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  1. KS3
  2. Biology
  3. Breathing and gas exchange
  4. The gas exchange system

Breathing and gas exchange · System

The gas exchange system

Breathe out into a bag and analyse it. There is four times more oxygen in there than carbon dioxide. Most people are surprised, and the surprise is worth taking seriously.

Start here

Mouth-to-mouth resuscitation works.

You breathe out into someone who has stopped breathing, and it keeps them alive. Their blood is re-oxygenated by air that has already been through your lungs.

What does that tell you about the air you breathe out?

At the bench · two bags of air

Predict the numbers, then see them

0 of 4 predicted

For each gas, say what happens to it between going in and coming out.

  • Nitrogen

  • Oxygen

  • Carbon dioxide

  • Water vapour

0 of 4 committed

The route in

Six parts, and only the last one exchanges anything.

  • Nose and mouth · Conditioning

    Air is warmed to body temperature, moistened, and filtered by hairs and mucus. Breathing through the nose does this far better than through the mouth, which is why cold dry air through an open mouth irritates the airways.

  • Trachea · Transport

    A single tube down the front of the neck, held open by C-shaped rings of cartilage so it cannot collapse when the pressure inside drops. Lined with mucus and cilia that sweep trapped dust and bacteria back up.

  • Bronchi · Transport

    The trachea divides into two, one bronchus to each lung. Still wide, still ringed with cartilage, still lined with cilia. No exchange happens here.

  • Bronchioles · Transport

    Each bronchus divides again and again — about twenty-three generations of branching in total — into ever narrower tubes with muscle in their walls instead of cartilage. That muscle is what narrows in an asthma attack.

  • Alveoli · Gas exchange — the only place

    Around 500 million tiny air sacs, each wrapped in capillaries, each with a wall one cell thick. This is where oxygen enters the blood and carbon dioxide leaves it, and once you are breathing air it is the only place in your body where that happens — before you were born, the placenta did all of it.

  • Ribs, intercostal muscles and diaphragm · Moving the air

    Not part of the airway at all — the machinery that changes the volume of the chest, and so moves air in and out. The lungs contain no muscle of their own and cannot move themselves. That is the next lesson.

Key fact

Breathing is the movement of air in and out. Gas exchange is oxygen and carbon dioxide swapping between air and blood, and it happens only in the alveoli. Respiration is a chemical reaction inside every cell. Three different things, three different words.

The gas exchange system: the route in, and the machinery around itTwo framed drawings above a key of six parts. The left frame is a chest seen from the front. At the top, two short tubes from the nose and from the mouth meet and become 02, the trachea, a single tube down the centre held open by a row of cartilage rings. At its foot the trachea divides in two: 03, the bronchi, one running into each lung. Inside each lung outline the airway keeps dividing — four divisions are drawn, each branch narrower and shorter than the one before, until the smallest tips end in little clusters of sacs, so that the lung is drawn full of branching rather than hollow. One cluster low in the right lung is ringed with a dashed circle. Around both lungs, outside them, are six pairs of ribs sweeping down and out from the centre, short hatched lines filling one gap between two ribs to show the intercostal muscles, and beneath the lungs a domed sheet, the diaphragm. The right frame magnifies the ringed cluster: a narrow bronchiole opens into a rosette of five air sacs; a blue capillary runs across the front of them; one arrow points from the air space into the blood, labelled oxygen in, and another points from the blood into the air space, labelled carbon dioxide out; the sac wall is marked one cell thick. Notes record about 500 million of these sacs, each wrapped in capillaries, and that four divisions are drawn where a real lung has about twenty-three. The key reads: 01 nose and mouth, air warmed, moistened and filtered. 02 trachea, held open by C-shaped rings of cartilage. 03 bronchi, one to each lung, no exchange here. 04 bronchioles, divide and divide again, about twenty-three times. 05 alveoli, gas exchange, the only place it happens. 06 ribs, intercostal muscles and diaphragm, not the airway but the machinery that moves the air.nosemouth01nose and mouth02trachea03bronchi04bronchioles05alveoli06ribsintercostalmusclesdiaphragmThe lungs are drawn full of branching because a lung is nota bag with air in it. It is closer to a sponge.ONE TIP, MAGNIFIEDoxygen incarbon dioxide outwall one cell thickabout 500 million of these,each wrapped in capillariesFour divisions are drawnopposite. A real lung hasabout twenty-three, andonly the last end in sacs.Blood in the capillaryTHE ROUTE IN, AND THE MACHINERY AROUND IT01Nose and mouthAir warmed, moistened and filtered02TracheaHeld open by C-shaped rings of cartilage03BronchiOne to each lung. No exchange here04BronchiolesDivide, and divide again, about 23 times05AlveoliGas exchange — the only place it happens06Ribs, intercostal muscles and diaphragmNot the airway. The machinery that moves airEvery one of those divisions exists to turn a bag into a surface.
The route in runs 01 to 05 and stops there: gas exchange happens at the sacs and nowhere earlier along the tubes. Each lung is drawn full of branching — four divisions, each branch narrower than the last — because a lung is not a hollow bag. The circled cluster is magnified on the right, where one capillary crosses the sacs and the wall between air and blood is a single cell thick. The ribs, the muscles between them and the diaphragm are drawn outside the lungs: 06 is the only part of this figure that is not the airway.

