BiologyCBSE Class 11 · Breathing and Exchange of Gases

Exchange of Gases and Transport of Oxygen and Carbon Dioxide

Updated 17 October 2026 · Fact-checked

Gases move by simple diffusion along partial pressure gradients at the alveoli and tissues. About 97 per cent of oxygen is carried as oxyhaemoglobin; about 70 per cent of carbon dioxide travels as bicarbonate, 20 to 25 per cent as carbamino-haemoglobin and 7 per cent dissolved in plasma. The rhythm centre in the medulla regulates respiration.

Each gas diffuses from higher to lower partial pressure: O₂ goes into blood at alveoli and into cells at tissues, while CO₂ moves the opposite way.

Illustration drawn for QuestionBuddy.

Understand Gas Exchange and Transport of Oxygen and CO2

Exchange. Primary sites: alveoli; also between blood and tissues. Gases move by simple diffusion by pressure/concentration gradient. Factors: pressure gradient, solubility of gases, thickness of the membrane. The solubility of CO₂ is 20 to 25 times that of O₂, so more CO₂ crosses the membrane despite a smaller gradient. The diffusion membrane has three layers: thin squamous epithelium of alveoli, endothelium of alveolar capillaries and the basement substance between them; total thickness is less than a millimetre.

Partial pressures (mm Hg). Atmospheric air: pO₂ 159, pCO₂ 0.3. Alveolar air: pO₂ 104, pCO₂ 40. Deoxygenated blood: pO₂ 40, pCO₂ 45. Oxygenated blood: pO₂ 95, pCO₂ 40. Tissues: pO₂ 40, pCO₂ 45. So O₂ moves from alveoli to blood (104 → 40) and CO₂ from blood to alveoli (45 → 40); in tissues the directions reverse.

Transport of oxygen. About 97 per cent by haemoglobin (red pigment in RBCs): each Hb can carry four O₂ (O₂ + Hb → HbO₂, oxyhaemoglobin); about 3 per cent dissolved in plasma. Binding depends on pO₂, pCO₂, H⁺ concentration and temperature. The plot of % saturation of Hb against pO₂ is the oxygen dissociation curve, sigmoid in shape. In alveoli (high pO₂, low pCO₂, low H⁺, lower temperature) binding is favoured; in tissues (low pO₂, high pCO₂, high H⁺, higher temperature) dissociation is favoured (Bohr effect; curve shifts right). Every 100 mL of oxygenated blood delivers about 5 mL of O₂ to tissues.

Transport of carbon dioxide. About 20 to 25 per cent as carbamino-haemoglobin (CO₂ bound to Hb amino groups); about 70 per cent as bicarbonate; 7 per cent dissolved. In RBCs the enzyme carbonic anhydrase (also in plasma at low levels) catalyses CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺. At the tissues (high pCO₂), the reaction goes forward; at the alveoli (low pCO₂) it goes in reverse, and CO₂ is released. Every 100 mL of deoxygenated blood delivers about 4 mL of CO₂ to alveoli.

Regulation of respiration. The respiratory rhythm centre in the medulla sets the basic rhythm. The pneumotaxic centre in the pons can moderate the rhythm (reducing duration of inspiration). A chemosensitive area adjacent to the rhythm centre is highly sensitive to CO₂ and H⁺; an increase activates this centre, increasing the rate. Receptors in the aortic arch and carotid artery also sense changes in CO₂ and H⁺ and signal the centre. Oxygen has a minor role.

Disorders. Asthma (inflammation of bronchi and bronchioles, wheezing and difficulty); emphysema (damage to alveolar walls, reducing surface for gas exchange; major cause cigarette smoking); occupational disorders (long exposure to dust in grinding or stone-breaking industries causes inflammation and fibrosis, e.g. silicosis, asbestosis).

Table: pO₂ and pCO₂ along the path: atmosphere, alveoli, blood, tissues.

