CO2 diffuses far faster than O2. Its solubility is 20 to 25 times higher, so even with a smaller gradient more CO2 crosses the diffusion membrane per unit pressure difference.
🧠 Diffusion speed is not decided by pressure gradient alone.

Solved Breathing and Exchange of Gases NEET PYQs

Try the real previous-year questions from this chapter — each with the answer and a full solution.

See all 27 Breathing and Exchange of Gases NEET PYQs ›
Next concept: Transport of Oxygen →Keep learning — 2 minFeeling ready? Solve the Breathing and Exchange of Gases NEET PYQs ›Or practice on your phone — get the free MedicNEET app ›

Frequently asked

What are the factors affecting diffusion of gases as per NCERT?

Pressure/concentration gradient, solubility of the gas, and thickness of the diffusion membrane. These are the only three factors NCERT mentions.

By how many times is CO2 more soluble than O2?

CO2 is 20 to 25 times more soluble than O2, which is why CO2 diffuses out easily despite a smaller pressure gradient.

How many layers make up the diffusion membrane?

Three: the squamous epithelium of alveoli, the endothelium of alveolar capillaries, and the basement substance between them. Its total thickness is less than a millimetre.

Does gas diffusion in lungs require energy?

No. It is simple (passive) diffusion driven by the partial pressure gradient, so no ATP is needed.

Related concepts:
Diffusion of Gases and the Diffusion MembranePartial Pressure of O2 and CO2 (Values at Alveoli, Blood, Tissues)Exchange of Gases at the Alveoli and TissuesTransport of Oxygen: Formation of OxyhaemoglobinAlveoli: Structure and Function as the Exchange Surface
Practise Biology for NEET:
Biology MCQs (free, chapter-wise)Breathing and Exchange of Gases PYQs