Gas Exchange in Humans
The four features of a gas exchange surface; structures of the breathing system; the composition difference between inspired and expired air; and Extended: ventilation mechanics (intercostal muscles and diaphragm), the CO₂ feedback mechanism for exercise, and goblet cells, mucus and cilia in airway protection.
Gas Exchange in Humans
CORE EXTENDEDFeatures of a Gas Exchange Surface
Alveoli are the site of gas exchange in the lungs. Their structure gives them four key features that make them highly efficient:
| Feature | How it aids gas exchange | How alveoli achieve it |
|---|---|---|
| Large surface area | More molecules can cross the surface simultaneously | Millions of alveoli in each lung create an enormous total surface area (~70 m² — the size of a tennis court) |
| Thin surface | Short diffusion distance → faster rate of diffusion | Alveolar wall + capillary wall together are only two cells thick (one cell each) |
| Good blood supply | Removes O₂ and delivers CO₂ continuously, maintaining steep concentration gradients | Dense network of capillaries surrounds every alveolus |
| Good ventilation with air | Replaces depleted O₂ and removes accumulated CO₂, maintaining steep gradients | Breathing movements continuously refresh alveolar air |
The Breathing System — Structures and Functions
| Structure | Function |
|---|---|
| Larynx | Voice box — contains vocal cords; connects pharynx to trachea |
| Trachea | Windpipe — carries air to and from the lungs; kept open by C-shaped cartilage rings |
| Bronchi (singular: bronchus) | Two branches of the trachea, one entering each lung; divide further into bronchioles |
| Bronchioles | Smaller airways branching from bronchi; no cartilage; lead to alveoli |
| Alveoli (singular: alveolus) | Tiny air sacs; the actual site of gas exchange between air and blood |
| Associated capillaries | Surround each alveolus; O₂ diffuses from alveolus into blood; CO₂ diffuses out |
| Lungs | Organs containing bronchi, bronchioles, and alveoli; housed in the thorax |
| Diaphragm | Dome-shaped muscle beneath the lungs; contracts flat to increase thorax volume (inspiration) |
| Ribs | Bony cage protecting the lungs; moved by intercostal muscles to change thorax volume |
| Intercostal muscles | Between the ribs; control rib movement during breathing |
The trachea is held open by C-shaped rings of cartilage. Without them, the trachea would collapse inward when air pressure drops during inhalation, blocking airflow. The C-shape (incomplete ring at the back) allows the oesophagus, which lies behind the trachea, to expand when food is swallowed.
Inspired vs Expired Air — Composition
| Gas | Inspired air (%) | Expired air (%) | Change |
|---|---|---|---|
| Oxygen | ~21% | ~16% | Decreases — O₂ diffuses from alveoli into blood for respiration |
| Carbon dioxide | ~0.04% | ~4% | Increases — CO₂ produced by cellular respiration diffuses from blood into alveoli |
| Water vapour | Variable (low) | High (saturated) | Increases — water evaporates from the moist lining of the airways |
| Nitrogen | ~78% | ~78% | Unchanged — not used or produced by the body |
Method: Bubble inspired air through one tube of limewater and expired air through another. Compare how quickly each turns cloudy.
Result: Expired air turns limewater cloudy (milky white) much faster than inspired air, showing expired air contains a higher concentration of CO₂.
Limewater test: Clear limewater + CO₂ → milky white (calcium carbonate precipitate). This is also the test for CO₂ in food tests (Topic 4) and gas exchange in plants (Topic 6).
Effect of Physical Activity on Breathing
During exercise, breathing rate and depth both increase. This can be investigated by counting breaths per minute at rest and after different intensities of exercise.
Ventilation Mechanism — Extended
| Phase | Muscles | Effect on thorax | Pressure change | Result |
|---|---|---|---|---|
| Inspiration (breathing in) |
Diaphragm contracts (flattens down); external intercostal muscles contract (ribs move up and out) | Volume of thorax increases | Pressure inside thorax decreases below atmospheric pressure | Air flows into lungs (down pressure gradient) |
| Expiration (breathing out) |
Diaphragm relaxes (domes up); internal intercostal muscles contract (ribs move down and in); elastic recoil of lung tissue | Volume of thorax decreases | Pressure inside thorax increases above atmospheric pressure | Air flows out of lungs (down pressure gradient) |
Exercise and Breathing Rate — Extended
1. During exercise, muscles respire faster, producing more CO₂.
2. CO₂ concentration in the blood increases.
3. This is detected by receptors (in the brain and aorta/carotid arteries).
4. The brain sends nerve impulses to the diaphragm and intercostal muscles.
5. Breathing rate increases and depth of each breath increases.
6. More CO₂ is exhaled; more O₂ is inhaled — blood CO₂ returns to normal.
Protecting the Breathing System — Goblet Cells, Mucus, Cilia
| Structure | What it does | How it protects |
|---|---|---|
| Goblet cells | Specialised epithelial cells that secrete mucus into the airway lining | Produce the sticky mucus layer that traps dust, bacteria, viruses, and other particles inhaled with air |
| Mucus | Sticky secretion lining the trachea, bronchi, and bronchioles | Traps pathogens and particles before they reach the alveoli (where they could cause infection) |
| Ciliated cells | Cells with hair-like cilia that beat rhythmically upward | Move the mucus layer (with trapped particles) continuously upward toward the throat, where it is swallowed or expelled — this is the mucociliary escalator |
Cigarette smoke paralyses and destroys cilia over time. Without functioning cilia, mucus and trapped pathogens/particles accumulate in the airways instead of being cleared upward. This leads to the characteristic “smoker’s cough” as the body attempts to clear mucus by coughing, and increases susceptibility to respiratory infections.
