આ MCQ મોડ્યુલ આના પર આધારિત છે: Exchange of Gases
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Exchange of Gases
🎓 Class 11
Biology
CBSE
Theory
Ch 14 – Breathing and Exchange of Gases
⏱ ~14 min
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આ મૂલ્યાંકન આના પર આધારિત હશે: Exchange of Gases
મૂલ્યાંકન બનાવવામાં તેમની સામગ્રી સામેલ કરવા ચિત્રો, PDF અથવા Word દસ્તાવેજ અપલોડ કરો.
Exchange of Gases and the Diffusion Membrane
Air has now reached the alveoli. Nothing pumps oxygen across into the blood — there is no carrier, no active transport, no energy spent. The gases move entirely by diffusion, and this part shows why every condition in the body happens to favour diffusion in precisely the directions life requires.
14.3 Exchange of Gases
Where exchange happens. Alveoli are the primary sites of exchange of gases. Exchange of gases also occurs between blood and tissues. At both sites, O₂ and CO₂ are exchanged by simple diffusion, mainly based on pressure or concentration gradient.
The factors that govern the rate
Besides the gradient, solubility of the gases, as well as the thickness of the membranes involved in diffusion, are also some important factors that can affect the rate of diffusion.
| Factor | Effect on rate | In the human body |
|---|---|---|
| Partial pressure gradient | Steeper gradient → faster diffusion | Favourable for O₂ alveoli→blood→tissues, and for CO₂ in the opposite direction |
| Solubility of the gas | More soluble → more diffuses per unit gradient | CO₂ is 20–25 times more soluble than O₂ |
| Thickness of the membrane | Thinner → faster diffusion | Total thickness much less than a millimetre |
Partial pressure
Pressure contributed by an individual gas in a mixture of gases is called partial pressure, and is represented as pO₂ for oxygen and pCO₂ for carbon dioxide.
This is the single idea the whole section rests on. Air is a mixture, and each gas in it exerts its own share of the total pressure. A gas diffuses from where its own partial pressure is higher to where it is lower — regardless of what the other gases are doing.
| Respiratory gas | Atmospheric air | Alveoli | Blood (deoxygenated) | Blood (oxygenated) | Tissues |
|---|---|---|---|---|---|
| O₂ | 159 | 104 | 40 | 95 | 40 |
| CO₂ | 0.3 | 40 | 45 | 40 | 45 |
What the table shows, in one sentence each. The data clearly indicates a concentration gradient for oxygen from alveoli to blood and blood to tissues. Similarly, a gradient is present for CO₂ in the opposite direction — i.e., from tissues to blood and blood to alveoli.
Reading the table properly — the four gradients
| Step | Gas | From | To | Gradient |
|---|---|---|---|---|
| At the alveolus | O₂ | Alveoli, 104 | Deoxygenated blood, 40 | 64 mm Hg — steep |
| At the alveolus | CO₂ | Deoxygenated blood, 45 | Alveoli, 40 | 5 mm Hg — shallow |
| At the tissue | O₂ | Oxygenated blood, 95 | Tissues, 40 | 55 mm Hg — steep |
| At the tissue | CO₂ | Tissues, 45 | Oxygenated blood, 40 | 5 mm Hg — shallow |
The puzzle in those numbers, and its answer. Look at the CO₂ column: the gradient is only 5 mm Hg at both sites, against 55–64 mm Hg for oxygen — barely a tenth. How can CO₂ be cleared as fast as O₂ is delivered on a gradient that shallow? The answer is solubility. As the solubility of CO₂ is 20–25 times higher than that of O₂, the amount of CO₂ that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher compared to that of O₂. A tenth of the gradient, times twenty times the solubility, more than makes up the difference. This is the single most examinable inference in the section.
The diffusion membrane
The diffusion membrane is made up of three major layers, namely:
- the thin squamous epithelium of alveoli;
- the endothelium of alveolar capillaries;
- the basement substance in between them — composed of a thin basement membrane supporting the squamous epithelium and the basement membrane surrounding the single layer endothelial cells of capillaries.
