AS 9 Gas exchange | A Level Biology 9700

Home › Learning Hub › A Level Biology › AS 9: Gas exchange

🎯What you need to be able to do

Describe the structure of the human gas exchange system: lungs, trachea, bronchi, bronchioles, alveoli and the capillary network.
Describe the distribution of cartilage, ciliated epithelium, goblet cells, squamous epithelium, smooth muscle and capillaries.
Recognise those tissues in slides, photomicrographs and electron micrographs.
Recognise trachea, bronchi, bronchioles and alveoli, and draw plan diagrams of the walls of the trachea and bronchus.
Describe how ciliated epithelial cells, goblet cells and mucous glands maintain the health of the system.
Describe the functions of cartilage, smooth muscle, elastic fibres and squamous epithelium.
Describe gas exchange between the air in the alveoli and the blood in the capillaries.

📚The biology

The route air takes

Air passes from the trachea into two bronchi, one to each lung, which divide repeatedly into bronchioles, ending in clusters of alveoli. Each alveolus is wrapped in a dense capillary network. As the tubes narrow, their walls change in a systematic way, and knowing the pattern lets you identify any section on sight.

What is in the wall, and where

Cartilage — C-shaped rings in the trachea, irregular plates in the bronchi, absent from bronchioles. It holds the airway open against the pressure changes of breathing, so it cannot collapse when pressure inside falls during inspiration. The rings are incomplete at the back so the oesophagus can expand when swallowing.
Smooth muscle — present throughout, and proportionally most important in the bronchioles, which have no cartilage. Contraction narrows the airway, constricting airflow; relaxation widens it. This is what happens in an asthma attack.
Elastic fibres — throughout, and abundant in the alveolar walls. They stretch during inspiration and recoil during expiration, helping to force air out; they also allow the airway to return to shape after the smooth muscle relaxes.
Ciliated epithelium — lines the trachea, bronchi and larger bronchioles; it becomes thinner and disappears in the smallest bronchioles.
Goblet cells and mucous glands — among the ciliated cells in the upper airways.
Squamous epithelium — the alveolar wall, one flattened cell thick.
Capillaries — a dense network over the alveoli.

The rule of thumb: as you go deeper, cartilage disappears, cilia and goblet cells disappear, epithelium becomes thinner, and the proportion of smooth muscle and elastic tissue rises.

Keeping the airways clean

Goblet cells and mucous glands secrete mucus, which is sticky and traps pathogens, dust and other particles in the inhaled air. Ciliated epithelial cells have cilia that beat rhythmically, sweeping the mucus with its trapped particles upwards to the throat, where it is swallowed and the pathogens are destroyed by stomach acid.

This is why smoking is so damaging: tar paralyses and destroys the cilia and stimulates goblet cells to produce more mucus, so mucus accumulates, is cleared only by coughing, and the trapped bacteria remain in the airways — hence chronic bronchitis and frequent infection.

Gas exchange at the alveolus

Oxygen dissolves in the film of moisture lining the alveolus and diffuses down its concentration gradient across the squamous alveolar epithelium, then across the capillary endothelium, into the blood plasma and into the red blood cell, where it binds to haemoglobin. Carbon dioxide diffuses the other way.

The adaptations all serve the same three quantities — large area, short distance, steep gradient:

Enormous total surface area — hundreds of millions of alveoli.
Very short diffusion distance — alveolar epithelium and capillary endothelium are each one flattened cell thick, so the barrier is well under a micrometre.
Dense capillary network, with capillaries narrower than a red blood cell so that cells are squeezed flat against the wall, reducing the distance further and slowing them so there is time to exchange.
A steep concentration gradient maintained by continuous ventilation (bringing fresh air) and continuous blood flow (removing oxygenated blood).
Moist lining, so gases dissolve before diffusing.

“Large surface area” on its own is half an answer. Diffusion rate depends on surface area and the concentration gradient and the thickness of the barrier. A full-mark answer names the feature, states which of the three it affects, and says why that increases the rate. Ventilation and blood flow are what maintain the gradient — without them the gradient collapses within seconds and the enormous surface area achieves nothing.

✏️Worked example

A photomicrograph shows a transverse section of a tube from the gas exchange system. The wall contains plates of cartilage, a layer of smooth muscle, and a lining of ciliated epithelium with numerous goblet cells. (a) Identify the tube and justify your answer. (b) A second section from the same lung shows a tube of similar diameter with no cartilage, a relatively thick layer of smooth muscle, and a thin epithelium with no goblet cells. Identify it. (c) Explain how the absence of cartilage in the second tube relates to its function, and what happens to it during an asthma attack.

(a) This is a bronchus. The presence of cartilage rules out a bronchiole immediately, since bronchioles have none. The cartilage is in irregular plates rather than a C-shaped ring, which distinguishes a bronchus from the trachea. Ciliated epithelium with goblet cells is consistent with both, so it is the shape and arrangement of the cartilage that identifies it.

(b) This is a bronchiole. No cartilage, a relatively high proportion of smooth muscle for the wall thickness, and a thin epithelium lacking goblet cells are all characteristic. In the smallest bronchioles the epithelium becomes cuboidal or squamous and cilia disappear entirely.

(c) Cartilage’s function is to hold an airway permanently open against the pressure changes of breathing. The bronchioles do not need this because they are held open by the surrounding lung tissue and by the elastic fibres in it, and because their diameter must be adjustable. Being free of a rigid ring lets the smooth muscle in the wall contract and relax to control the volume of air reaching the alveoli — constricting to reduce airflow, dilating to increase it, for example during exercise.

