Gas Exchange — The Lungs
Where the circulatory system meets the atmosphere. A branching network of airways with specialised tissues for cleaning, support, and control, ending in 300 million alveoli that maximise the surface area for diffusion. The structural design that lets us extract enough oxygen to power an active mammalian body.
The gas exchange system
The gas exchange system is responsible for taking up O2 from the atmosphere into the blood, and removing CO2 from the blood to be exhaled. In mammals, this is achieved by a branching network of airways ending in millions of microscopic alveoli — together giving the surface area, thin barrier, and steep concentration gradient required for efficient diffusion.
This topic links closely with Topic 8: Topic 8 dealt with how the blood transports gases between lungs and tissues; Topic 9 deals with how O2 gets into the blood at the lungs in the first place, and how CO2 is released.
For 9700 (2025–2027), the following are NOT in the 9.1 syllabus:
- The mechanism of breathing — do NOT describe diaphragm contraction, intercostal muscle action, or pressure changes in the thorax
- Lung volumes — tidal volume, vital capacity, residual volume, spirometer traces
- Effects of exercise on ventilation rate / depth
Stay focused on what IS required: structures and tissues, their distributions, their functions, and gas exchange at alveoli. Some of the excluded items appear in IGCSE 0610 or in other A-level boards (OCR/Edexcel/IB) but are not part of CAIE 9700.
Overview of the gas exchange system
Air enters through the nose/mouth, then passes through a series of progressively narrowing tubes that branch like an inverted tree, ending in tiny alveoli where exchange takes place:
- Trachea (windpipe): single tube from larynx down to chest
- Bronchi (singular: bronchus): two main branches, one to each lung
- Bronchioles: progressively smaller branching tubes within the lung
- Alveoli: clusters of tiny air sacs at the end of the smallest bronchioles — the actual gas-exchange surface
- Capillary network: dense web of capillaries surrounding each alveolus
Two spongy organs in the thoracic cavity, surrounded by the pleural membranes (a thin double-layered sheet with lubricating fluid in between — reduces friction during breathing). Each lung contains the airway tree and around 300 million alveoli, giving a total alveolar surface area of about 70–100 m2 in an adult human — comparable to half a tennis court.
Trachea and bronchi — structure
The trachea and bronchi share a similar wall structure with several distinct tissue layers. Their walls are larger and more rigid than those of bronchioles.
| Layer (lumen → outside) | Tissue | Function |
|---|---|---|
| Innermost (lining the lumen) | Ciliated epithelium & goblet cells | Trap and remove dust, microbes, debris (cleaning system) |
| Beneath the epithelium | Mucous glands in connective tissue | Secrete additional mucus to support the goblet cell layer |
| Layer of smooth muscle | Smooth muscle | Adjusts airway diameter (more important in smaller airways) |
| Layer of cartilage (largest layer) | Cartilage — C-shaped rings in trachea, irregular plates in bronchi | Holds the airway open against negative pressure during inhalation; prevents collapse |
| Outermost | Connective tissue with elastic fibres | Allows stretch and recoil during breathing |
The cartilage rings are open at the back (where the trachea sits against the oesophagus). The C-shape provides rigid support to keep the airway open while still allowing the oesophagus to expand backwards when food is swallowed. Plain rings of cartilage all the way around would compress the oesophagus.
Bronchioles — structure
As airways branch into smaller bronchioles, the wall structure changes:
- No cartilage — the bronchioles' diameter is small enough that they are kept open by the surrounding lung tissue (held open by elastic fibres pulling outwards)
- Smooth muscle dominant — the most prominent tissue in the bronchiole wall; allows large changes in diameter (bronchodilation / bronchoconstriction)
- Ciliated epithelium still present in larger bronchioles, becoming progressively thinner and disappearing in the smallest ones close to alveoli
- Few or no goblet cells in smaller bronchioles — less mucus needed at this depth
- Walls are very thin and lined with squamous epithelium in the smallest ducts approaching alveoli
The smooth muscle in bronchiole walls can contract or relax to control airflow into different parts of the lung. In asthma, this smooth muscle contracts excessively (bronchoconstriction), narrowing the lumen and making it hard to breathe. Bronchodilator inhalers (e.g. salbutamol) cause the smooth muscle to relax, widening the airways. This is a common AS application question.