Gas Exchange and Transport of Oxygen and CO2: key facts

Partial pressures: Alveoli: pO2 104, pCO2 40 | Deoxygenated blood: pO2 40, pCO2 45; mm Hg
O2 transport: 97% as HbO2
3% dissolved
1 Hb binds 4 O2; Oxyhaemoglobin
CO2 transport: $70$; Carbonic anhydrase
Delivery: 100 mL oxygenated blood gives 5 mL O2 | 100 mL deoxygenated blood gives 4 mL CO2; To tissues and to alveoli

How to answer Gas Exchange and Transport of Oxygen and CO2 questions

Questions give partial pressures or conditions and ask for the direction or the effect.

1Compare the pressures to find the direction: high to low.
2For transport, state the percentages and forms.
3For the dissociation curve, say whether conditions favour binding or release.
4Remember carbonic anhydrase and the reverse reaction at alveoli.
5Name the centre and its location for regulation.

Gas Exchange and Transport of Oxygen and CO2 in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Mountain trek

Suppose a trekker at high altitude breathes faster and deeper, and after a few days feels better.

Explain the initial breathing change using regulation of respiration.

Try it first, then show the solutionSolution

At altitude the air pressure is lower, so alveolar and arterial pO2 fall.
The receptors in the carotid artery and aortic arch sense the low pO2 and send signals to the rhythm centre in the medulla.
The rhythm centre increases the rate and depth of breathing, so ventilation rises and more O2 reaches the alveoli.
Over days the body adapts, for example by making more red blood cells and haemoglobin, so the person feels better.

Answer: Low pO2 at altitude stimulates the carotid and aortic receptors, which signal the rhythm centre in the medulla. Breathing becomes faster and deeper, so more O2 reaches the alveoli. Over days, more RBCs and haemoglobin are produced.

Memory hook: Low O2 at altitude: carotid and aortic receptors sense it, so breathe faster and deeper.

Tricks to remember Gas Exchange and Transport of Oxygen and CO2

Short memory aids. Say each one aloud once, then use it on a question the same day.

97-3
97% as HbO2, 3% dissolved.
Use it when: O2 transport
70-25-7
CO2: bicarbonate 70, carbamino 20-25, dissolved 7.
Use it when: CO2 transport
Medulla leads, pons trims
Rhythm centre in medulla; pneumotaxic in pons.
Use it when: Regulation

Gas Exchange and Transport of Oxygen and CO2: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Diffusion membrane · 2 marks · Very short answerBoard level

Name the three layers of the diffusion membrane and its thickness.

Try it first, then show the solutionSolution

Squamous epithelium of alveoli, endothelium of capillaries, basement substance (1 mark).
Total thickness less than a millimetre (1 mark).

Answer: Three thin layers, under 1 mm.

Example 2: CO2 transport · 3 marks · Short answerBoard level

How is CO2 transported in blood? Give the percentages and the role of carbonic anhydrase.

Try it first, then show the solutionSolution

About 70 per cent as bicarbonate, 20 to 25 per cent as carbamino-haemoglobin, 7 per cent dissolved (1 mark).
Carbonic anhydrase catalyses CO2 + H2O to H2CO3 and then HCO3- + H+ in RBCs (1 mark).
At alveoli the reaction reverses and CO2 is released (1 mark).

Answer: Bicarbonate mostly; enzyme speeds the reaction both ways.

Example 3: Direction of diffusion · 4 marks · Short answerHarder board level

Using pO2 and pCO2 values, show the direction of movement of both gases at the alveoli and at the tissues.

Try it first, then show the solutionSolution

Alveoli: pO2 104 to blood 40, so O2 moves into blood (1 mark).
Alveoli: blood pCO2 45 to alveolar 40, so CO2 moves out (1 mark).
Tissues: blood pO2 95 to tissue 40, so O2 moves to tissue (1 mark).
Tissues: pCO2 45 to blood 40, so CO2 moves into blood (1 mark).

Answer: O2 into blood then tissue; CO2 into alveoli.

Example 4: Bohr effect · 3 marks · Short answerHarder board level

Explain how an increase in pCO2, H+ and temperature changes oxygen binding and why this is useful in exercising muscle.

Try it first, then show the solutionSolution

These changes shift the dissociation curve to the right (1 mark).
Haemoglobin binds oxygen less strongly, so releases more O2 (1 mark).
Active muscle makes CO2, acid and heat, so receives more O2 where needed (1 mark).

