The gas exchange system

1. Overview

The gas exchange system delivers air deep into the lungs and brings it into close contact with the blood, so that oxygen can diffuse into the blood and carbon dioxide can diffuse out. Air travels through the trachea, then the two bronchi, then many smaller bronchioles, and finally reaches millions of alveoli, each wrapped in a network of capillaries.

Along the way the airways must be held open, kept clean and moist, and able to widen and narrow. Several tissues are spaced along the system to do these jobs:

• cartilage holds the large airways open
• ciliated epithelium and goblet cells keep the airways clean
• smooth muscle and elastic fibres adjust airway width
• squamous epithelium and capillaries form the gas exchange surface

Their arrangement creates a surface that is large, thin and well supplied with blood for rapid diffusion.

Key Definitions

• Gas exchange system: the lungs and airways (trachea, bronchi, bronchioles and alveoli) that move air in and out and allow oxygen and carbon dioxide to be exchanged with the blood.
• Alveoli: tiny air sacs at the ends of the bronchioles where gas exchange takes place, giving a very large total surface area.
• Ciliated epithelium: a tissue of column-shaped cells bearing many cilia that line the trachea, bronchi and larger bronchioles and sweep mucus upwards.
• Goblet cell: a cell in the airway epithelium that secretes mucus to trap dust and microbes.
• Squamous epithelium: a single layer of very thin, flattened cells forming the wall of an alveolus, giving a short diffusion distance for gases.
• Cartilage: a firm, flexible supporting tissue forming C-shaped rings in the trachea and irregular blocks in the bronchi that hold the airways open.
• Smooth muscle: involuntary muscle in the walls of the airways that contracts to narrow them and relaxes to widen them.
• Elastic fibres: stretchy protein fibres in airway and alveolar walls that recoil after stretching to help push air out and restore the original shape.
• Capillaries: the smallest blood vessels, with walls one cell thick (endothelium), forming a dense network around each alveolus for gas exchange.

Content

The pathway of air through the system

Air enters through the nose and mouth and passes down the trachea (windpipe). The trachea divides into two bronchi, one to each lung. Inside each lung the bronchus branches repeatedly into narrower bronchioles. The smallest bronchioles end in clusters of alveoli, the tiny air sacs where gas exchange happens. Each alveolus is surrounded by a dense capillary network, so the air in the alveolus and the blood in the capillary are kept extremely close together. This branching, tree-like layout means a single trachea supplies hundreds of millions of alveoli, creating an enormous total surface area for diffusion.

Distribution of the tissues

The tissues are not spread evenly - their amounts change as the airways get smaller. The table below shows where each tissue is present (✓) and absent (✗):

Tissue	Trachea	Bronchi	Bronchioles	Alveoli
Cartilage	✓ (C-shaped rings)	✓ (irregular blocks/plates)	✗	✗
Ciliated epithelium	✓	✓	✓ (fewer in smallest)	✗
Goblet cells	✓	✓	✓ (fewer in smallest)	✗
Smooth muscle	✓	✓	✓ (greatest proportion here)	✗
Elastic fibres	✓	✓	✓	✓ (around alveoli)
Squamous epithelium	✗	✗	✗	✓ (forms the wall)
Capillaries	✗	✗	✗	✓ (dense network)

A few points worth remembering:

• Cartilage disappears once you reach the bronchioles - this is the single most reliable way to tell a bronchiole from a bronchus.
• Because bronchioles have no cartilage, smooth muscle makes up the greatest proportion of their walls, which is why they can change diameter so easily.
• The lining changes from ciliated epithelium in the airways to squamous epithelium in the alveoli, and the capillary lining is a separate tissue called endothelium.

