IGCSE Biology Topic 11: Gas Exchange in Humans

11.1 Surfaces & Breathing System 11.2 Inspired & Expired Air 11.3 Ventilation 11.4 Exercise & Clean Airways 11.5 Exam Technique

Hi Tara. One piece of honesty before you start: Cambridge numbers this whole topic as a single sub-topic, 11.1. The five sections below are our split, not theirs — the syllabus content is identical, it is just cut into pieces you can finish in one sitting. If a past paper says “11.1” it means the lot.

Here is the shape of it. 11.1 is the four features every gas exchange surface in biology has, and the tour of the breathing system from larynx down to alveolus. 11.2 is the one table you must know by heart — what changes between the air going in and the air coming out — plus the limewater test that shows it. 11.3 is the Supplement explanation Cambridge marks hardest: ribs, two sets of intercostal muscles and the diaphragm producing volume and pressure changes. 11.4 is what exercise does, why it happens, and how your airways keep themselves clean. 11.5 is the checklist for the night before.

Two warnings that are each worth several marks. First: expired air is not “air with the oxygen taken out”. It still contains 16 % oxygen — more than three quarters of what you breathed in comes straight back out, which is exactly why mouth-to-mouth resuscitation works. Second: air is never sucked into your lungs. Nothing in your chest can pull on air. Your thorax gets bigger, the pressure inside falls below the pressure outside, and the atmosphere pushes air in. Every ventilation mark scheme is written in those terms.

11.1 Gas Exchange Surfaces and the Breathing System ▼

▶ Watch: Gas Exchange Surfaces and the Breathing System

Opens on YouTube in a new tab. Watch one, then come back and do the Check Yourself questions — watching without testing yourself feels like learning but is not.

The Problem a Lung Solves

An amoeba has no lungs and does not miss them. Oxygen diffuses in through its surface, and because every part of it is within a fraction of a millimetre of that surface, diffusion is fast enough on its own. You are about 30 cm thick. Oxygen diffusing in through your skin would take years to reach the middle of you, and you would be dead long before it arrived.

You met the reason in Topic 3. As an organism gets bigger, its volume grows faster than its surface area, so the surface area available per unit of volume falls. A big organism has too little skin for too much inside. The answer is not to grow more skin — it is to grow a specialised surface, folded up and packed inside the body, whose only job is exchange. Your lungs are that surface. So are a fish’s gills, so is the lining of your small intestine, and so is a root hair cell.

The four features, in Cambridge’s own words

Every gas exchange surface has: a large surface area, a thin surface, a good blood supply, and good ventilation with air.

Four words to hold them: big, thin, well plumbed, well aired. Learn them as a set, because a four-mark question is almost always four features and nothing else. And notice that the last two do the same job from opposite sides — see the box below.

What “good blood supply” is actually for

The commonest half-answer is that a good blood supply means “more oxygen gets carried away”. True, but that is the consequence, not the reason it earns a mark. Write this instead: the blood carries oxygen away as fast as it arrives, so the concentration of oxygen in the blood stays low, and the concentration gradient between the air in the alveolus and the blood stays steep.

Good ventilation does the identical job from the other side: fresh air arriving keeps the oxygen concentration in the alveolus high and the carbon dioxide concentration low. Both features exist to maintain the concentration gradient, and if you say that phrase you will pick up the mark every time.

The Tour: Larynx to Alveolus

Air takes exactly one route in and the same route out. Learn it as an ordered list, because “put these structures in the order air passes through them” is a free mark that people throw away by putting bronchioles before bronchi.

nose / mouth → larynx → trachea → bronchi → bronchioles → alveoli

bronchus (one) → bronchi (two or more). alveolus (one) → alveoli (many). Getting the singular and plural right is worth doing; getting them wrong looks careless in a written answer.The tubes get narrower and more numerous as you go down, which is why the total cross-sectional area goes up even though each tube gets smaller.

Every structure Cambridge names in Topic 11 is on this diagram. Note that both sets of intercostal muscles are Supplement material and both can be asked for by name.