And the crucial dimension. However, its total thickness is much less than a millimetre. Therefore, all the factors in our body are favourable for diffusion of O₂ from alveoli to tissues and that of CO₂ from tissues to alveoli.
📐 Activity 14.3 — Work out every gradient from Table 14.1
What to do. Copy Table 14.1 into your notebook. Then, for each of the four exchange steps, calculate the partial pressure difference and write an arrow showing the direction of net diffusion. Finally answer three questions: (i) Why is alveolar pO₂ only 104 when atmospheric pO₂ is 159? (ii) Why is oxygenated blood at pO₂ 95 rather than 104, if it has just left the alveolus? (iii) Why are the tissue values (40 and 45) identical to the deoxygenated blood values?
Predict: before calculating, guess whether the CO₂ gradients will be larger or smaller than the O₂ gradients — and whether that should worry you.
The four gradients. At the alveolus: O₂ 104 → 40, a difference of 64, into the blood; CO₂ 45 → 40, a difference of 5, out into the alveolus. At the tissue: O₂ 95 → 40, a difference of 55, into the tissue; CO₂ 45 → 40, a difference of 5, into the blood.
Should the small CO₂ gradients worry you? No. The solubility of CO₂ is 20–25 times higher than that of O₂, so the amount of CO₂ that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher compared to that of O₂. Twenty times the solubility on a tenth of the gradient still clears the CO₂ comfortably.
(i) Why alveolar pO₂ is only 104. Three reasons. Inspired air is humidified in the conducting part, and the added water vapour takes its own share of the total pressure, lowering every other partial pressure. It then mixes with the large volume of air already in the lungs — recall the functional residual capacity of Part 2, some 2300 mL of stale air that a 500 mL breath cannot flush out. And oxygen is being continuously removed into the blood while CO₂ is added, which is why alveolar pCO₂ has risen from 0.3 to 40.
(ii) Why oxygenated blood is 95, not 104. Because diffusion needs a gradient to proceed and equilibrium is never quite reached, and because a small amount of blood from the bronchial circulation, which has already given up its oxygen to the lung tissue itself, mixes back into the pulmonary veins. The 9 mm Hg shortfall is normal.
(iii) Why tissue and deoxygenated-blood values coincide. Because blood leaving a tissue has come very close to equilibrating with it. The tissue is continuously consuming O₂ and producing CO₂, so it holds the low pO₂ of 40 and high pCO₂ of 45; blood that has just exchanged with it carries away almost exactly those values. This is also the neatest demonstration that tissues, not lungs, set the composition of venous blood.
🎯 Interactive: Set the site and see the gradient
pO₂ / pCO₂ (mm Hg): 159 / 0.3
The air outside the body. Note how high pO2 is and how nearly absent CO2 is - by the time this air reaches the alveoli, humidification, mixing with residual air and exchange with blood have already brought pO2 down to 104 and raised pCO2 to 40.
🎯 Competency-Based Questions
Scenario: Three patients are investigated. Patient A has pulmonary fibrosis, in which the basement substance of the diffusion membrane is thickened. Patient B has severe emphysema, in which many alveolar walls have been destroyed. Patient C is a healthy mountaineer at 5000 m, where atmospheric pO₂ is roughly half its sea-level value.
Q1. Which factor affecting diffusion is disturbed in each patient? L3 Apply
The chapter names three factors: the pressure/concentration gradient, the solubility of the gases, and the thickness of the membranes involved in diffusion. Patient A — the thickness of the diffusion membrane is increased; normally its total thickness is much less than a millimetre, and thickening it slows diffusion. Patient B — the area of the exchange surface is lost, since alveolar walls are damaged, due to which the respiratory surface is decreased. Patient C — the partial pressure gradient is reduced, because a lower atmospheric pO₂ lowers alveolar pO₂ and so shrinks the 104-to-40 gradient that normally drives oxygen into the blood. Solubility is a property of the gases and cannot be altered by disease or altitude.