Alveoli — structure and adaptations
Each alveolus is roughly 200–300 μm across — a tiny hollow sphere of cells. Their structure is the textbook example of an exchange surface optimised for diffusion:
The alveolar wall consists of a single layer of squamous epithelial cells — extremely thin and flat. This minimises the diffusion distance for gases (typically <1 μm from alveolar air to blood plasma).
Each alveolus is wrapped in a dense network of capillaries, also one cell thick. Combined with the alveolar wall, the total barrier between alveolar air and red blood cell is just two cells thick — an exceptionally short diffusion path.
Roughly 300 million alveoli per lung give a combined surface area of around 70–100 m2 (adult human). This vast area accommodates simultaneous diffusion of large volumes of O2 and CO2.
A thin film of moisture lines each alveolus. Gases dissolve in this fluid before diffusing across the cell membrane — gases must be in solution to cross. The moisture also reduces surface tension; specialised surfactant produced by alveolar cells prevents alveolar collapse (premature babies often lack surfactant).
Networks of elastic fibres surround each alveolus. They stretch when the alveolus inflates and recoil during exhalation, helping push air out. This recoil also helps maintain alveolar shape.
Constant breathing renews alveolar air, keeping its O2 concentration high and CO2 concentration low. Constant blood flow through capillaries removes oxygenated blood and brings in deoxygenated blood, keeping the blood-side gradient steep. Together, ventilation + perfusion keep diffusion gradients maximally steep.
Tissue functions in the gas exchange system
The 9700 syllabus requires explicit functions of cartilage, smooth muscle, elastic fibres, and squamous epithelium:
| Tissue | Found in | Function |
|---|---|---|
| Cartilage | Trachea (C-shaped rings), bronchi (irregular plates) | Provides structural support; keeps airways open against negative pressure during inhalation; flexible enough to allow some deformation |
| Smooth muscle | Trachea, bronchi, bronchioles (most abundant in bronchioles) | Contracts or relaxes to alter airway diameter (bronchoconstriction / bronchodilation); adjusts airflow distribution to different lung regions |
| Elastic fibres | Bronchioles, alveoli, surrounding all airways | Stretch during inhalation; recoil passively during exhalation to help expel air; help keep small airways open |
| Squamous epithelium | Alveolar walls, smallest bronchioles | Single layer of very flat cells — provides a short diffusion distance for gases at the gas-exchange surface |
The airway cleaning system — goblet cells, mucous glands, ciliated epithelium
Inhaled air contains dust, pollen, pathogens, and other particles. The airway lining intercepts these before they reach the delicate alveoli:
Large flask-shaped cells embedded in the airway epithelium. They secrete mucus — a sticky glycoprotein-rich solution — onto the airway surface. Mucus traps dust, pollen, microbes, and other particles, preventing them from reaching the alveoli.
Glands in the connective tissue beneath the epithelium produce additional mucus, supplementing the goblet cells. Together they coat the entire airway with a thin layer of mucus.
The columnar epithelial cells lining airways have cilia — tiny hair-like extensions on their surface. The cilia beat rhythmically (like a wave), sweeping the mucus layer (carrying its trapped debris) upwards from the lungs towards the throat. The mucus is then either swallowed (so debris is destroyed by stomach acid) or coughed out.
This whole system is sometimes called the mucociliary escalator: mucus + cilia continuously transport trapped material upwards, keeping the lower airways and alveoli clean. Without it, every breath would deliver foreign particles deep into the lungs.
Tobacco smoke contains chemicals (especially tar and acrolein) that damage the cilia — they are paralysed or destroyed. Mucus is no longer cleared from the airways, so it accumulates with trapped pathogens. This is why smokers develop a chronic cough (the body's only remaining way to clear mucus), more frequent chest infections, and conditions such as chronic bronchitis. This is a frequent AS application question.
Airway comparison — trachea vs bronchus vs bronchiole
| Feature | Trachea | Bronchus | Bronchiole |
|---|---|---|---|
| Diameter | Largest (~2.5 cm) | Large (~1–2 cm) | Small (<1 mm) |
| Cartilage | C-shaped rings (regular) | Irregular plates (less complete) | None |
| Smooth muscle | Some | More than trachea | Most prominent (dominates wall) |
| Ciliated epithelium | Yes (extensive) | Yes | Larger bronchioles only; reduces toward alveoli |
| Goblet cells | Many | Many | Few or absent in smaller bronchioles |
| Wall thickness | Thick | Thick | Thin |
Gas exchange at the alveoli
Gas exchange between alveolar air and capillary blood occurs by simple diffusion down concentration (partial pressure) gradients. There is no active transport of gases.