Structure	What it is	What it does
Larynx	The voice box, at the top of the trachea	Air passes through it; the vocal cords in it produce sound
Trachea	The windpipe, held open by C-shaped rings of cartilage	Carries air to the two bronchi; the cartilage stops it collapsing
Bronchi	Two tubes, one to each lung, still with cartilage	Carry air into each lung
Bronchioles	Fine branching tubes, no cartilage, with muscle in their walls	Distribute air throughout the lung tissue
Alveoli	Tiny air sacs, wall one cell thick, wrapped in capillaries	This is where gas exchange happens — nowhere else
Lungs	Two elastic organs in the thorax — no muscle that can inflate them	House the alveoli; they are inflated by the thorax around them
Ribs	Bones forming a cage around the thorax	Protect the lungs and heart; moved by the intercostal muscles
External intercostal muscles	The outer muscle layer between the ribs	Contract to raise the ribs up and out during inhalation
Internal intercostal muscles	The inner muscle layer, fibres running the other way	Contract to pull the ribs down and in during forced exhalation
Diaphragm	A dome-shaped sheet of muscle below the lungs	Contracts and flattens to increase the volume of the thorax

Supplement

The one-mark question about cartilage

Cambridge asks for the function of cartilage in the trachea by name. The answer is that the rings of cartilage hold the trachea open — they stop it collapsing. The good version adds when: during inhalation the pressure inside the airway falls below the pressure outside it, and without a stiff wall the tube would be squashed shut by the atmosphere, exactly like sucking too hard on a paper straw.

The rings are C-shaped, not complete circles, and the gap is at the back where the oesophagus lies, so that a swallowed mouthful can bulge into it. That is a nice detail and worth knowing, but the marking point is keeps the airway open. Say that first.

Cartilage is also flexible, which is why you can bend your neck without shutting off your own air supply. And the reason bronchioles have no cartilage is that they are deep inside the lung, surrounded and held open by the lung tissue itself, and they need to be able to change their diameter to control where the air goes.

The Alveolus: Where the Topic Actually Happens

There are roughly 300–500 million alveoli in a pair of human lungs, and together they have a surface area of about 70 m² — a badminton court, folded into your chest. You are not required to memorise those figures. You are required to be able to say why the number matters: a larger surface area means more diffusion can happen at once, so more oxygen enters the blood per second.

Notice that features 3 and 4 are the same idea approached from the two sides of the wall. Both maintain the concentration gradient; neither “makes” oxygen move faster by itself.

Three words, three meanings, and Cambridge marks all three separately

Ventilation is the movement of air in and out of the lungs. It is a mechanical process and it happens in the thorax.

Gas exchange is the diffusion of oxygen and carbon dioxide across the alveolus wall. It happens at the alveolus and nowhere else in the breathing system.

Respiration is the chemical reaction inside cells that breaks down nutrient molecules and releases energy. It happens in every living cell in your body, including the ones in your lungs.

“Respiration takes place in the lungs” is the single most expensive sentence in this topic. It scores nothing, and in a question about the difference between the three it actively costs you.

Worked Example 1 A biologist compares two animals. Animal P is a cube-shaped hypothetical organism 2 mm along each edge with no lungs. Animal Q is the same shape but 20 mm along each edge. (a) Calculate the surface area to volume ratio of each. [3] (b) Explain why Q cannot survive on diffusion through its surface alone but P can. [3]

Step 1: a cube has six faces

Animal P: surface area = 6 × (2 × 2) = 24 mm²; volume = 2³ = 8 mm³; ratio = 24 ÷ 8 = 3 : 1.

Animal Q: surface area = 6 × (20 × 20) = 2400 mm²; volume = 20³ = 8000 mm³; ratio = 2400 ÷ 8000 = 0.3 : 1.

Step 2: read what the numbers mean

Q is ten times longer but its surface area per unit of volume is ten times smaller. Every cubic millimetre of Q is respiring and needs oxygen, but there is only a tenth as much surface per cubic millimetre through which oxygen can enter.