Q2. In patient A, oxygen uptake is severely impaired while CO₂ removal remains almost adequate. Explain. L4 Analyse
Because the two gases are not equally handicapped by a thicker membrane. The solubility of CO₂ is 20–25 times higher than that of O₂, so the amount of CO₂ that can diffuse per unit difference in partial pressure is much higher. CO₂ therefore has a large reserve of diffusing capacity and can still cross a thickened membrane on its usual 5 mm Hg gradient. Oxygen has no such reserve: it is the poorly soluble gas, it depends on a steep gradient, and slowing its transfer immediately shows up as inadequate loading of haemoglobin. This asymmetry — low oxygen with near-normal carbon dioxide — is the classic signature of a diffusion barrier.
Q3. Fill in the blanks: The diffusion membrane has three layers: the thin ______ of alveoli, the ______ of alveolar capillaries, and the ______ between them. Its total thickness is ______. L1 Remember
squamous epithelium; endothelium; basement substance; much less than a millimetre.
Q4. A student notices that atmospheric pCO₂ is 0.3 mm Hg but alveolar pCO₂ is 40 — more than a hundredfold higher. Is something wrong with the lungs? L4 Analyse
Nothing is wrong — this is exactly what a working lung must look like. Alveolar air is not a sample of the atmosphere. It is atmospheric air that has been humidified and warmed in the conducting part, then mixed with the large volume of air already in the lungs — the functional residual capacity — which a single 500 mL tidal breath cannot replace, and which is continuously receiving CO₂ from the blood and losing O₂ to it. The rise from 0.3 to 40 is the evidence that CO₂ is being excreted. Had alveolar pCO₂ stayed at 0.3, no CO₂ would be leaving the body at all. The same logic explains the fall in pO₂ from 159 to 104. A useful way to see it: alveolar values are set by the balance between ventilation and blood flow, not by the composition of outside air.
Q5. “Since gas exchange is only diffusion, the body plays no active part in it.” Evaluate. L5 Evaluate
The premise is right and the conclusion is wrong. The chapter is unambiguous that O₂ and CO₂ are exchanged at these sites by simple diffusion, mainly based on pressure/concentration gradient — no pump, no carrier, no ATP is spent at the membrane itself. But diffusion is passive only at the moment of crossing. Everything that makes it possible is actively maintained:
(i) The gradients are created and defended by work. Alveolar pO₂ of 104 is held up only by continuous breathing, which is muscular work by the diaphragm and intercostals; tissue pO₂ of 40 is held down by cellular respiration. Stop either and the gradient collapses within a minute.
(ii) The membrane is actively kept thin and clean. Its total thickness is much less than a millimetre only because the conducting part clears the air of foreign particles on every breath. Fibrosis shows what happens when that defence is overwhelmed.
(iii) Blood flow must match. The gradient at the alveolus exists only because deoxygenated blood at pO₂ 40 keeps arriving — which requires the heart.
(iv) Carriage is not diffusion. Once across, the gases are handled by haemoglobin and carbonic anhydrase, as Part 4 shows — and it is that carriage which keeps dissolved levels low and the gradient steep.
The accurate formulation: the crossing is passive, but the conditions that permit it are actively and expensively maintained. Calling gas exchange “just diffusion” is like calling a waterfall “just gravity” — true of the last step, silent about everything that put the water up there.
(i) The gradients are created and defended by work. Alveolar pO₂ of 104 is held up only by continuous breathing, which is muscular work by the diaphragm and intercostals; tissue pO₂ of 40 is held down by cellular respiration. Stop either and the gradient collapses within a minute.
(ii) The membrane is actively kept thin and clean. Its total thickness is much less than a millimetre only because the conducting part clears the air of foreign particles on every breath. Fibrosis shows what happens when that defence is overwhelmed.
(iii) Blood flow must match. The gradient at the alveolus exists only because deoxygenated blood at pO₂ 40 keeps arriving — which requires the heart.
(iv) Carriage is not diffusion. Once across, the gases are handled by haemoglobin and carbonic anhydrase, as Part 4 shows — and it is that carriage which keeps dissolved levels low and the gradient steep.
