NCERT grounding
NCERT Class XI, Chapter 14 (§14.3 Exchange of Gases) opens with one sentence that is examined verbatim: "Alveoli are the primary sites of exchange of gases." It immediately specifies that exchange between the alveolar air and pulmonary blood, and between blood and the systemic tissues, occurs by simple diffusion. The drivers are a pressure or concentration gradient for each gas; the modifiers are the solubility of the gas and the thickness of the membrane through which it must travel. Table 14.1 of NCERT lists the partial pressures of O2 and CO2 at four locations — atmospheric air, alveoli, deoxygenated blood and oxygenated blood — and these four columns supply nearly every numerical PYQ on this subtopic. The NIOS Biology lesson on respiration (§14.2.2) reinforces the same picture and adds that the alveolus and the capillary together build a wall less than a micrometre thick.
Partial pressures, the diffusion membrane and the gradients
In a mixture of gases, the pressure contributed by each individual gas is called its partial pressure, written pO2 for oxygen and pCO2 for carbon dioxide. Dalton's law assures us that the total pressure of the gas mixture is just the sum of these contributions, which means each gas behaves at the diffusion membrane as if the other gases were not there. Gas exchange at the alveolus is therefore not one process but two parallel processes — O2 moves down its own pressure gradient, CO2 moves down its own pressure gradient, and the two move in opposite directions across the same physical wall at the same time.
The atmospheric air entering the trachea is not the same air that bathes the alveolus. As fresh air mixes with residual gas in the conducting passages it picks up water vapour, loses some O2 to absorption, and accumulates some CO2 from the alveolar reservoir. The result is the four-column data set that NCERT Table 14.1 makes the centrepiece of this subtopic — and that NEET 2021 Q.155 asked verbatim.
| Site | pO2 (mm Hg) | pCO2 (mm Hg) |
|---|---|---|
| Atmospheric air | 159 | 0.3 |
| Alveoli | 104 | 40 |
| Deoxygenated blood (pulmonary artery) | 40 | 45 |
| Oxygenated blood (pulmonary vein) | 95 | 40 |
| Tissues | 40 | 45 |
Read the alveolus row against the deoxygenated-blood row and the two gradients fall out at once. For oxygen, pO2 drops from 104 in the alveolus to 40 in the entering blood — a 64 mm Hg gradient that pushes O2 into the capillary. For carbon dioxide the direction is reversed: pCO2 is 45 in the deoxygenated blood and only 40 in the alveolus, so the 5 mm Hg gradient pushes CO2 out of the capillary and into the alveolar lumen, from where it will be exhaled.
O2 gradient — alveolus → blood
pO2 falls from 104 mm Hg in alveolar air to 40 mm Hg in deoxygenated blood. A large pressure head drives O2 across.
CO2 gradient — blood → alveolus
pCO2 falls from 45 mm Hg in deoxygenated blood to 40 mm Hg in alveolar air. The gradient is small but adequate because CO2 is highly soluble.
The three-layer diffusion membrane
What the gases physically cross is called the respiratory membrane or diffusion membrane. NCERT names three layers, in order from gas to blood:
Diffusion membrane (alveolus → pulmonary capillary)
-
Layer 1
Squamous alveolar epithelium
A single sheet of thin Type I pneumocytes lining the air space. About 0.05–0.10 µm thick.
Air-side wall -
Layer 2
Basement substance
A fused basement membrane that supports the alveolar epithelium and surrounds the capillary endothelium. The thinnest layer.
Connective interface -
Layer 3
Capillary endothelium
A single layer of endothelial cells lining the pulmonary capillary lumen. About 0.05–0.10 µm thick.
Blood-side wall
NCERT is emphatic on the total thickness: it is "much less than a millimetre" — in fact below one micrometre in healthy lungs, roughly fifty times thinner than a sheet of household paper. Two thin epithelial sheets and a fused basement membrane are the only thing standing between alveolar air and the haemoglobin of a passing erythrocyte. This dimensional fact is half of the reason why diffusion at this surface is so efficient; the other half is the very large area over which it occurs.
Total alveolar surface area
Approximately 300–500 million alveoli per pair of lungs fold roughly 100 square metres of respiratory surface into a thoracic volume of just a few litres. Large area · thin wall = high diffusion capacity.
An inline view of the alveolus and the gradients
Figure 1. A single alveolus with a wrapping pulmonary capillary. O2 diffuses inward along its 64 mm Hg gradient; CO2 diffuses outward along its 5 mm Hg gradient. The three-layer diffusion membrane between them — squamous alveolar epithelium, basement substance and capillary endothelium — is less than 1 µm thick.
Why CO2 wins despite a tiny gradient
A student looking at the gradients in the table will reasonably ask: if CO2 only has a 5 mm Hg gradient against O2's 64, how does the body export anywhere near enough of it? The answer is the second factor in Fick's law of diffusion — solubility. The solubility of CO2 in the aqueous tissue of the diffusion membrane is roughly 20–25 times that of O2. The amount of a gas that crosses per unit gradient is proportional to its solubility, so CO2's small head of pressure is multiplied many-fold by its much higher solubility. The two gases end up moving comparable molar quantities per breath in opposite directions.
