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Detailed notes on Gas Exchange and Respiration for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
Gas exchange is the physical diffusion of respiratory gases across surfaces. In humans, this happens in the alveoli of the lungs. Examiners want features that increase efficiency: large surface area, thin membrane, moist surface, good blood supply.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
All gas exchange surfaces share four key features that maximise diffusion rate.
By Fick's Law, diffusion rate ∝ (surface area × concentration difference) / diffusion distance.
Gas exchange surfaces maximise this by:
| Feature | Example in alveoli | Why it helps |
|---|---|---|
| Large surface area | ~700 m² (size of a tennis court) | More sites for diffusion simultaneously |
| Thin membrane | One epithelial cell + one capillary cell (~0.5 µm total) | Shorter diffusion distance |
| Moist surface | Mucus lining alveoli | Gases must dissolve to diffuse across membrane |
| Maintained concentration gradient | Rich blood supply (continuous flow) + ventilation | Prevents equilibrium — keeps gradient steep |
These four features are shared by ALL gas exchange surfaces: alveoli in humans, gills in fish, leaves in plants (mesophyll cells), skin in amphibians.
Air travels: nasal cavity → trachea → bronchi → bronchioles → alveoli. Each structure is adapted to its role.
Pathway of air: nasal cavity → pharynx → larynx → trachea → bronchi (one per lung) → bronchioles → alveoli.
| Structure | Features | Function |
|---|---|---|
| Nasal cavity | Moist, ciliated, capillary-rich | Warms, moistens, filters air |
| Trachea | C-shaped cartilage rings, ciliated mucosa | Keeps airway open; mucus traps particles |
| Bronchi | Smaller cartilage rings | Air conduction to lungs |
| Bronchioles | Smooth muscle, no cartilage | Control airflow by constriction/dilation |
| Alveoli | Thin walls, capillary network, moist | Gas exchange surface |
Ciliated mucosa in trachea/bronchi: Goblet cells secrete mucus to trap pathogens and particles; cilia beat upward (toward throat) to remove mucus — the mucociliary escalator. Smoking paralyses cilia, causing mucus build-up and smoker's cough.
Breathing changes lung volume, which changes air pressure, driving airflow. Inspiration is active; expiration is mainly passive.
Inspiration (breathing in):
Expiration (breathing out):
Key principle: It is volume change → pressure change → air flow. The lungs themselves contain no muscle — they move passively as the thorax volume changes.
Lung volumes:
O₂ diffuses from alveolus to blood; CO₂ from blood to alveolus — both by diffusion down concentration gradients maintained by ventilation and blood flow.
At the alveolus wall:
Why gradients are maintained:
CO₂ in blood: Mostly transported as hydrogencarbonate ions (HCO₃⁻) in plasma (formed by CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻, catalysed by carbonic anhydrase in red cells). In alveoli, the reaction reverses, releasing CO₂ for exhalation.
Smoking damages the mucociliary escalator, destroys alveoli, causes cancer, and constricts airways — all reducing gas exchange efficiency.
Harmful substances in tobacco smoke:
| Substance | Effect |
|---|---|
| Tar | Coats cilia, destroys them; carcinogen → lung cancer |
| Carbon monoxide | Binds haemoglobin with 200× affinity of O₂ → carboxyhaemoglobin → less O₂ delivered |
| Nicotine | Addictive; increases heart rate and blood pressure |
| Particulates | Block airways; trigger immune response → inflammation |
Chronic obstructive pulmonary disease (COPD):
Lung cancer: Tar carcinogens damage DNA in epithelial cells → uncontrolled cell division.
Smoker's cough: Cilia paralysed by tar → mucus accumulates → must cough to clear.
Verbatim phrases and definitions Cambridge mark schemes credit.
Paper 4 asks: 'Explain how the alveolus is adapted for gas exchange' (4 marks — list and explain each adaptation). 'Explain how smoking causes breathlessness' (3 marks — tar, cilia, emphysema, surface area). Breathing mechanism diagrams are common: 'Explain what happens to diaphragm and ribs during inspiration'. Data interpretation on lung volumes and spirometry traces appears in Paper 4 structured questions.
Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (B11); 0654/42 May/Jun 2023 — Q5 (gas exchange); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.
Step-by-step solutions to past-paper-style questions on gas exchange , written exactly the way a tutor would explain them at the board.
Question
Explain how the alveoli are adapted for efficient gas exchange.
Step-by-step solution
Step 1
Large surface area: the lungs contain millions of alveoli arranged in clusters, providing a combined surface area of approximately 70 m² in adults.
Step 2
Thin walls: alveolar walls are one cell thick; capillary walls are also one cell thick → very short diffusion distance for gases.
Step 3
Moist surface: the inner surface of alveoli is lined with moisture; gases (O2 and CO2) dissolve in this moisture to diffuse across.
Step 4
Rich capillary blood supply: a dense network of capillaries surrounds each alveolus; blood flowing continuously maintains a steep concentration gradient for both O2 (into blood) and CO2 (into alveolus).