In an asthma attack the smooth muscle of the bronchioles contracts, narrowing the lumen, while the lining becomes inflamed and secretes excess mucus, narrowing it further. Airflow to the alveoli falls, less oxygen reaches the gas exchange surface, and breathing out becomes difficult. Inhaled drugs relax that smooth muscle to reverse the constriction.

Check it. Run the checklist down the airway in order and see whether the section fits: trachea has C-shaped cartilage rings; bronchus has irregular cartilage plates; bronchiole has no cartilage. Then confirm with the epithelium, which thins and loses goblet cells as you go deeper. If a section shows cartilage and a very thin squamous lining, you have misread one of them — those two do not occur together.

Using diameter to identify the tube. A large bronchiole and a small bronchus can look the same size in section, and a section cut obliquely distorts the apparent diameter of anything. Identification rests on what is in the wall, not how wide the lumen is. Second trap: do not confuse the smooth muscle of a bronchiole with cartilage — cartilage stains differently and its cells sit in distinct spaces (lacunae) within a matrix, while smooth muscle forms bands of elongated cells.

📝Practise

Work through these, then reveal the answer. Each question targets a different objective from the list above.

1. Explain how the alveolus is adapted for efficient gas exchange, referring to three factors that affect the rate of diffusion.

Surface area: there are hundreds of millions of alveoli, giving an enormous total surface across which diffusion can occur, and the greater the area the greater the rate. Diffusion distance: the alveolar wall is a squamous epithelium one flattened cell thick and the capillary wall is a single endothelial cell, so oxygen crosses a barrier well under a micrometre; the shorter the distance, the faster the diffusion. Capillaries are also narrower than red blood cells, so the cells are squeezed against the wall, shortening the distance further. Concentration gradient: continuous ventilation replaces the alveolar air, keeping the oxygen concentration high and the carbon dioxide concentration low, while continuous blood flow removes oxygenated blood; both maintain a steep gradient. The moist lining also allows gases to dissolve before diffusing.

2. Describe how goblet cells and ciliated epithelial cells work together, and explain the effect of smoking on this system.

Goblet cells (and mucous glands beneath the epithelium) secrete mucus, which is sticky and traps pathogens, dust and other particles carried in on inhaled air, preventing them reaching the alveoli. Ciliated epithelial cells bear cilia that beat rhythmically, sweeping the mucus and its trapped material upwards towards the throat, where it is swallowed and any pathogens destroyed by stomach acid. Smoking disrupts both halves: tar paralyses and eventually destroys the cilia, so mucus can no longer be moved, while irritation causes goblet cells to increase in number and secrete more mucus. Mucus therefore accumulates in the airways and can only be cleared by coughing (smoker’s cough); trapped bacteria remain, so infections are frequent, and persistent inflammation leads to chronic bronchitis.

3. State where cartilage is found in the gas exchange system and explain its function.

Cartilage is found as incomplete C-shaped rings in the trachea and as irregular plates in the bronchi. It is absent from bronchioles and from alveoli. Its function is to provide support, holding the airway open so that it does not collapse when the pressure inside falls during inspiration, and preventing it being crushed by surrounding structures. It is strong but flexible, so the airways can still bend as the neck and chest move. The rings in the trachea are incomplete at the back, where the trachea lies against the oesophagus, so that the oesophagus can expand into the gap when food is swallowed.

4. Compare the function of smooth muscle and elastic fibres in the airways.

Smooth muscle contracts to narrow the lumen of the airway, particularly in the bronchioles, controlling the volume of air reaching the alveoli; when it relaxes, the airway widens. Its action is active and is under involuntary control. Elastic fibres do not contract. They stretch as the lungs inflate during inspiration and store energy, then recoil during expiration, helping to force air out of the lungs. They also pull an airway back to its original diameter after the smooth muscle has relaxed — without them the airway would not reopen fully. In the alveolar walls, elastic fibres are what allow alveoli to inflate and then spring back; their destruction in emphysema is why expiration becomes so difficult in that disease.

5. A student writes that oxygen ‘is pumped from the alveolus into the blood’. Correct this and describe the actual process.

Nothing is pumped: gas exchange at the alveolus is entirely by diffusion, a passive process requiring no ATP. Oxygen in the alveolar air dissolves in the film of moisture lining the alveolus, then diffuses down its concentration gradient — from the high concentration in the alveolus to the lower concentration in the blood — across the squamous alveolar epithelium and the capillary endothelium, into the plasma and then into the red blood cell, where it binds to haemoglobin. Binding to haemoglobin removes dissolved oxygen from the plasma and so maintains the gradient, allowing diffusion to continue. Carbon dioxide diffuses in the opposite direction, down its own gradient, from blood to alveolus. Ventilation and blood flow maintain both gradients.

6. A section of lung tissue from a person with emphysema shows fewer, larger alveoli with reduced elastic tissue. Predict two consequences and explain each.

First, the total surface area for gas exchange is greatly reduced, because many small alveoli have merged into fewer large ones and a sphere’s surface area to volume ratio falls as it enlarges. Less surface means a lower rate of diffusion of oxygen into the blood, so the person becomes breathless on exertion and blood oxygen saturation falls. Second, loss of elastic fibres means the alveoli cannot recoil during expiration. Normally the stretched elastic fibres recoil to help force air out; without them, expiration becomes an active, effortful process and air is trapped in the alveoli, so the volume of fresh air reaching the exchange surface with each breath falls. This also reduces the concentration gradient, compounding the first effect.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

Histology atlases from any medical school — labelled sections of trachea, bronchus and bronchiole side by side, which is the fastest way to learn the identification pattern
British Lung Foundation — accessible accounts of asthma, bronchitis and emphysema that connect the histology to real disease
Any respiratory system animation showing the branching from trachea to alveolus with the wall composition changing — the pattern is much easier to see than to memorise from a table