Gas Exchange, Oxygen and Carbon Dioxide Transport, Regulation and Disorders
Updated 17 October 2026 · Fact-checked
Gases move by diffusion along partial pressure gradients across the thin respiratory membrane. About 97% of the oxygen is carried by haemoglobin; carbon dioxide travels mainly as bicarbonate (70%), as carbamino-haemoglobin (20 to 25%) and dissolved in plasma (7%). Breathing is controlled by the rhythm centre in the medulla, moderated by the pneumotaxic centre.
Understand Gas Exchange, Transport of Gases and Regulation
Gas exchange. Alveoli are the primary sites of exchange of gases, and also the tissues. Diffusion depends on the partial pressure gradient, solubility of the gas, and the thickness of the membrane. Partial pressures (mm Hg): in the alveolar air pO₂ = 104 and pCO₂ = 40; deoxygenated blood in the pulmonary artery pO₂ = 40, pCO₂ = 45; oxygenated blood pO₂ = 95, pCO₂ = 40; tissues pO₂ = 40, pCO₂ = 45. So oxygen diffuses from alveoli to blood and from blood to tissues; CO₂ moves the other way. The CO₂ is 20 to 25 times more soluble than O₂, so a smaller gradient suffices. The diffusion membrane (alveolar epithelium, capillary endothelium and basement substance) is less than a millimetre thick.
Transport of oxygen. About 97% is carried bound to haemoglobin in the red blood cells as oxyhaemoglobin (each Hb binds 4 O₂; 1 g of Hb binds about 1.34 mL of O₂), and about 3% is dissolved in plasma. The oxygen-haemoglobin dissociation curve is sigmoid: high pO₂, high saturation in lungs; low pO₂ in tissues, release of O₂ (the point moves along the curve towards unloading; the curve itself does not shift). Factors shifting the curve to the right (more release): high pCO₂, high H⁺ (low pH), high temperature (together the Bohr effect, in tissues) and high 2,3-BPG.
Transport of CO₂. About 20 to 25% as carbamino-haemoglobin, about 70% as bicarbonate ions (CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ catalysed by carbonic anhydrase in the RBCs; HCO₃⁻ moves out and Cl⁻ moves in: the chloride shift), and about 7% dissolved in plasma. In the alveoli the reactions reverse and CO₂ is released.
Regulation. The respiratory rhythm centre in the medulla oblongata sets the rhythm; the pneumotaxic centre in the pons moderates it (shortens inspiration); a chemosensitive area near the rhythm centre is sensitive to CO₂ and H⁺; receptors in the aortic arch and carotid artery recognise changes in CO₂ and H⁺. O₂ has little effect on the regulation.
Disorders. Asthma: inflammation of bronchi and bronchioles with difficulty in breathing. Emphysema: damage of the alveolar walls, reducing the surface area, commonly caused by cigarette smoking. Occupational respiratory disorders (silicosis, asbestosis) from dust in industries cause fibrosis.
Gas Exchange, Transport of Gases and Regulation: key facts
- Partial pressures (mm Hg)
Alveoli pO₂ 104, pCO₂ 40 Deoxygenated blood 40, 45 Oxygenated blood 95, 40 - Gradients drive diffusion.
- Oxygen carriage
- 97% as oxyhaemoglobin
- 3% dissolved
- 1 g Hb binds about 1.34 mL O₂
- Each Hb binds up to 4 O₂.
- CO₂ carriage
- Carbonic anhydrase in RBCs.
- Regulation
Rhythm centre medulla Pneumotaxic centre pons Chemosensitive area near the rhythm centre - CO₂ and H⁺ are the main stimuli.
Gas Exchange, Transport of Gases and Regulation 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.
Oxygen carried by blood
Suppose a patient has 15 g of haemoglobin per 100 mL of blood, which is 97% saturated (1 g of Hb binds 1.34 mL of O₂).
Find the volume of O₂ carried by haemoglobin in each 100 mL of blood and in a litre.
Try it first, then show the solutionSolution
- Maximum binding = 15 × 1.34 = 20.1 mL per 100 mL.
- At 97% saturation = 0.97 × 20.1 = 19.5 mL per 100 mL.
- In 1 litre = 10 × 19.5 = 195 mL of O₂.
