MrBadmusBreathing and gas exchange

Breathing and gas exchange · System

The gas exchange system

Start here

If you had to guess, how much of the oxygen in a breath is still there when you breathe it out?

Key words

  • The single tube from the throat to the lungs, held open by rings of cartilage.

  • The narrowest branches of the airway, with muscle in their walls instead of cartilage.

  • The tiny air sacs at the end of the airway where gas exchange happens.

  • Oxygen and carbon dioxide swapping between the air in the alveoli and the blood.

Key knowledge

Air travels down a branching set of tubes to the alveoli.

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.
Air travels from 01 to 05. Gas exchange happens only in the air sacs.

The parts and what they do

RowWhat it isWhat it does
Nose and mouth
Trachea
Bronchi
Bronchioles
Alveoli
Ribs
Diaphragm and intercostals

Mucus in the airways traps dust and bacteria.

Cilia are tiny hairs on the cells of the airway.

They sweep the mucus up and out of the lungs.

Three words to keep apart

RowWhat it isWhere it happens
Breathing
Gas exchange
Respiration

Air in and air out

RowAir breathed inAir breathed out
Nitrogen
Oxygen
Carbon dioxide
Water vapour
Temperature

  • Wrong: We breathe in oxygen and breathe out carbon dioxide.

    Right: Air in and air out are both mostly nitrogen. Air out has less oxygen and more carbon dioxide.

  • Wrong: Breathing and respiration are the same thing.

    Right: Breathing moves air. Respiration is a chemical reaction in cells.

Key fact

Air travels down the trachea, bronchi and bronchioles to the alveoli. In the alveoli, oxygen moves into the blood and carbon dioxide moves out.

Practice

Not started yet.

Rungs 3 and 4 you mark yourself.

Rung 1

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

Rung 2

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

Rung 3

A student writes: “We breathe in oxygen and breathe out carbon dioxide.” Explain what is wrong with this statement about the air. Then write a correct version. Use figures.

Rung 4

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.

Air you breathe out is still about 16% oxygen.

That is enough oxygen to keep another person alive.

Rings of cartilage hold the trachea open.

They stop it closing when the pressure inside falls.

The airways divide about 23 times between the trachea and the alveoli.

Before this lesson

  • This is the first lesson in this unit.

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 MrBadmus

Still mixing up breathing, gas exchange and respiration?

These are typical values for a person at rest.

Lesson content © MrBadmus.

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