Answer: Right shift: more oxygen release in active tissues.

Example 5: Carbon monoxide · 5 marks · Long answerHOTS / competency

Suppose haemoglobin binds carbon monoxide about 200 times more tightly than oxygen. Explain why CO poisoning reduces oxygen delivery even when pO2 is normal, and why the person may not show deeper breathing.

Try it first, then show the solutionSolution

CO competes for the same sites on haemoglobin (1 mark).
Haemoglobin tied to CO cannot carry oxygen, so oxygen-carrying capacity falls (1 mark).
Dissolved oxygen (3 per cent) is a small supplement (1 mark).
Breathing is driven mainly by CO2 and H+, which may not rise, so no extra drive to breathe (1 mark).
Tissues become oxygen-starved despite normal alveolar pO2 (1 mark).

Answer: CO blocks Hb; breathing drive stays normal.

Where students lose marks in Gas Exchange and Transport of Oxygen and CO2

The slips examiners see most often, and what to write instead.

Mistake: pO2 in alveoli 40
Values swapped.
Wrong working
Alveolar pO2 is 40.
Right working
Alveolar pO2 is 104; deoxygenated blood is 40.
Do this: 104 alveoli.
Usually costs: 1 mark.
Mistake: CO2 mostly as carbamino
Percent confusion.
Wrong working
Most CO2 is as carbamino-Hb.
Right working
About 70% is bicarbonate.
Do this: 70% bicarbonate.
Usually costs: 1 mark.
Mistake: Carbonic anhydrase only in tissues
Reversibility missed.
Wrong working
Works only at tissues.
Right working
Catalyses reversible reaction in both sites.
Do this: Reversible.
Usually costs: 1 mark.

Exam tips

Quote the pressures in a table.
Name the three layers of the membrane.
Explain the sigmoid curve in terms of conditions.
Mention aortic and carotid receptors.

Practice questions on Gas Exchange and Transport of Oxygen and CO2

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Most of the CO2 in blood is carried as
(a) carbaminohaemoglobin (b) dissolved CO2 (c) bicarbonate (d) carbonic acid in plasma
Hint
About 70 per cent.
Answer
(c).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The diffusion membrane of the lung is extremely thin.
Reason (R): It consists of three layers with a total thickness of less than a millimetre.
(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.
Hint
Is R a description or an explanation of A?
Answer
(b).
Q3 · 2 marks · Very short answerBoard level
Where is the respiratory rhythm centre and what activates it most?
Hint
Medulla.
Answer
In the medulla; a rise in CO2 and H+ concentration acting on the chemosensitive area activates it.
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a graph shows the sigmoid dissociation curve shifting right during exercise.
(a) Name the shape. (1)
(b) Name the effect. (1)
(c) What happens to O2 release? (2)
Hint
Bohr.
Answer
(a) Sigmoid. (b) Bohr effect (right shift). (c) More oxygen is released at a given pO2 because Hb affinity falls.
Q5 · 3 marks · Short answerHOTS / competency
Suppose a smoker's alveoli are damaged (emphysema). Predict the effect on diffusion area, gas exchange and breathing effort, and explain.
Hint
Surface area reduced.
Answer
Alveolar walls break down, so surface area falls and diffusion is reduced. Oxygen uptake and CO2 removal fall, and the person breathes harder yet may remain short of oxygen.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.O2 as HbO2?Show answer
About 97%.
2.CO2 as bicarbonate?Show answer
About 70%.
3.Pons centre?Show answer
Pneumotaxic.
4.O2 per 100 mL blood delivered?Show answer
5 mL.
5.Emphysema cause?Show answer
Smoking damages alveoli.
6.Alveolar pO2?Show answer
104 mm Hg.

Gas Exchange and Transport of Oxygen and CO2: frequently asked questions

How do gases cross the alveoli?

By diffusion along pressure gradients.

What is oxyhaemoglobin?

Haemoglobin bound to oxygen.

What does carbonic anhydrase do?

Speeds CO2 and water to carbonic acid.

What controls breathing rate?

Medulla rhythm centre responding to CO2 and H+.

What is asthma?

Inflamed airways causing wheezing.