Step 3: add the distance argument, because it is the second half of the mark

There is also the diffusion distance. In P nothing is more than 1 mm from the surface. In Q the centre is 10 mm away, and diffusion over that distance is far too slow to keep up with the rate at which the cells use oxygen. Q therefore needs a specialised gas exchange surface with a large area, a thin wall and a blood supply to carry the oxygen the rest of the way.

(a) P = 3 : 1, Q = 0.3 : 1. (b) Q has a much smaller surface area per unit volume and a much greater diffusion distance, so diffusion through the surface alone cannot supply its cells fast enough.

Worked Example 2 A student writes: “The alveoli have a large surface area so that more oxygen is made available to the blood, and they have a good blood supply so that the oxygen can be carried around the body.” Explain why this answer would score badly, and rewrite it. [4]

Fault 1: “made available”

Nothing makes oxygen available. A larger area does not create oxygen; it provides more surface across which diffusion can occur at the same time. That phrase is the mark.

Fault 2: the blood supply reason is the wrong reason

“So the oxygen can be carried round the body” is true, and it is the answer to a question about transport in animals, not this one. In this topic the blood supply matters because it removes oxygen from the capillary as fast as it arrives, keeping the concentration gradient steep.

The rewrite

“The alveoli give a very large surface area, so more diffusion can take place at once [1]. Their walls are one cell thick, giving a short diffusion distance of about 1 µm [1]. The dense capillary network carries oxygen away as quickly as it diffuses in, so the concentration of oxygen in the blood stays low and the concentration gradient is maintained [1]. Ventilation replaces the air in the alveolus, keeping the oxygen concentration in the air space high, which maintains the gradient from the other side [1].”

Four features, each with the word gradient or distance attached to it. That is what a four-mark version of this question is looking for.

Check Yourself: 11.1 Gas Exchange Surfaces and the Breathing System

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

Which list gives the four features of a gas exchange surface as Cambridge states them?

A large surface area, thick surface, good blood supply, good ventilation

B large surface area, thin surface, muscular wall, good ventilation

C large surface area, thin surface, good blood supply, good ventilation with air

D large surface area, thin surface, good blood supply, a supply of energy from respiration

These four are worth memorising word for word, because a four-mark question is usually one mark each. Option B is the trap worth noticing: the alveolus wall contains no muscle at all, and the lungs have no muscle that could inflate them — that misconception comes back in 11.3 as “the lungs pull themselves open”. Option D drags in active transport, which is not how gases cross the alveolus wall.

Question 2

Which sequence gives the order in which inhaled air passes through the breathing system?

A larynx → trachea → bronchioles → bronchi → alveoli

B larynx → trachea → bronchi → bronchioles → alveoli

C trachea → larynx → bronchi → alveoli → bronchioles

D trachea → bronchi → larynx → bronchioles → alveoli

The tubes get narrower and more numerous as you go down, so the big ones come first: bronchi (two) before bronchioles (thousands). Option A is the standard slip — it reads plausibly because “bronchiole” is the longer word, but the -ole ending means “little”, and little tubes come last.

Question 3

What is the function of the cartilage in the trachea?

A it holds the trachea open so that it does not collapse

B it contracts to push air down towards the lungs

C it produces mucus that traps dust particles

D it warms the air before it reaches the alveoli

During inhalation the pressure inside the airway falls below atmospheric pressure, and a floppy tube would be squashed shut — the cartilage rings prevent that. Option B is a misconception worth naming: cartilage is not muscle and cannot contract, and nothing in the airways pushes air along. Producing mucus is the job of the goblet cells, which you meet in 11.4.

Question 4

Gas exchange in a human takes place

A throughout the trachea, bronchi and bronchioles

B in the muscle cells, where the oxygen is used

C across the walls of the alveoli only

D in the capillaries of every organ in the body

The tubes are plumbing; they have thick walls and no capillaries against them, so nothing can diffuse across. Option B confuses gas exchange with respiration, and option D confuses it with the delivery of oxygen to the tissues, which belongs to transport in animals. Three different processes, three different places — keep them apart and this topic gets much easier.

Question 5

Why does a good blood supply increase the rate of gas exchange at an alveolus?