The accurate formulation: the crossing is passive, but the conditions that permit it are actively and expensively maintained. Calling gas exchange “just diffusion” is like calling a waterfall “just gravity” — true of the last step, silent about everything that put the water up there.
🧠 Assertion–Reason Questions
For each pair choose: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.
Assertion (A): CO₂ is removed efficiently although its partial pressure gradient is only about 5 mm Hg.
Reason (R): The solubility of CO₂ is 20–25 times higher than that of O₂, so more of it diffuses per unit difference in partial pressure.
Answer: A. Both are true and the reason is exactly the explanation. Solubility compensates for the shallow gradient.
Assertion (A): Oxygen diffuses from the alveoli into the blood.
Reason (R): pO₂ is 104 mm Hg in the alveoli and 40 mm Hg in deoxygenated blood.
Answer: A. Both are true and the reason gives the gradient that drives the diffusion — a difference of 64 mm Hg in the direction alveoli → blood.
Assertion (A): Alveolar air has the same composition as atmospheric air.
Reason (R): Alveoli are the primary sites of exchange of gases.
Answer: D. The assertion is false — atmospheric pO₂ is 159 and pCO₂ 0.3, while alveolar values are 104 and 40. The reason is true, and is in fact why the assertion must be false: because exchange occurs there, alveolar air is continuously depleted of O₂ and enriched with CO₂.
Coming next. Part 4 takes up Sections 14.4 to 14.6 — how oxygen is carried as oxyhaemoglobin and what the oxygen dissociation curve reveals, the three modes of CO₂ transport with carbonic anhydrase, the neural regulation of respiration, and the disorders of the respiratory system.
Frequently Asked Questions - Exchange of Gases and the Diffusion Membrane
Where does exchange of gases take place?
Alveoli are the primary sites of exchange of gases, and exchange also occurs between blood and tissues. At both sites O2 and CO2 are exchanged by simple diffusion, mainly based on pressure or concentration gradient.
What is partial pressure?
The pressure contributed by an individual gas in a mixture of gases. It is written pO2 for oxygen and pCO2 for carbon dioxide, and a gas diffuses from where its own partial pressure is higher to where it is lower.
What are the partial pressures of O2 and CO2 at the different sites?
For O2: atmospheric air 159, alveoli 104, deoxygenated blood 40, oxygenated blood 95, tissues 40 mm Hg. For CO2: atmospheric air 0.3, alveoli 40, deoxygenated blood 45, oxygenated blood 40, tissues 45 mm Hg.
Which factors affect the rate of diffusion of gases?
The pressure or concentration gradient, the solubility of the gases, and the thickness of the membranes involved in diffusion. In the human body all three are favourable for O2 moving from alveoli to tissues and CO2 from tissues to alveoli.
How is carbon dioxide removed efficiently on a gradient of only 5 mm Hg?
Because the solubility of CO2 is 20 to 25 times higher than that of O2, so the amount of CO2 that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher than for oxygen. High solubility compensates for the shallow gradient.
What are the three layers of the diffusion membrane?
The thin squamous epithelium of the alveoli, the endothelium of the alveolar capillaries, and the basement substance between them, which is composed of a thin basement membrane supporting the squamous epithelium and the basement membrane surrounding the single layer of capillary endothelial cells. Its total thickness is much less than a millimetre.
Why is alveolar pO2 lower than atmospheric pO2?
Because inspired air is humidified in the conducting part, mixes with the large volume of air already present in the lungs which a single tidal breath cannot flush out, and is continuously losing oxygen to the blood while gaining carbon dioxide. Hence pO2 falls from 159 to 104 and pCO2 rises from 0.3 to 40.
Why is diffusion of gases restricted to the alveolar region?
Because only the alveoli and their ducts form the exchange part, having walls thin enough and a diffusion membrane of less than a millimetre in thickness, closely applied to capillaries. Everything from the nostrils to the terminal bronchioles is the conducting part, whose thicker, cartilage-supported walls are not vascularised for exchange.
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