The gas exchange system

Syllabus points

• Describe the structure of the human gas exchange system.
• Recognise and describe the distribution of cartilage, ciliated epithelium, goblet cells, smooth muscle and elastic fibres.
• Relate the structure of the alveoli to gas exchange.
• Describe the functions of the airway tissues.

The route air takes

Trachea → bronchi → bronchioles → alveoli.

Air is drawn in through the nose and mouth, down the trachea, which divides into two bronchi, one to each lung. Each bronchus branches repeatedly into progressively narrower bronchioles, and the finest of these end in clusters of alveoli, which is where gas exchange happens.

Everything before the alveoli is conducting airway. No exchange happens there, and its job is to deliver clean, warm, moist air to the alveoli without collapsing on the way.

The tissues, and where each one is

This is the part Cambridge tests most precisely, usually as a table asking which tissue is present in which airway.

Cartilage

Present in the trachea and bronchi. Absent from bronchioles.

In the trachea it forms C-shaped rings, incomplete at the back. In the bronchi it is present as irregular blocks.

Its function is support: it holds the airway open and stops it collapsing when the pressure inside falls during inhalation. The rings are C-shaped rather than complete so that the oesophagus, immediately behind, can expand when food passes down it.

Bronchioles have no cartilage because they are held open by the surrounding lung tissue, and because cartilage would prevent them changing diameter.

Smooth muscle

Present in the trachea, bronchi and bronchioles.

It contracts to narrow the airway and relaxes to widen it, controlling the flow of air. It matters most in the bronchioles, which have no cartilage to hold them at a fixed width, so their diameter is controlled entirely by this muscle.

In asthma, the smooth muscle of the bronchioles contracts excessively, narrowing the airway and making breathing difficult.

Elastic fibres

Present throughout, including in the alveolar walls.

They stretch during inhalation as the airways and alveoli expand, and recoil during exhalation, helping to push air out. Elastic recoil is a significant part of what makes normal breathing out passive.

Emphysema destroys these fibres, so the lungs lose their recoil and exhalation becomes difficult.

Ciliated epithelium

Present in the trachea, bronchi and larger bronchioles. Absent from alveoli.

The cilia beat in a coordinated wave, sweeping mucus upward towards the throat, where it is swallowed. The mucus carries with it the dust, pollen and bacteria trapped in it. This is the mucus escalator, and it is the lung's main cleaning mechanism.

Alveoli have no cilia because a ciliated cell would be far too thick for gas exchange.

Goblet cells

Present in the trachea, bronchi and larger bronchioles. Absent from alveoli.

They secrete mucus, which traps dust and microorganisms and keeps the airway surface moist.

Goblet cells and ciliated cells work as a pair: one produces the mucus, the other moves it. Smoking paralyses the cilia, so mucus accumulates and has to be moved by coughing, which is what a smoker's cough is.

Summary table

Two rows carry most of the marks: cartilage stops at the bronchi, and cilia and goblet cells stop before the alveoli.

The alveoli

Each lung contains roughly 300 million alveoli. Every feature is about making diffusion fast, and Fick's law is the frame to hang them on: rate depends on surface area and concentration difference, divided by distance.

A very large surface area. The alveoli together give a surface of around 70 m², which is more area for diffusion.

A very short diffusion distance. The alveolar wall is a single layer of squamous epithelium, extremely flattened. The capillary wall is a single layer of endothelium. Together they are less than 1 µm thick, so oxygen crosses two thin cells and no more.

A steep concentration gradient, maintained by two things at once:

• ventilation constantly replaces the air in the alveoli, so the oxygen concentration inside stays high and the carbon dioxide concentration stays low
• a good blood supply constantly removes oxygenated blood and brings deoxygenated blood, so the gradient across the wall is never allowed to disappear

A question asking how the gradient is maintained is asking for both of those, not one.

A moist lining. The alveolar surface is coated with a thin film of fluid, and gases must dissolve before they can diffuse across a membrane.

Capillaries in close contact. The capillary network wraps the alveolus so closely that red cells are pressed against the wall, shortening the diffusion path further, and they travel slowly, allowing time for exchange.

Surfactant. A film of phospholipid on the alveolar surface reduces surface tension. Without it, the water lining would pull the alveolus closed as it emptied. Premature babies are sometimes born before they can make enough surfactant, which is why their lungs are prone to collapse.

Elastic fibres in the wall let the alveolus expand on inhalation and recoil on exhalation.

Which way each gas goes

In the alveolus, the partial pressure of oxygen is high and in the blood arriving it is low, so oxygen diffuses into the blood.

The partial pressure of carbon dioxide is high in the arriving blood and low in the alveolus, so carbon dioxide diffuses out of the blood.

Both are simple diffusion down a gradient. No energy is used, and there is no active transport in the lung.

Common mistakes

• Saying bronchioles contain cartilage. They do not, and this is the single most tested fact in the topic.
• Saying alveoli have goblet cells or cilia. They have neither, because both would be too thick.
• Saying the alveolar wall is "thin" without saying it is one cell thick.
• Saying a large surface area alone explains fast exchange. The short distance and the maintained gradient matter just as much.
• Saying ventilation maintains the gradient, and stopping there. The blood supply is the other half.