A it removes oxygen as fast as it arrives, so the concentration gradient stays steep

B it increases the surface area available for diffusion

C it supplies the energy needed to move oxygen across the wall

D it makes the alveolus wall thinner

Diffusion needs no energy at all, which kills option C — it is powered by the random movement of the molecules themselves. The examinable idea is the gradient, and it is worth writing the whole phrase: oxygen is carried away, its concentration in the blood stays low, the gradient between alveolar air and blood is maintained.

Question 6

The diffusion distance between the air in an alveolus and the blood in a capillary is about 1 µm. This is because

A the alveolus wall is one cell thick and the capillary lies inside the alveolus

B the alveolus wall is very thin and the capillary wall is made of cartilage

C both the alveolus wall and the capillary wall are one cell thick

D the gases are pumped across by protein carriers, so the distance does not matter

Two walls, each a single flattened cell, is the whole answer — the capillary lies against the alveolus, not inside it, so option A gets the anatomy wrong. Option D is active transport smuggled in; gases cross by diffusion, with no carrier and no energy cost.

Question 7

A student says the lungs need muscle so that they can pull themselves open. The best correction is that

A the lungs contain a thin layer of muscle but it is too weak to be seen

B the lungs contain no muscle and are stretched by the thorax enlarging around them

C the alveoli contract and relax to pump air in and out

D the cartilage rings in the bronchioles pull the lungs open

This is the central misconception of the whole topic. The lungs are elastic bags with no muscle that can inflate them; the diaphragm and the intercostal muscles enlarge the thorax and the lungs follow. Option D contains a second error worth spotting — bronchioles have no cartilage, only the trachea and bronchi do.

Question 8

Which structure in the human body has a role most similar to that of an alveolus?

A a villus in the small intestine

B a vein carrying blood back to the heart

C a red blood cell

D the septum of the heart

A villus is the same design argument in a different organ: huge surface area from millions of folds, a wall one cell thick, and a rich blood supply that carries the absorbed substance away to maintain the gradient. The red blood cell is a tempting choice because red blood cells carry oxygen, but carrying is transport, not exchange — the red blood cell is on the receiving end.

Question 9

Bronchioles differ from bronchi in that bronchioles

A are wider and fewer in number

B are lined with alveoli along their whole length

C contain complete rings of bone that hold them open

D are narrower, far more numerous, and have no cartilage

Cartilage stops at the bronchi; deeper in, the lung tissue itself holds the bronchioles open and their muscular walls let them change diameter. Option C contains the misconception that the rings are bone — they are cartilage, which is firm but flexible, which is why you can bend your neck and still breathe.

Question 10

Ventilation, gas exchange and respiration are three different processes. Which row is correct?

A ventilation happens at the alveolus; gas exchange happens in the thorax; respiration happens in the lungs

B ventilation is the movement of air; gas exchange is diffusion at the alveolus; respiration is a reaction in cells

C ventilation and breathing are different; gas exchange and respiration mean the same thing

D all three describe the same process at different levels of detail

These three words are the marks in this topic. Ventilation is mechanical and involves the whole thorax; gas exchange is diffusion and happens only across the alveolus wall; respiration is a chemical reaction happening inside every living cell you own. Option C is the commonest confusion of the three, and a mark scheme will refuse it every time.

Question 11

A lung disease destroys the walls between neighbouring alveoli, so that many small air sacs merge into a few large ones. The immediate effect on gas exchange is that

A the diffusion distance falls, so gas exchange becomes faster

B the volume of air in the lungs falls, so less oxygen is available

C the blood supply to the lungs is cut off completely

D the total surface area falls, so less oxygen diffuses into the blood per second

Merging small sacs into large ones keeps roughly the same volume of air but slashes the surface area — which is exactly why option B is the tempting wrong answer. It is area, not volume, that limits the rate of diffusion, and this is the reasoning behind every question about emphysema.

Question 12

Which pair of structures is correctly matched with the muscle layers that move the ribs?