Which combination of features makes alveoli efficient surfaces for gas exchange?
- A. Thick walls, large surface area, poor blood supply
- B. Thin walls, small surface area, good blood supply
- C. Thin walls, large surface area, good blood supply, good ventilation
- D. Thick walls, large surface area, good ventilation
Describe the events that occur during inspiration (breathing in). Include the role of the diaphragm, external intercostal muscles, and how air enters the lungs. [5 marks]
- The diaphragm contracts and flattens (moves downward) [1 mark]
- The external intercostal muscles contract, pulling the ribs upward and outward [1 mark]
- These movements increase the volume of the thorax [1 mark]
- The pressure inside the thorax/lungs decreases below atmospheric pressure [1 mark]
- Air flows into the lungs down the pressure gradient (from higher pressure outside to lower pressure inside) [1 mark]
Comprehensive Practice Questions
Mixed questions across Topic 11.
A student breathes out through limewater and it turns milky white. Which gas caused this change and in which direction does it diffuse in the alveoli?
- A. Oxygen; diffuses from blood into alveoli
- B. Carbon dioxide; diffuses from blood into alveoli
- C. Carbon dioxide; diffuses from alveoli into blood
- D. Nitrogen; diffuses from alveoli into blood
Explain how the alveoli are adapted for efficient gas exchange. Give four adaptations. [4 marks]
- Millions of alveoli provide a large total surface area for diffusion [1 mark]
- Alveolar walls are only one cell thick (+ capillary wall one cell thick) providing a thin surface / short diffusion distance [1 mark]
- Dense capillary network provides a good blood supply that continuously removes O₂ and delivers CO₂, maintaining steep concentration gradients [1 mark]
- Breathing movements provide good ventilation that replaces O₂-depleted air with fresh air, maintaining the gradient [1 mark]
A student’s breathing rate increases from 15 to 28 breaths per minute during vigorous exercise. Explain, in terms of CO₂ concentration and brain activity, why this increase occurs. [4 marks]
- During exercise, muscles respire faster, producing more CO₂ → CO₂ concentration in the blood increases [1 mark]
- This increased CO₂ concentration is detected by receptors in the brain (and/or carotid arteries/aorta) [1 mark]
- The brain sends increased nerve impulses to the diaphragm and intercostal muscles [1 mark]
- This causes both an increased rate and greater depth of breathing — more CO₂ is exhaled and more O₂ is taken in [1 mark]
High-Frequency Mistakes — Topic 11 Overall
- 👘"Expired air contains no oxygen"Expired air still contains about 16% oxygen — only about 5% has been removed. It is lower than inspired air (21%), not zero. Saying "no oxygen" is incorrect.
- ⬆"Diaphragm moves up during inspiration"During inspiration, the diaphragm contracts and flattens/moves downward, increasing thorax volume. It moves upward only during expiration (when it relaxes and returns to its dome shape).
- 🧠Ext: Confusing internal and external intercostal musclesExternal intercostal muscles contract during inspiration (ribs up and out). Internal intercostal muscles contract during forced expiration (ribs down and in). Remember: External = Entry of air (inspiration).
- 📈Ext: Saying exercise increases breathing because "oxygen runs out"The trigger for increased breathing rate is rising CO₂ concentration in the blood — detected by the brain. Oxygen levels do not drop enough to trigger the response directly. Always link the mechanism to CO₂, not O₂.
- 💧Ext: Forgetting the sequence: CO₂ rises → detected by brain → nerve impulses → breathing increasesFull chain required for Paper 4 marks. Stopping at "CO₂ rises" without linking to brain detection and nerve signals to muscles loses 2 of the 4 available marks.
- 🚪Ext: Saying cilia produce mucusCilia do NOT produce mucus — they move it. Mucus is produced by goblet cells. Cilia are on ciliated cells, which are a different cell type from goblet cells. Both types are found in the same airway lining but have distinct roles.
- 🚨Calling the process "breathing" when asked about "gas exchange"Breathing (ventilation) is the mechanical movement of air in and out of the lungs. Gas exchange is the diffusion of O₂ and CO₂ across the alveolar surface. They are related but distinct processes. Use the correct term for the question asked.
Highest-yield Core items: the four features of a gas exchange surface (large SA / thin / blood supply / ventilation — always link each to how it improves diffusion); the composition table for inspired vs expired air (O₂, CO₂, water vapour — values not required, just direction of change); and the limewater practical. For Extended: the ventilation mechanism is a reliable Paper 4 question (learn the full chain: muscles contract → volume increases → pressure decreases → air flows in); the CO₂-brain feedback mechanism for exercise; and the goblet cell/mucus/cilia trio for respiratory protection (frequently appears alongside Topic 10 immunity questions).