- O2 uptake: alveolar air has a high pO2 (~13–14 kPa); deoxygenated blood arriving in the capillaries has a low pO2 (~5 kPa). O2 diffuses from alveolar air across the squamous epithelium and capillary endothelium into the plasma, then into red blood cells where it binds haemoglobin (Topic 8.2)
- CO2 release: deoxygenated blood arriving has a high pCO2 (~6 kPa); alveolar air has a low pCO2 (~5 kPa). CO2 diffuses from blood (where it has been transported as HCO3−, carbaminohaemoglobin, or dissolved — reversed at the lungs as in Topic 8.2) into the alveolar air, ready to be exhaled
- By the time blood leaves the capillary, its pO2 has reached ~13 kPa (oxygenated) and its pCO2 has fallen to ~5 kPa — ready to leave for the heart and the systemic circulation
Maintaining steep concentration gradients
Diffusion rate depends on the steepness of the concentration gradient. The lung's structure and operation maintain steep gradients for both gases:
- Continuous ventilation — breathing constantly replaces alveolar air, so alveolar O2 stays high and alveolar CO2 stays low. Even in resting humans, ~12–15 breaths per minute exchange enough air to maintain steep gradients. (Note: 9.1 does not require the breathing mechanism, but does ask candidates to appreciate that ventilation maintains the gradient.)
- Continuous blood flow through capillaries — oxygenated blood is constantly removed and deoxygenated blood is constantly arriving, keeping the blood-side gradient steep
- Short diffusion distance — the alveolar wall (one squamous cell) + capillary wall (one squamous cell) + barely any plasma between RBC and capillary wall means total diffusion distance is well under 1 μm
- Huge surface area — ~70–100 m2 means simultaneous diffusion across enormous total area; total diffusion rate is the sum across this whole surface
Although Fick's law is not named in the 9700 syllabus, the idea behind it underlies all exam answers about gas exchange efficiency:
Rate of diffusion ∝ (surface area × concentration gradient) ÷ diffusion distance
Lungs maximise rate by maximising surface area, maximising the gradient (via ventilation + blood flow), and minimising distance (single-cell-thick walls). Use this framework to construct any "explain the efficiency of the alveoli" answer.
Which feature of an alveolus contributes most directly to the SHORT DIFFUSION DISTANCE for gases between alveolar air and red blood cells?
- A. The presence of elastic fibres surrounding the alveolus
- B. The huge total surface area of all alveoli combined
- C. The squamous epithelium of the alveolar wall and the single-cell-thick capillary wall
- D. The thin film of moisture lining the alveolus
A patient is diagnosed with chronic bronchitis after years of smoking. Which combination of changes best describes the cellular damage caused by smoke?
- A. Cartilage rings collapse; alveoli rupture
- B. Cilia are paralysed or destroyed; goblet cells produce excess mucus that cannot be cleared
- C. Smooth muscle relaxes permanently; airway diameter becomes too wide
- D. Squamous epithelium thickens; gas exchange is more efficient
The structure of the alveoli and the surrounding capillary network is highly adapted for efficient gas exchange.
(a) Describe THREE features of an alveolus that adapt it for efficient gas exchange. [3]
(b) Explain how the structure and the operation of the gas exchange system maintain a steep diffusion gradient for oxygen between alveolar air and the blood. [4]
(c) The bronchioles do not contain cartilage, but they have abundant smooth muscle. Suggest the functional advantage of this difference compared to the trachea. [2]
(a) Three alveolar adaptations [3 marks; any three]
- Squamous epithelium (one cell thick) — minimises diffusion distance for gases [1]
- Surrounded by a dense capillary network — capillary walls also one cell thick; maintains close blood supply for diffusion [1]
- Huge total surface area (~70–100 m2) — large area available for simultaneous diffusion [1]
- Moist lining — gases dissolve before diffusing across the cell membrane [1]
- Elastic fibres — alveolus expands during inhalation and recoils during exhalation, helping ventilation [1]
(b) Maintaining the O2 gradient [4 marks]
- Continuous ventilation constantly renews alveolar air, keeping alveolar pO2 high [1]
- Continuous blood flow through capillaries removes oxygenated blood and brings deoxygenated blood with low pO2, keeping blood-side pO2 low [1]
- Together this maintains a steep partial-pressure difference between alveolar air and blood [1]
- Haemoglobin in red blood cells binds O2, removing dissolved O2 from plasma quickly — maintaining low plasma pO2 and therefore the gradient (Topic 8.2 link) [1]
(c) Bronchiole smooth muscle vs trachea cartilage [2 marks]
- Bronchioles have small diameter and are kept open by surrounding lung tissue, so cartilage support is not needed; cartilage in such small tubes would limit flexibility [1]
- The smooth muscle allows the bronchiole's diameter to be actively adjusted (bronchoconstriction or bronchodilation), controlling airflow distribution between regions of the lung — cartilage is rigid and cannot do this [1]
Topic 9 Practice — Comprehensive
Mixed practice in 9700 P1/P2 style, including synoptic links to Topic 8. Try each before revealing the answer.