A the diaphragm is the outer layer and the intercostal muscles the inner layer

B the external intercostal muscles lie outside the internal intercostal muscles, between the same two ribs

C the internal intercostal muscles lie inside the lungs

D there is only one layer of intercostal muscle, named external because it lies outside the lungs

Two layers, one on top of the other in the same gap, with their fibres running in opposite directions — that opposite arrangement is why they can pull the ribs in opposite directions. Option D is the version most students carry from Core-level teaching; naming both layers is Supplement material and Cambridge asks for it by name.

11.2 Inspired and Expired Air, and the Limewater Test ▼

▶ Watch: Inspired vs Expired Air and Limewater

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The One Table You Have to Know

Cambridge limits this to oxygen, carbon dioxide and water vapour. Nitrogen and temperature come up constantly anyway, so they are in the table too — but if a question says “limited to”, give the three.

Component	Inspired air (in)	Expired air (out)	Change
Oxygen	21 %	16 %	Falls by 5 percentage points
Carbon dioxide	0.04 %	4 %	Rises about 100-fold
Nitrogen	78 %	78 %	Unchanged
Water vapour	Variable, usually low	Saturated — high	Rises
Temperature	That of the surroundings	About 37 °C, body temperature	Usually rises

Expired air is not “air with the oxygen used up”

16 % is a lot of oxygen. Of every 100 parts of air you breathe in, 21 are oxygen and 16 come straight back out — you keep about five, which is less than a quarter of what went in. This is not inefficiency; it is what makes mouth-to-mouth resuscitation work. The air you blow into someone else’s lungs still has three quarters of its oxygen in it.

Any option in a multiple choice paper claiming that expired air contains no oxygen, or only a trace, is wrong for that reason. Learn 21 and 16 as a pair.

Supplement

Explaining each difference — the marks are in the because

Oxygen falls from 21 % to 16 % because oxygen diffuses from the air in the alveoli, down its concentration gradient, into the blood, where it is carried away and used by cells in respiration. Only the air that reaches an alveolus loses any oxygen at all, which is part of why so much comes back out.

Carbon dioxide rises from 0.04 % to 4 % because carbon dioxide produced by respiring cells is carried in the blood to the lungs, where its concentration in the blood is higher than in the alveolar air, so it diffuses out into the alveolus and is breathed away.

Water vapour rises because the lining of the alveoli and airways is moist, and water evaporates from that moist surface into the air. That moist lining is not an accident — gases must dissolve before they can diffuse across a membrane — so losing water is the unavoidable price of being able to exchange gases at all.

Temperature rises because the air is warmed to body temperature by contact with the warm surfaces of the airways and lungs.

Nitrogen does not change because the body does not use nitrogen gas and does not produce it. Do not be tempted by tables that show it changing very slightly: any apparent change is only because everything is expressed as a percentage and the other percentages have shifted.

Testing It: Limewater

Limewater is the standard test for carbon dioxide, and it does exactly one thing: it turns cloudy, or milky. It does not “go white”, it does not “change colour”, and it does not “turn to chalk”. Cloudy or milky. Nothing else is accepted.

Tube A is the control: it proves that the cloudiness in B is caused by something in the expired air and not by bubbling air through limewater as such.

How to write the method so it scores

1. Set up two boiling tubes with the same volume of limewater at the same concentration.

2. Connect them to a mouthpiece through a T-piece with two one-way valves, so that breathing in draws room air through tube A and breathing out pushes expired air through tube B.

3. Breathe gently in and out through the mouthpiece, counting the breaths.

4. Record how many breaths it takes for each tube to turn cloudy.

5. Result: tube B turns cloudy after only a few breaths; tube A stays clear far longer. Conclusion: expired air contains much more carbon dioxide than inspired air.

Two things this experiment does not show

It does not show that respiration happens in the lungs. It shows that expired air is richer in carbon dioxide. The carbon dioxide was made by respiring cells all over your body and carried to the lungs in the blood — the lungs are where it leaves, not where it is made.

It does not measure how much carbon dioxide there is, only that there is more. If a question asks how you would make it quantitative, say: use a carbon dioxide sensor or gas syringe, or count and compare the number of breaths needed rather than just observing which goes cloudy first.