Oxygen (O2)
64 mm Hg
Alveolus → blood gradient
- Direction: alveolus → capillary
- pO2: 104 (alveolus) → 40 (blood)
- Solubility in membrane: low (baseline)
- Carriage in blood: 97% bound to haemoglobin
- Needs a large gradient to deliver enough
Carbon dioxide (CO2)
5 mm Hg
Blood → alveolus gradient
- Direction: capillary → alveolus
- pCO2: 45 (blood) → 40 (alveolus)
- Solubility in membrane: ~20–25× that of O2
- Carriage in blood: 70% as bicarbonate
- Small gradient is enough, thanks to solubility
Factors that set the rate of diffusion
NCERT collapses Fick's law of diffusion into one sentence — exchange depends on the partial-pressure gradient, the solubility of the gas and the thickness of the membrane — and adds elsewhere that the surface area available also matters. Four factors, then. In a healthy adult lung at rest all four are tuned in favour of efficient exchange; in disease, one or more of them is what fails.
Rule: Rate of diffusion ∝ (gradient × solubility × area) ÷ thickness. Healthy lungs maximise the numerator and minimise the denominator. Disease attacks the denominator (fibrosis, oedema) or the area (emphysema).
Partial-pressure gradient
64 / 5 mm Hg
O2 in · CO2 out
The driving head for each gas. A bigger gradient pushes more molecules across per unit time. Lost in high-altitude hypoxia when atmospheric pO2 falls.
NEET 2021 · Q.155Solubility of the gas
~25× for CO2
CO2 : O2
CO2 dissolves much more readily in the membrane fluid. This is why a tiny CO2 gradient still clears a normal day's metabolic load.
NCERT §14.3Surface area
≈ 100 m²
Two adult lungs
300–500 million alveoli unfold a tennis-court of membrane. Surface area is destroyed by emphysema, which is why exchange fails despite normal gradients.
Trap · NEET 2018 Q.169Membrane thickness
< 1 µm
All three layers combined
Rate is inversely proportional to thickness. Thickened by pulmonary fibrosis or pulmonary oedema; both raise the diffusion barrier even when area is intact.
NIOS §14.2.2The hidden assumption — passive, no ATP at the membrane
One last quality of alveolar exchange that NEET likes to slip into wrong-statement items: the diffusion step itself is passive. No carrier, no pump, no ATP is spent at the alveolar wall. The energy budget of breathing is consumed earlier — by the contraction of the diaphragm and external intercostals that ventilates the alveoli, and by the right ventricle that perfuses the pulmonary capillaries. Once both reservoirs are stocked, the molecules themselves travel free.
What happens at the tissues mirrors the alveolus
The same physics governs the secondary exchange at the systemic tissues, but with the gradients flipped. Tissue pO2 is about 40 mm Hg and tissue pCO2 is about 45 mm Hg, so O2 now leaves the blood and enters the cell while CO2 moves from the cell into the blood. NCERT closes §14.3 with the line that summarises this symmetry — "all the factors in our body are favourable for diffusion of O2 from alveoli to tissues and that of CO2 from tissues to alveoli." The alveolus is one half of a loop; this is the other half.
Figure 2. Partial-pressure ladder for O2 (teal) and CO2 (coral) across the four sites of NCERT Table 14.1. pO2 drops from 159 in the atmosphere to 104 in the alveolus (because of mixing with residual air and water vapour); the alveolus-to-blood and blood-to-alveolus gradients are the difference between adjacent bars.
Worked examples
A student is told that the alveolar pO2 and pCO2 values are 104 and 40 mm Hg. The deoxygenated blood entering the pulmonary capillary has pO2 40 and pCO2 45. State the direction of net movement of each gas and the gradient driving it.
O2: 104 (alveolus) − 40 (blood) = +64 mm Hg gradient; net diffusion is alveolus → blood. CO2: 45 (blood) − 40 (alveolus) = +5 mm Hg gradient; net diffusion is blood → alveolus. The 5 mm Hg CO2 gradient is small but adequate because CO2 is roughly 20–25 times more soluble than O2 in the diffusion membrane.
List the three layers of the diffusion membrane at the alveolus, in the order in which an O2 molecule crosses them when moving from alveolar air to capillary blood. State the order-of-magnitude total thickness.
The crossing order is (1) the thin squamous epithelium of the alveolus (Type I pneumocyte sheet), (2) the basement substance — a fused basement membrane supporting the alveolar epithelium and surrounding the capillary endothelium, and (3) the endothelium of the pulmonary capillary. Total thickness is less than 1 µm — far less than a millimetre as NCERT states.
A patient develops pulmonary fibrosis with thickening of the alveolar basement membrane to roughly three times normal, while alveolar surface area and partial-pressure gradients are unchanged. Predict the effect on the rate of O2 diffusion and justify with the relevant factor of diffusion.
The rate of diffusion is inversely proportional to membrane thickness. Tripling the thickness will reduce the rate of O2 transfer per breath to about one-third of normal, even with gradients and area intact. This is why interstitial lung disease produces arterial hypoxia despite normal ventilation — the bottleneck is the denominator of Fick's relation, not the numerator.