Recognising the tissues under the microscope

You should be able to identify each tissue in slides, photomicrographs and electron micrographs:

• Cartilage - pale, glassy background (matrix) with rounded cells (chondrocytes) sitting in small spaces.
• Ciliated epithelium - tall, column-shaped cells in a row, with a fringe of fine cilia along the free (top) edge.
• Goblet cells - flask- or cup-shaped cells set among the epithelium, with a pale, swollen region full of mucus near the top.
• Squamous epithelium of the alveoli - extremely thin, flattened cells; in section they look like a fine line, often with a slight bulge where the nucleus is.
• Smooth muscle - sheets or rings of spindle-shaped cells, each with a single central nucleus, with no stripes (no striations).
• Elastic fibres - thin, wavy or branching strands running through the wall, usually stained darker than the surrounding tissue (you are not expected to identify these as confidently as the main tissues).
• Capillaries - very narrow tubes whose wall is a single layer of flattened endothelial cells, often containing red blood cells.

Recognising the airways and making plan diagrams

You should also recognise whole structures: the trachea (large tube with C-shaped cartilage and a ciliated lining), a bronchus (smaller tube with blocks of cartilage), a bronchiole (small tube with no cartilage but a noticeable ring of smooth muscle) and alveoli (clusters of thin-walled air sacs surrounded by capillaries). A plan diagram (tissue map) of a transverse section of the trachea or a bronchus should show only the outlines of the tissue layers - for example, the positions of the cartilage, smooth muscle and the epithelium - using clear, labelled boundaries. A plan diagram must not contain any individual cells; cells are only drawn in a high-power detailed diagram of a small area. The drawing marks, though, are won on how you draw it: make the diagram large, keep the tissue layers in their correct relative proportions (for example show the cartilage as a thick band and the epithelium as a thin inner line), and draw each label line with a ruler so it touches the structure it names.

Keeping the airways healthy: cilia, goblet cells and mucous glands

The lining of the airways protects the lungs from dust, pollen and microbes carried in the air. Goblet cells in the epithelium and mucous glands in the airway walls secrete mucus, a sticky fluid that traps these particles before they can reach the delicate alveoli. The cilia on the ciliated epithelial cells then beat rhythmically in one direction, sweeping the mucus and its trapped debris upwards towards the throat, where it is swallowed or coughed out. Together this "mucus escalator" keeps the airways clear and reduces the chance of infection.

Functions of cartilage, smooth muscle, elastic fibres and squamous epithelium

• Cartilage is firm yet flexible. It holds the trachea and bronchi open so air can always flow, and it stops them collapsing when the air pressure inside falls during inhalation. The C-shape (rather than a complete ring) in the trachea leaves room for the food pipe (oesophagus) behind it to expand when swallowing, and still lets the trachea bend as you move.
• Smooth muscle can contract to narrow the airways (reducing airflow) and relax to widen them (increasing airflow), adjusting how much air reaches the alveoli. It works involuntarily (without conscious control).
• Elastic fibres are stretched when the airways and alveoli fill with air during inhalation. When the smooth muscle relaxes, the fibres recoil to their original length, helping to push air out during exhalation and returning the airways to their normal width.
• Squamous epithelium forms the alveolar wall from extremely thin, flattened cells, giving a short diffusion distance between the air and the blood so that gases cross quickly.

Gas exchange between alveolar air and capillary blood

The alveoli are perfectly built for fast diffusion. Three features matter:

1. Large surface area - the millions of alveoli together give a huge area for gases to cross.
2. Short diffusion distance - the alveolar wall is a single layer of thin squamous epithelium and the capillary wall is a single layer of endothelium, so oxygen and carbon dioxide cross only two thin cells plus a thin film of moisture.
3. Steep concentration (diffusion) gradients - these are maintained by ventilation (breathing constantly refreshes the air in the alveoli) and by the blood flow through the capillaries.

Blood arriving in the alveolar capillaries comes from the heart and is deoxygenated - it has a low oxygen concentration and a high carbon dioxide concentration. So oxygen diffuses down its gradient from the alveolar air, across the alveolar wall and capillary wall, into the blood, where it is picked up by haemoglobin in the red blood cells. At the same time carbon dioxide diffuses the other way, out of the blood into the alveolar air, to be breathed out. The blood leaving the capillaries is therefore oxygenated.