Which combination of tissues is most characteristic of the wall of a small bronchiole, but NOT of the trachea?
- A. Cartilage and ciliated epithelium
- B. Cartilage and smooth muscle
- C. Smooth muscle and elastic fibres, with no cartilage
- D. Squamous epithelium and goblet cells
Goblet cells in the trachea and bronchi secrete mucus. What is the role of cilia in the same tissue layer?
- A. To secrete additional mucus that supplements the goblet cells
- B. To kill pathogens trapped in the mucus by direct contact
- C. To beat rhythmically and sweep mucus and trapped debris upward toward the throat
- D. To absorb water and small molecules across the epithelium
The cartilage in the wall of the trachea is in C-shaped rings rather than complete rings. What is the most likely reason?
- A. To allow the trachea to constrict during exhalation
- B. To allow the oesophagus, lying behind the trachea, to expand backwards when food is swallowed
- C. To save the body's resources by using less cartilage
- D. To allow ciliated epithelium to extend into the gap
Oxygen passes from atmospheric air to a respiring muscle cell via the gas exchange system and the circulatory system.
(a) Describe the path taken by an oxygen molecule from the trachea to a red blood cell in an alveolar capillary. [4]
(b) Explain why the partial pressure of oxygen in the alveoli is higher than that in the deoxygenated blood arriving in alveolar capillaries. [2]
(c) Explain how the Bohr shift, occurring in active muscle, increases the rate of O2 delivery to that muscle. [3]
(a) Path of an O2 molecule [4 marks]
- From the trachea, O2 passes into the bronchi (left or right), then into progressively smaller bronchioles [1]
- It enters one of the alveoli at the end of the bronchiole tree [1]
- O2 dissolves in the moist lining of the alveolus and diffuses across the squamous epithelium of the alveolar wall and the single-celled wall of the surrounding capillary [1]
- It enters the plasma, then crosses the red blood cell membrane and binds to a haem group of haemoglobin (forming oxyhaemoglobin) [1]
(b) Why alveolar pO2 is higher than capillary blood pO2 [2 marks]
- Continuous ventilation replaces alveolar air with fresh inhaled air rich in O2, keeping alveolar pO2 high [1]
- Blood arriving from the body via the pulmonary artery has had its O2 removed by respiring tissues, so it has low pO2; continuous flow brings new low-pO2 blood into the capillaries [1]
(c) Bohr shift effect on O2 delivery to active muscle [3 marks]
- Active muscle produces more CO2 by aerobic respiration, raising local CO2 partial pressure / lowering pH [1]
- The Bohr shift moves the haemoglobin oxygen dissociation curve to the right at high CO2, lowering Hb's affinity for O2 at any given pO2 [1]
- So at the same pO2 as in resting tissue, more O2 is released from Hb to the active muscle, matching supply to demand without external regulation [1]
Synoptic note: This question integrates Topic 9 (gas exchange at alveoli) with Topic 8.2 (Hb behaviour in tissues). Such cross-topic structured questions are increasingly common in 9700 P2.
In the genetic disease cystic fibrosis (CF), the mucus produced by goblet cells and mucous glands in the airways is abnormally thick and sticky. CF patients commonly suffer recurrent lung infections.
