At the alveoli, O2 and CO2 move only by SIMPLE DIFFUSION (no energy used), driven by the partial pressure gradient of each gas. They cross a very thin diffusion membrane made of 3 layers: (1) squamous epithelium of the alveoli, (2) endothelium of the alveolar capillary, and (3) the basement substance in between. Memory hook: "SEB" — Squamous, Endothelium, Basement — and its total thickness is less than 1 mm, so gases slip across fast.
O2 and CO2 cross the alveoli by simple diffusion down their partial pressure gradients. The diffusion membrane has 3 layers — squamous epithelium of the alveoli, capillary endothelium, and the basement substance between — with a total thickness under 1 mm.
Your doubts, answered
Is gas exchange at the alveoli diffusion or active transport?
It is SIMPLE DIFFUSION, a passive process. No ATP is spent. Gases move down their own partial pressure gradient: O2 from high pO2 in alveoli to low pO2 in blood, and CO2 the opposite way. NEET often traps students who write active transport.
What are the three layers of the diffusion membrane?
In order from air to blood: (1) the thin squamous epithelium of the alveoli, (2) the endothelium of the alveolar capillary, and (3) the basement substance between them (a basement membrane supporting the squamous epithelium plus the basement membrane around the capillary endothelium). Total thickness is less than a millimetre.
Why does CO2 diffuse faster than O2 through the same membrane?
Because the solubility of CO2 is about 20 to 25 times higher than that of O2. So for the same difference in partial pressure, far more CO2 crosses the membrane per unit time. This is why a small pCO2 gradient (only about 5 mm Hg) is enough to remove CO2.
In which direction do O2 and CO2 move at the alveoli?
O2 moves from the alveoli into the blood (alveolar pO2 104 mm Hg is higher than blood pO2 40 mm Hg). CO2 moves from the blood into the alveoli (blood pCO2 45 mm Hg is higher than alveolar pCO2 40 mm Hg). Each gas simply follows its own gradient.
Where exactly does diffusion occur — is it in the whole respiratory tract?
No. Diffusion of O2 and CO2 happens only in the respiratory (exchange) part — the alveoli and their ducts. The conducting part (nostrils to terminal bronchioles) only carries, cleans, warms and humidifies air; it has no gas exchange.
⚠️ The NEET trap ✗ A higher H+ concentration (or gas exchange being an energy-using active transport) helps gas movement at the alveoli. ✓ Gas exchange at alveoli is passive SIMPLE DIFFUSION down partial pressure gradients. At the alveoli conditions are high pO2, LOW pCO2, LOW H+ and low temperature — these favour oxyhaemoglobin formation, not a high H+. 🧠 Alveoli = high O2, low everything-else (low CO2, low H+, low temp). If an option says 'high H+ in alveoli favours O2 loading', it is the wrong statement — exactly what NEET 2020 asked.
Real NEET questions
2021
The partial pressures (in mm Hg) of oxygen (O2) and carbon dioxide (CO2) at alveoli (the site of diffusion) are:
A · pO2 = 95 and pCO2 = 40
B · pO2 = 159 and pCO2 = 0.3
C · pO2 = 104 and pCO2 = 40 ✓
D · pO2 = 40 and pCO2 = 45
Solution: From NCERT Table 14.1, at the alveoli pO2 = 104 mm Hg and pCO2 = 40 mm Hg. The pair 159 and 0.3 is atmospheric air, 95/40 is oxygenated blood, and 40/45 is tissues. The alveolar values create the gradients that drive diffusion across the diffusion membrane.
2016
The partial pressure of oxygen in the alveoli of the lungs is
A · Equal to that in the blood
B · More than that in the blood ✓
C · Less than that in the blood
D · Less than that of carbon dioxide
Solution: Alveolar pO2 is 104 mm Hg, while the deoxygenated blood arriving at the alveoli has pO2 of only 40 mm Hg. Because alveolar pO2 is MORE than blood pO2, O2 diffuses from the alveoli into the blood down this gradient.
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.
Three layers: the squamous epithelium of the alveoli, the endothelium of the alveolar capillary, and the basement substance between them. Its total thickness is less than a millimetre, which speeds up diffusion.
Why is the diffusion membrane so thin?
A thin membrane means a very short distance for gases to travel, so O2 and CO2 diffuse quickly. Rate of diffusion falls if this membrane thickens, as happens in some lung diseases.
What factors decide the rate of diffusion of gases?
Three main factors: the partial pressure (concentration) gradient of the gas, the solubility of the gas, and the thickness of the diffusion membrane. In a healthy body all three favour O2 moving in and CO2 moving out.
Does gas exchange need energy?
No. It is simple diffusion, a passive process. Gases just move from higher to lower partial pressure without using ATP.