Answer
Alveoli are adapted by: (1) large surface area — millions of alveoli; (2) thin walls (one cell) — short diffusion distance; (3) moist lining — gases dissolve for diffusion; (4) dense capillary network — maintains steep concentration gradients for O₂ and CO₂.
Question
Describe the changes in the thorax that occur during inspiration.
Step-by-step solution
Step 1
The diaphragm contracts and flattens (moves downward from its domed resting position).
Step 2
The external intercostal muscles contract, pulling the ribs up and outward.
Step 3
Both actions increase the volume of the thorax (chest cavity).
Step 4
Increased volume → decreased pressure inside the lungs (below atmospheric pressure).
Step 5
Air rushes in through the airways from the atmosphere (higher pressure) into the lungs (lower pressure) until pressures equalise.
Answer
Diaphragm contracts (flattens); external intercostal muscles contract (ribs move up and out); thorax volume increases; lung pressure falls below atmospheric; air moves into the lungs down the pressure gradient.
Question
State TWO differences between inhaled and exhaled air.
Step-by-step solution
Step 1
Inhaled air contains approximately 21% oxygen; exhaled air contains approximately 16% oxygen (less oxygen).
Step 2
Inhaled air contains approximately 0.04% carbon dioxide; exhaled air contains approximately 4% carbon dioxide (more CO₂).
Step 3
Exhaled air is also more saturated with water vapour than inhaled air. Nitrogen content remains approximately the same (~78%) in both.
Answer
Question
Explain how carbon monoxide in cigarette smoke affects oxygen transport in the blood.
Step-by-step solution
Step 1
Carbon monoxide (CO) has a much higher affinity for haemoglobin than oxygen does.
Step 2
CO binds irreversibly (or very tightly) to haemoglobin, forming carboxyhaemoglobin.
Step 3
Haemoglobin bound to CO cannot carry oxygen. The oxygen-carrying capacity of the blood is reduced.
Step 4
Tissues receive less oxygen → cells cannot respire aerobically at full rate → reduced energy production, especially dangerous during exercise or in people with heart disease.
Answer
CO binds permanently to haemoglobin, forming carboxyhaemoglobin; haemoglobin is no longer able to carry oxygen. The oxygen-carrying capacity of the blood is reduced, so tissues receive less oxygen.
Question
A patient has emphysema. Explain why they have difficulty exercising.
Step-by-step solution
Step 1
Emphysema is caused by destruction of the alveolar walls (often due to smoking). Alveoli merge into fewer, larger air sacs.
Step 2
Destruction reduces the total surface area available for gas exchange in the lungs.
Step 3
Less O2 diffuses into the blood per breath. The rate of O2 absorption cannot meet the increased demand during exercise.
Step 4
Muscles receive insufficient O2 for aerobic respiration at the required rate → fatigue and breathlessness during exercise.
Answer
Emphysema destroys alveolar walls, reducing the surface area for gas exchange. Less O₂ is absorbed per breath; during exercise, muscles need more O₂ for respiration. The lungs cannot supply sufficient O₂, causing breathlessness and early fatigue.
Definitions to memorise and the exact keywords mark schemes credit for gas exchange answers — sharpened from recent examiner reports for the 2026 0654 sitting.
The diffusion of oxygen from the air into the blood and carbon dioxide from the blood into the air across a gas exchange surface (alveoli in mammals).
Tiny air sacs in the lungs with thin, moist, well-supplied walls that provide a large surface area for gas exchange between air and blood.
The process of moving air into and out of the lungs (breathing in = inspiration; breathing out = expiration) to maintain concentration gradients for gas exchange.
A sheet of muscle below the lungs that contracts (flattens) during inspiration to increase thorax volume and reduce lung pressure, drawing air in.
A lung disease in which the walls of the alveoli are destroyed, reducing the surface area for gas exchange and causing chronic breathlessness. Often caused by smoking.
The stable compound formed when carbon monoxide binds to haemoglobin; it cannot carry oxygen, reducing the blood's oxygen-carrying capacity.
The traps other students keep falling into on gas exchange questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.
Why it happens
Students describe breathing in everyday language rather than the mechanism.
How to avoid it
Air moves passively down a pressure gradient: diaphragm and ribs create more volume → pressure inside lungs falls below atmospheric → air flows in. The lungs do not actively suck air.
Why it happens
Students know red blood cells carry oxygen and assume they carry CO₂ too.
How to avoid it
Most CO2 is transported dissolved in blood plasma (as hydrogen carbonate ions, HCO3−). Some is carried by haemoglobin, and a small amount dissolves directly in plasma.
Why it happens
Students know smoking causes cancer but mix up which component is responsible.
How to avoid it
Tar in cigarette smoke is the carcinogen — it damages cilia and DNA, leading to lung cancer. Nicotine is the addictive component that raises blood pressure. CO reduces O₂ transport.
Why it happens
Students focus on ventilation but overlook surface area and diffusion distance.
How to avoid it
Rate of gas exchange depends on: surface area, diffusion distance, concentration gradient (maintained by ventilation and blood flow), and surface moisture. Breathing rate affects the concentration gradient only.
The things students keep getting wrong in this sub-topic, answered.