Worked Example 3 Inspired air contains 0.04 % carbon dioxide; expired air contains 4.0 %. (a) Calculate the percentage increase in the carbon dioxide content. [2] (b) A student concludes from this that almost all of the oxygen breathed in has been converted to carbon dioxide. Evaluate that conclusion using the oxygen figures. [3]

Step 1: percentage increase is increase divided by the ORIGINAL

The increase is 4.00 − 0.04 = 3.96 percentage points. The original value is 0.04. So 3.96 ÷ 0.04 × 100 = 9900 %, which is the same as saying carbon dioxide is 100 times more concentrated in expired air.

The commonest error is dividing by 4.0 instead of 0.04, which answers a different question entirely. Percentage change is always taken of the starting value.

Step 2: check the claim against the oxygen numbers

Oxygen falls from 21 % to 16 % — a fall of only 5 percentage points, which is under a quarter of the oxygen that went in. So the great majority of the inhaled oxygen is breathed straight back out unused. The conclusion is wrong.

Step 3: say what the numbers do show, and be careful with the arithmetic trap

The 5 percentage-point fall in oxygen and the roughly 4 percentage-point rise in carbon dioxide are of a similar size, which is consistent with oxygen being used and carbon dioxide produced in respiration. But 9900 % sounds enormous only because the starting figure for carbon dioxide is tiny. A big percentage change of a small number is still a small absolute change. Spotting that is usually the third mark.

(a) 9900 % increase (a hundredfold rise). (b) The conclusion is not supported: oxygen falls only from 21 % to 16 %, so less than a quarter of the inhaled oxygen is used; the huge percentage rise in carbon dioxide reflects its very low starting value, not a large absolute change.

Check Yourself: 11.2 Inspired and Expired Air

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

Approximately what percentage of oxygen is present in expired air?

A 0 %

B 4 %

C 16 %

D 21 %

21 % goes in, 16 % comes out, so under a quarter of the oxygen is actually used — which is why mouth-to-mouth resuscitation works at all. Option B is the figure for carbon dioxide in expired air, and mixing the two up is the commonest slip in this sub-topic.

Question 2

Why is the percentage of nitrogen the same in inspired and expired air?

A nitrogen is neither used nor produced by the body

B nitrogen molecules are too large to diffuse across the alveolus wall

C the nitrogen used in respiration is exactly replaced by nitrogen from the blood

D nitrogen dissolves in the moisture lining the alveoli and is then released again

The body has no use for nitrogen gas, so none of it is taken up and none is added. Option C invents a role for nitrogen in respiration that does not exist, and option B invents a size rule that is not how diffusion across a membrane works here — the simple answer really is the right one.

Question 3

Limewater is bubbled with expired air. What is observed?

A it turns from colourless to blue

B it turns bright orange

C it turns cloudy or milky

D it stays clear but becomes warm

Cloudy or milky is the only accepted wording. “Goes white” and “changes colour” are both refused, because limewater does not gain a colour — it loses its transparency. Notice that all three wrong options describe a colour change, which is the habit this question is testing.

Question 4

In the two-tube limewater experiment, what is the purpose of the tube through which inspired air is drawn?

A to warm the air before it is breathed in

B to act as a control, showing the cloudiness is caused by something in the expired air

C to remove carbon dioxide from the room air before it enters the lungs

D to measure how much oxygen has been absorbed

Without the control tube, someone could argue that bubbling any gas through limewater turns it cloudy. The control shows it does not, in any reasonable time. Option C describes something the apparatus incidentally does a little of, but a control is defined by what it lets you conclude, not by its side effects.

Question 5

Expired air is warmer and contains more water vapour than inspired air. This is because

A water is produced when oxygen combines with carbon dioxide in the alveoli

B the mucus in the trachea is boiled off by the warmth of the body

C water diffuses out of the blood into the alveoli through protein carriers

D the air is warmed by the body and water evaporates from the moist lining of the airways and alveoli

The alveoli have to be moist because gases must dissolve before they can diffuse across a membrane, and the price of that moisture is that water evaporates into the air and is lost. Option A invents a chemical reaction that does not happen; option C smuggles in active transport, which plays no part in gas exchange.

