Gases are exchanged at the alveoli and the tissues by simple diffusion, driven by the partial pressure gradient of each gas. At the alveoli O2 moves from air into blood and CO2 moves from blood into air; at the tissues the direction reverses. Memory hook: "gas always walks downhill" — every gas moves from high partial pressure to low partial pressure, no energy (ATP) is used.
O2 and CO2 diffuse at the alveoli and tissues along partial pressure gradients (values in mm Hg). O2: alveoli to blood to tissues; CO2: tissues to blood to alveoli. No energy is used.
Your doubts, answered
Is the exchange of gases active transport or diffusion?
It is simple diffusion, not active transport. No ATP is spent. Each gas moves only because its partial pressure is higher on one side than the other. At the alveoli pO2 is high in the air and low in the blood, so O2 diffuses into blood. Do not confuse this with active transport of ions, which needs energy and moves against the gradient.
Why does O2 move from the alveoli into the blood and not the other way?
Because of the pressure gradient. In the alveoli pO2 is about 104 mm Hg, while in the deoxygenated blood arriving there it is only about 40 mm Hg. Gas moves from high to low partial pressure, so O2 diffuses from alveoli into blood. CO2 does the opposite: it is higher in blood (about 45 mm Hg) than in the alveoli (about 40 mm Hg), so CO2 leaves the blood.
How much O2 and CO2 does blood actually deliver?
NCERT gives two exact numbers you must memorise. Every 100 mL of oxygenated blood delivers about 5 mL of O2 to the tissues. Every 100 mL of deoxygenated blood delivers about 4 mL of CO2 to the alveoli. Students often swap these, so link 5 to O2 and 4 to CO2.
What are the three layers of the diffusion membrane?
The diffusion membrane has three major layers: (1) the thin squamous epithelium of the alveoli, (2) the endothelium of the alveolar capillaries, and (3) the basement substance in between. Its total thickness is much less than a millimetre, which is why gases cross it so easily.
Why can more CO2 diffuse across the membrane than O2?
Because CO2 is 20 to 25 times more soluble than O2. Solubility is one of the factors that decides the rate of diffusion. So even though the pCO2 gradient (about 45 to 40 mm Hg) is small, the high solubility of CO2 lets a large amount of it diffuse per unit pressure difference.
⚠️ The NEET trap ✗ Gases are pumped across the alveoli by active transport using energy. ✓ Gases are exchanged by simple diffusion along the partial pressure gradient, using no energy at all. 🧠 NEET loves the word 'diffusion'. If an option says active transport, carrier protein or ATP for gas exchange at alveoli or tissues, it is a trap. Only diffusion (based on partial pressure, solubility and membrane thickness) moves O2 and CO2.
Real NEET questions
NEET 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: In NCERT Table 14.1, pO2 at the alveoli is 104 mm Hg, while in the deoxygenated blood reaching the alveoli it is only 40 mm Hg. This gradient (alveoli > blood) drives O2 diffusion from alveoli into blood, so alveolar pO2 is more than that in the blood.
NEET 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 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.
NEET 2022
Under normal physical conditions in human beings every 100 ml oxygenated blood can deliver ____ ml of O2 to the tissues.
A · 2 ml
B · 5 ml ✓
C · 4 ml
D · 10 ml
Solution: Every 100 mL of oxygenated blood delivers around 5 mL of O2 to the tissues. The distractor 4 ml is the amount of CO2 delivered by 100 mL of deoxygenated blood to the alveoli.
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.
By what process are gases exchanged at the alveoli and tissues?
By simple diffusion, based mainly on the partial pressure or concentration gradient. Solubility of the gases and the thickness of the diffusion membrane also affect the rate.
What decides the direction of gas movement?
The partial pressure gradient. Each gas always moves from where its partial pressure is high to where it is low. O2 moves alveoli to blood to tissues; CO2 moves tissues to blood to alveoli.
What is the diffusion membrane made of?
Three layers: the squamous epithelium of the alveoli, the endothelium of the alveolar capillaries, and the basement substance between them. Its total thickness is less than a millimetre.
Why is CO2 exchange so efficient despite a small gradient?
CO2 is 20 to 25 times more soluble than O2, so a large amount of CO2 diffuses per unit difference in partial pressure even though its pressure gradient is small.
How much O2 and CO2 does blood transfer per 100 mL?
100 mL of oxygenated blood delivers about 5 mL O2 to tissues; 100 mL of deoxygenated blood delivers about 4 mL CO2 to the alveoli.