Answer: 19.5 mL per 100 mL; 195 mL per litre.
Memory hook: Oxygen carrying capacity depends on haemoglobin.
Tricks to remember Gas Exchange, Transport of Gases and Regulation
Short memory aids. Say each one aloud once, then use it on a question the same day.
Bohr shift: tissues are hot, acidic and CO₂-rich
These conditions make haemoglobin give up O₂.
Use it when: Dissociation curve questions.
Bicarbonate 70, carbamino 20, dissolved 7
Learn the CO₂ transport percentages.
Use it when: Transport of CO₂.
Gas Exchange, Transport of Gases and Regulation: solved examples
Ordered from board level to HOTS. Try each one before you open the solution.
Example 1: Mode of CO₂ transport · 1 mark · MCQBoard level
Most of the CO₂ in blood is carried as:
(a) dissolved CO₂
(b) carbamino-haemoglobin
(c) bicarbonate
(d) carbonic acid
Try it first, then show the solutionSolution
- About 70% of CO₂ is carried as bicarbonate in plasma.
Answer: (c) bicarbonate
Example 2: Exchange of gases · 3 marks · Short answerBoard level
Describe the exchange of O₂ and CO₂ between alveoli and blood using partial pressures. Why does CO₂ diffuse with a smaller gradient?
Try it first, then show the solutionSolution
- In the alveoli pO₂ is 104 mm Hg and in the deoxygenated blood 40 mm Hg, so O₂ diffuses into the blood. (1 mark)
- pCO₂ is 45 mm Hg in the blood and 40 mm Hg in the alveoli, so CO₂ diffuses out. (1 mark)
- CO₂ is 20 to 25 times more soluble than O₂, so a smaller gradient is enough. (1 mark)
Answer: O₂ goes into the blood and CO₂ out along their gradients.
Example 3: Oxygen capacity of blood · 4 marks · Short answerHarder board level
Blood contains 14 g of haemoglobin per 100 mL and each gram binds 1.34 mL of O₂. Calculate the maximum O₂ carried per 100 mL and per litre, and the O₂ delivered per minute by a cardiac output of 5 L min⁻¹ if the blood is 97% saturated and the tissues extract 25% of it.
Try it first, then show the solutionSolution
- Maximum = 14 × 1.34 = 18.76 mL per 100 mL; per litre = 187.6 mL. (1 mark)
- At 97%: 18.2 mL per 100 mL; per litre 182 mL. (1 mark)
- Delivered per minute by 5 L: 5 × 182 = 910 mL min⁻¹. (1 mark)
- Tissue extraction of 25%: 0.25 × 910 = 228 mL O₂ per minute. (1 mark)
Answer: 18.8 mL per 100 mL; 187.6 mL per litre; about 228 mL of O₂ used per minute.
Example 4: The oxygen dissociation curve · 4 marks · Short answerHarder board level
Draw (describe) the oxygen-haemoglobin dissociation curve. State four factors that shift it to the right and explain why this is useful in active tissues.
Try it first, then show the solutionSolution
- The curve is sigmoid: a steep rise in saturation at low pO₂ and a plateau at high pO₂. (1 mark)
- Right shift: increased pCO₂, increased H⁺ (lower pH), increased temperature (together the Bohr effect) and increased 2,3-BPG. Low pO₂ in the tissues does not shift the curve; it makes O₂ unload along the curve. (1.5 marks)
- In active tissues these conditions occur together, so haemoglobin releases more O₂ to the tissues. (1.5 marks)
Answer: Sigmoid; right shift with CO₂, H⁺, temperature and 2,3-BPG; more O₂ released in active tissues.
Example 5: Transport of CO₂ and chloride shift · 5 marks · Long answerHOTS / competency
Explain how CO₂ is carried from the tissues to the lungs, with the reactions in the RBC and the chloride shift. How does the process reverse in the alveoli? State the centre that responds to rising CO₂ and its action.