(a) Explain why CF patients suffer recurrent lung infections. [3]
(b) Suggest why CF patients may also have difficulty exchanging gases efficiently. [3]
(a) Why CF causes recurrent lung infections [3 marks]
- The thick mucus is too viscous for cilia to sweep upward effectively — the mucociliary escalator fails [1]
- Trapped dust, microbes, and pathogens accumulate in the airways instead of being cleared [1]
- Bacteria multiply in the stagnant mucus, leading to repeated infections such as pneumonia [1]
(b) Why gas exchange is impaired [3 marks]
- Thick mucus accumulating in bronchioles narrows the airways and partially blocks airflow to alveoli, reducing ventilation [1]
- The mucus layer adds to the diffusion distance between alveolar air and the capillary blood, slowing diffusion [1]
- Repeated infections damage alveolar walls and reduce the total surface area for gas exchange [1]
Topic 9 — Common Mistakes
- ⚠Describing the breathing mechanism in a 9.1 answerThe 9700 (2025-2027) syllabus explicitly excludes the breathing mechanism. Mentioning diaphragm contraction, intercostal muscles, or thoracic pressure changes wastes time and may not be credited. Stay with structures, tissues, and gas exchange.
- 🧬Confusing “respiration” with “gas exchange”Gas exchange is the diffusion of O2 and CO2 between alveolar air and blood. Respiration is the chemical release of energy from glucose inside cells (Topic 12). Don't say "respiration takes place at the alveoli" — it doesn't. Use the right term.
- 🎯Saying alveoli have ciliated epithelium or goblet cellsWrong. Alveoli have squamous epithelium only — very thin and flat, optimised for diffusion. Cilia and goblet cells belong to the larger conducting airways (trachea, bronchi, larger bronchioles). Putting cilia in alveoli would block diffusion.
- 🏪"Bronchioles have cartilage"Bronchioles do NOT have cartilage. They are kept open by elastic fibres in the surrounding lung tissue, and their wall is dominated by smooth muscle (allowing diameter control). Putting cartilage in bronchioles would prevent active diameter control.
- 🧸Saying smooth muscle "pumps air" or "contracts to push air"Smooth muscle in airways alters diameter, not airflow directly. Air movement is driven by the breathing mechanism (excluded from 9.1). Smooth muscle contraction narrows the bronchiole; relaxation widens it. It controls distribution of airflow, not generation of airflow.
- ❓Counting the moisture layer as part of the diffusion barrierSquamous epithelium + capillary endothelium = the diffusion barrier. The thin film of moisture is functionally essential (gases must dissolve to diffuse across membranes) but is generally considered part of the alveolar surface, not added to the “wall thickness”. Frame the moisture's role as enabling diffusion, not adding distance.
- 🤗"Smoking destroys alveoli" used as an answer about chronic bronchitisTwo different smoking-related conditions: chronic bronchitis = damaged cilia + excess mucus + airway narrowing. Emphysema = alveolar wall destruction + loss of surface area. Don't confuse them. The 9700 syllabus may ask about cilia and mucus specifically.
- 🌏Saying the alveoli have huge surface area without explaining what that doesThe exam expects you to link surface area to rate of diffusion. Saying "large surface area for gas exchange" without further reason often loses the explanation mark. Always link: surface area → rate of diffusion (more area means more gas exchanged per unit time).
- ➡Forgetting to mention BOTH ventilation and blood flow when discussing gradientsFor a steep diffusion gradient, both sides of the alveolar barrier must be kept “fresh”. Ventilation keeps alveolar air's O2 high and CO2 low; blood flow keeps blood plasma's O2 low and CO2 high. Mentioning only one usually loses a mark.
- 🤯Saying CO2 "is in the blood as gas" before reaching the alveoliMost CO2 in the blood is transported as HCO3− ions in plasma; ~10% as carbaminohaemoglobin; only ~5% dissolved as CO2 gas (Topic 8.2). At the lungs, the chemistry reverses to release CO2 for diffusion. Be specific about which form — especially in synoptic Q.
Topic 9 is one of the smaller AS topics (one sub-section) but is heavily synoptic with Topic 8 (transport in mammals) and connects to Topic 11 (lung pathogens like the TB bacterium) and Topic 12 (respiration generates the CO2 being exhaled and consumes the O2 being inhaled). Highest-yield items: tissue distribution and functions in trachea / bronchus / bronchiole / alveolus, recognising these in micrographs, the alveolar adaptations for diffusion, the mucociliary cleaning system, and effects of smoking. Practical assessment may include identifying tissues from prepared slides and drawing plan diagrams of TS sections. Always link adaptations to rate of diffusion via surface area, gradient, and distance.