Think again

“You breathe in oxygen and breathe out carbon dioxide.”

Read that sentence as a statement about the contents of the two bags and it is badly wrong. You breathe in air that is 21% oxygen and breathe out air that is 16% oxygen — so the largest gas in both directions is nitrogen, and the second largest in both directions is oxygen. Carbon dioxide is the fourth thing in the bag, behind water vapour on a warm day. What is true is that oxygen moves into the blood and carbon dioxide moves out of it, which is a claim about direction of movement rather than about composition. Write “oxygen diffuses from the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveoli” and you have said something defensible.

“Breathing and respiration are the same thing.”

They are separated by about a metre and by the whole of chemistry. Breathing is a mechanical process performed by muscles, moving air between the atmosphere and your alveoli. Respiration is a chemical reaction — glucose and oxygen releasing energy — happening in the mitochondria of every one of your cells, including the ones in your toes. A plant respires and does not breathe. A person on a ventilator is being breathed for, and is respiring entirely on their own. If you use the word respiration to mean breathing in an exam, you have made a mistake examiners consider significant, because the two are studied in different chapters for a reason.

“Your lungs are hollow bags that fill up like balloons.”

A lung is not a bag with air in it — it is a solid-feeling sponge, and a piece of fresh lung tissue floats. The bronchi divide about twenty-three times, each branch narrower than the last, ending in some 500 million alveoli. Nothing in that structure is a single open cavity. The reason matters: a balloon has almost no surface area for its volume, and gas exchange is limited entirely by surface area. Every one of those twenty-three divisions exists to turn a bag into a surface.

Mastery ladder

Not started yet.

Rungs 3 and 4 you mark yourself.

Rung 1 · Three words

A plant in a dark cupboard is releasing carbon dioxide. Which of the three processes is it carrying out?

Rung 2 · The one that catches people

Which gas makes up the largest part of the air you breathe OUT?

Rung 3 · Rewrite the sentence

A student writes: “We breathe in oxygen and breathe out carbon dioxide.” Explain what is wrong with it as a statement about the air, and write a version that is defensible. Use figures.

Rung 4 · Take it somewhere new

Explain why mouth-to-mouth resuscitation can keep someone alive, using the composition figures. Then say why a rescuer’s own breathing rate rises while they do it, and which of the three processes that change belongs to.

Key note

Air travels through the trachea, bronchi and bronchioles to about 500 million alveoli, where gas exchange happens. Inhaled air is 21% oxygen and 0.04% carbon dioxide; exhaled air is about 16% oxygen and 4% carbon dioxide, warmer and saturated with water vapour. Nitrogen is unchanged. Breathing, gas exchange and respiration are three different processes.

Going further

Nitrogen is 78% of every breath you have ever taken and your body does nothing with any of it — you exhale exactly what you inhaled. Yet nitrogen is in every protein and every strand of DNA in you, so you plainly acquire it somehow. The answer is that you get all of it from food, because breaking the triple bond in N₂ is beyond any animal. Certain bacteria can do it, some of them living in nodules on the roots of pea and bean plants, and everything else on Earth ultimately depends on that chemistry. You are breathing your own most essential element in and straight back out again, twenty thousand times a day, unable to touch it.

Before this lesson

  • Nothing — this is where the unit starts.

Connects to

At GCSE this becomes

  • The gas exchange system in detail, ventilation rates, and the link to aerobic and anaerobic respiration.

Where to next

Ask Mr Badmus AI

Still mixing up breathing, gas exchange and respiration?

Composition figures are rounded typical values for a person at rest; exhaled composition changes with activity and with how long the breath is held.

Lesson content © MrBadmusAI.

Mr. Badmus AI

KS3 Science Tutor

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