Try it first, then show the solutionSolution
- In the tissues CO₂ enters the RBCs where carbonic anhydrase catalyses CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻. (1.5 marks)
- HCO₃⁻ diffuses out into plasma and Cl⁻ moves in to keep electrical balance: the chloride shift. (1 mark)
- Some CO₂ combines with haemoglobin to form carbamino-haemoglobin, and some dissolves in plasma. (1 mark)
- In the alveoli, with low pCO₂, the reactions reverse: HCO₃⁻ re-enters the RBC, forms CO₂ and is exhaled. (0.75 mark)
- The chemosensitive area and the rhythm centre in the medulla increase the rate and depth of breathing when CO₂ rises. (0.75 mark)
Answer: Bicarbonate formation with carbonic anhydrase and chloride shift; reversed in the alveoli; medulla increases the breathing rate.
Where students lose marks in Gas Exchange, Transport of Gases and Regulation
The slips examiners see most often, and what to write instead.
Mistake: Wrong partial pressure direction
Stating that O₂ diffuses from blood to alveoli.
Do this: O₂ moves from the alveoli (104 mm Hg) to the blood (40 mm Hg).
Usually costs: 2 marks.
Mistake: Percentages of CO₂ transport
Giving 70% for carbamino-haemoglobin.
Do this: 70% bicarbonate, 20 to 25% carbamino-haemoglobin, 7% dissolved.
Usually costs: 1 mark.
Mistake: Putting the rhythm centre in the pons
Wrong location.
Do this: Rhythm centre: medulla; pneumotaxic centre: pons.
Usually costs: 1 mark.
Exam tips
- Draw the sigmoid curve and label the shift.
- Show the partial pressure table.
- Write the reactions of CO₂ transport in sequence.
- State the numbers: 97%, 70%, 20 to 25% and 7%.
Practice questions on Gas Exchange, Transport of Gases and Regulation
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
The enzyme which catalyses the formation of carbonic acid in RBCs is:
(a) pepsin
(b) carbonic anhydrase
(c) amylase
(d) lipaseHint
It speeds up CO₂ + H₂O → H₂CO₃.
Answer
(b) carbonic anhydrase
Q2 · 2 marks · Short answerBoard level
Name the respiratory centres of the brain and their location.
Hint
Medulla and pons.
Answer
The respiratory rhythm centre in the medulla oblongata and the pneumotaxic centre in the pons.
Q3 · 3 marks · Short answerHarder board level
What is emphysema? Name its major cause.
Hint
Think of the alveoli.
Answer
A chronic disorder in which the alveolar walls are damaged, decreasing the surface area for gas exchange; cigarette smoking is the major cause.
Q4 · 3 marks · Short answerHarder board level
Why does CO₂ diffuse more readily than O₂ across the respiratory membrane although its partial pressure gradient is smaller?
Hint
Think of solubility.
Answer
CO₂ is 20 to 25 times more soluble than O₂ in the membrane and fluids, so more CO₂ diffuses for a given gradient.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): The oxygen-haemoglobin curve shifts to the right in active muscles. Reason (R): Active muscles have a higher pCO₂, lower pH and higher temperature. Choose: (a) both true and R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.
Hint
Bohr effect.
Answer
(a) Both true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a mountaineer at a high altitude breathes faster.
(i) Name the centre that sets the rhythm. (1 mark)
(ii) State what the receptors in the carotid and aortic bodies detect. (1 mark)
(iii) State the effect of low pO₂ on the haemoglobin saturation. (1 mark)
(iv) State one adaptation that increases oxygen carrying capacity over time. (1 mark)Hint
Think of acclimatisation.
Answer
(i) The respiratory rhythm centre in the medulla. (ii) Changes in CO₂ and H⁺ concentration. (iii) The saturation falls. (iv) An increase in the number of red blood cells (haemoglobin).
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.Percent of O₂ carried by Hb?Show answer
97%.
2.Bicarbonate share of CO₂ transport?Show answer
About 70%.
3.Pneumotaxic centre?Show answer
In the pons.
4.Cause of emphysema?Show answer
Smoking.
Gas Exchange, Transport of Gases and Regulation: frequently asked questions
What is the chloride shift?
The movement of Cl⁻ into the RBC as HCO₃⁻ moves out, maintaining electrical neutrality.
Why is haemoglobin said to be the respiratory pigment?
It binds oxygen reversibly and carries most of it in blood.
What is the Bohr effect?
The decrease in oxygen affinity of haemoglobin at higher CO₂ or H⁺ concentrations, which favours oxygen release in the tissues.