Question 6

By roughly how many times is carbon dioxide more concentrated in expired air than in inspired air?

A 4 times

B 10 times

C 40 times

D 100 times

4.0 ÷ 0.04 = 100. The reason people answer 4 or 10 is that they read the 4 % figure and forget how tiny 0.04 % is. It is worth remembering that this hundredfold rise is still only about four percentage points in absolute terms — big ratio, small absolute change.

Question 7

A student breathes out through limewater and it turns cloudy after 5 breaths. She concludes that respiration takes place in the lungs. This conclusion is

A correct, because carbon dioxide is only found where respiration occurs

B wrong, because the carbon dioxide was produced in respiring cells all over the body and carried to the lungs in the blood

C wrong, because limewater tests for oxygen rather than carbon dioxide

D correct, but only if a control tube is also used

The lungs are where carbon dioxide leaves, not where it is made. This is the ventilation / gas exchange / respiration distinction reappearing as a data-evaluation question, which is exactly how a challenge paper likes to test it. Option D is tempting because a control genuinely is needed — but a control would not rescue a conclusion that is about the wrong process.

Question 8

Which change would you expect in the expired air of a person during vigorous exercise compared with at rest?

A more oxygen and less carbon dioxide

B more nitrogen and less oxygen

C less oxygen and more carbon dioxide

D no change, because the composition of expired air is fixed

Muscle cells are respiring faster, so more oxygen is removed from the alveolar air and more carbon dioxide is added to it. Option D is a genuine misconception: the 16 % and 4 % figures are typical resting values, not constants. Option B breaks the rule that the body neither uses nor makes nitrogen.

Question 9

Oxygen moves from the air in an alveolus into the blood by

A diffusion, down a concentration gradient

B active transport, using energy from respiration

C osmosis, through a partially permeable membrane

D mass flow, pushed by the pressure of the air in the alveolus

Gas exchange is diffusion and nothing else — no energy, no carriers. Osmosis is the movement of water, so option C is out by definition. Option D is a nice trap because mass flow is how air gets into the alveolus in the first place; it is just not how the gases cross the wall.

Question 10

In a limewater experiment, one tube contains 25 cm³ of limewater and the other contains 10 cm³. The most serious consequence is that

A the limewater will react with the glass of the larger tube

B the bubbles will be a different size in each tube

C a difference in the time taken to turn cloudy could be caused by the volume rather than by the air

D the experiment will not work at all

This is what an uncontrolled variable actually costs you: not a failed experiment, but a result you cannot interpret. A smaller volume of limewater would go cloudy sooner whatever gas you bubbled through it. Option D overstates it — in evaluation questions, say what the flaw makes uncertain, not that everything is ruined.

Question 11

Why does expired air still contain a large percentage of oxygen?

A because the alveolus wall is too thick for all the oxygen to cross

B because oxygen is produced in the lungs as well as being absorbed

C because the blood is already saturated with oxygen before it reaches the lungs

D because only some of the oxygen diffuses out before the air is breathed out again, and some air never reaches an alveolus

Diffusion continues only while there is a gradient and only while the air is in contact with the surface, and a good deal of each breath stays in the trachea and bronchi where no exchange happens at all. Option C is exactly backwards: blood arriving at the lungs is deoxygenated, which is what makes the gradient steep in the first place.

Question 12

Which statement about the composition of expired air is correct?

A it contains more carbon dioxide than oxygen

B it contains more nitrogen than any other gas

C it contains equal volumes of oxygen and carbon dioxide

D it contains no water vapour, because the water has been absorbed by the blood

Nitrogen is 78 % of expired air, four times as much as everything else put together — a useful sanity check whenever you are handed a table of figures. Option A is a common feeling rather than a fact: expired air is 16 % oxygen against 4 % carbon dioxide, so there is still four times as much oxygen as carbon dioxide coming out.