Worked example

Exam-style question: The wall of an alveolus and the wall of the capillary next to it are each only one cell thick. Explain how the structure of the alveoli allows efficient gas exchange between the air and the blood. [3]

Model answer:

• The alveolar wall is made of thin, flattened squamous epithelium and the capillary wall is one cell thick (endothelium), giving a short diffusion distance so gases cross quickly.
• The very large number of alveoli provides a large surface area for diffusion.
• Ventilation and the continuous blood flow maintain steep concentration gradients for oxygen and carbon dioxide, keeping the rate of diffusion high.

Worked example

Exam-style question: A student views a photomicrograph of a goblet cell taken at a magnification of $\times 1500$. On the photomicrograph the cell measures $30$ mm from top to bottom. Calculate the actual length of the goblet cell. Give your answer in micrometres (µm). [3]

Model answer:

• Here the image size is the size you measure on the page ($30$ mm) and the actual size is what you are calculating. Always put the larger measured image size on top and divide by the magnification - dividing the wrong way round is a common slip that gives an impossible answer.
• Use the magnification equation, rearranged for actual size: $$\text{actual size} = \frac{\text{image size}}{\text{magnification}} = \frac{30\ \text{mm}}{1500} = 0.02\ \text{mm}$$
• Convert millimetres to micrometres ($1\ \text{mm} = 1000\ \mu\text{m}$): $$0.02\ \text{mm} \times 1000 = 20\ \mu\text{m}$$
• Actual length = 20 µm - a sensible size for a single cell, which confirms the units are correct.

Key Equations

This topic is mostly qualitative, but you may need the magnification equation when working with micrographs and scale bars:

$$\text{magnification} = \frac{\text{image size}}{\text{actual size}}$$

Diffusion across the alveolar surface is best explained using surface area, diffusion distance and concentration gradient rather than a formula.

Common Mistakes to Avoid

• Calling capillary walls "squamous epithelium". The lining of any blood vessel, including a capillary, is endothelium. Squamous epithelium is the term for the thin alveolar wall; endothelium is the term for the vessel lining - keep the two words separate.
• Saying bronchioles contain endothelium. Endothelium only ever lines blood vessels. Bronchioles contain ciliated epithelium, smooth muscle and elastic fibres - and no cartilage.
• Stating that all airways contain cartilage. Cartilage is found in the trachea (C-shaped rings) and bronchi (irregular blocks) but is absent from bronchioles and alveoli.
• Writing that the blood entering the alveolar capillaries is oxygenated. It arrives deoxygenated (low oxygen, high carbon dioxide) and leaves oxygenated - this is what drives the diffusion gradients.
• Drawing individual cells on a plan diagram. A plan (tissue map) shows only the outlines of the tissue layers; cells belong only in a detailed high-power drawing.
• Saying cilia "trap" the dust. It is the mucus (from goblet cells and mucous glands) that traps particles; the cilia move the mucus upwards.
• Muddling units when measuring cell sizes. Convert carefully between mm, micrometres (µm) and nanometres (nm) - remember $1\ \text{mm} = 1000\ \mu\text{m}$ and $1\ \mu\text{m} = 1000\ \text{nm}$ - and check that your answer is a sensible size for the structure.

Exam Tips

• Use the precise tissue names the question expects: squamous epithelium for the alveolar wall and endothelium for capillary (and other blood vessel) linings.
• When asked about gas exchange, always link the three classic adaptations - large surface area, short diffusion distance and steep concentration gradient - and say what maintains the gradient (ventilation and blood flow).
• For "describe the distribution" questions, organise your answer by where each tissue is and is not (e.g. cartilage in trachea and bronchi but not bronchioles), not just a list of tissues.
• For plan-diagram questions, draw smooth, continuous outlines of the layers, label them clearly, and resist the temptation to add cells.
• Distinguish the jobs clearly: smooth muscle contracts to narrow airways, while elastic fibres recoil to widen them again and aid exhalation - don't say muscle widens the airway.
• In micrograph questions, give a visible reason for each identification (e.g. "ciliated epithelium because of the fringe of cilia on tall column-shaped cells").
• In magnification calculations, convert everything to the same unit first, then rearrange $\text{magnification} = \frac{\text{image size}}{\text{actual size}}$, and finally convert your answer to the unit the question asks for.