11.3 Ventilation — Ribs, Intercostal Muscles and the Diaphragm ▼

▶ Watch: Ventilation — Ribs, Intercostal Muscles and Diaphragm

Opens on YouTube in a new tab. Watch one, then come back and do the Check Yourself questions — watching without testing yourself feels like learning but is not.

Supplement — and the hardest-marked part of Topic 11

One Rule Runs the Whole Thing

There is a single chain here, and every mark in the sub-topic is a link in it:

muscles move → volume changes → pressure changes → air flows

Air always flows from high pressure to low pressure. Nothing in your chest touches the air directly — the muscles change the size of the box, the pressure inside the box changes as a result, and the atmosphere does the rest.If your answer names the muscles and jumps straight to “air goes in”, you have skipped the two marks in the middle.

Air is pushed in, never sucked in

“The lungs suck in air” scores nothing, and it is not a wording quibble — it is the wrong physics. Nothing can pull on a gas. When your thorax enlarges, the pressure inside falls below atmospheric pressure, and the air outside is pushed in by the higher pressure of the atmosphere, down a pressure gradient.

The same goes for the lungs themselves. They contain no muscle. They are elastic bags that are stretched by the thorax enlarging around them, and they recoil when it shrinks. If you ever catch yourself writing that the lungs expand to draw air in, turn the sentence round: the thorax expands, so the lungs are stretched, so air is pushed in.

Inhalation and Exhalation, Side by Side

Six things change in each direction: two muscle groups, the ribs, the diaphragm, the volume and the pressure. A five-mark ventilation question is usually five of those six.

	Inhalation (breathing in)	Exhalation (breathing out)
External intercostal muscles	Contract	Relax
Internal intercostal muscles	Relax	Contract (in forced breathing out)
Ribs	Move up and out	Move down and in
Diaphragm muscle	Contracts, so the diaphragm flattens and moves down	Relaxes, so the diaphragm returns to its dome and moves up
Volume of thorax	Increases	Decreases
Pressure in thorax	Decreases — falls below atmospheric	Increases — rises above atmospheric
Air	Pushed in by the atmosphere	Pushed out of the lungs

Two things that are easy to get backwards

The diaphragm. At rest it is a dome, bulging upwards into the chest. When its muscle contracts it flattens, which pulls the floor of the thorax downwards and makes the box bigger. So contracting makes it go down, and relaxing makes it go up. Everyone gets this the wrong way round at least once, usually because “contract” sounds as though something should get smaller and higher.

The two intercostals. External muscles are for expanding the chest — they contract when you breathe in. The internal ones are underneath and pull the ribs the other way. They are an antagonistic pair: one contracts while the other relaxes, exactly like the biceps and triceps at your elbow, because a muscle can only pull and never push.

Quiet breathing out is mostly free

When you breathe out gently, the internal intercostals barely do anything. The external intercostals and the diaphragm simply relax, and the stretched, elastic lungs and rib cage recoil back to their resting size all by themselves. That is why breathing out at rest takes no effort.

The internal intercostals earn their keep in forced exhalation — blowing out candles, coughing, playing a wind instrument, breathing hard during exercise. If a question mentions forced or deep breathing out, name them. In a written answer about breathing out, always name both layers: the external intercostal muscles relax and the internal intercostal muscles contract, pulling the ribs down and in. At rest most of breathing out is elastic recoil, but that is the answer the mark scheme credits.

The Bell Jar Model, and Where It Lies to You

You will meet a bell jar with a glass tube through the bung, two balloons inside, and a rubber sheet stretched across the open bottom. Pull the sheet down and the balloons inflate; let it go and they deflate. It is a good model of one thing and a poor model of several others, and challenge papers like asking which is which.

Part of the model	Represents
Bell jar	The thorax / rib cage
Glass tube	The trachea
Y-piece and side tubes	The bronchi
Balloons	The lungs
Rubber sheet	The diaphragm

Three fair criticisms of the model

The jar is rigid. A real rib cage moves — the ribs swing up and out — so the model shows only the diaphragm’s contribution and has no intercostal muscles at all.

The balloons are thick and empty. Real lungs are not hollow bags; they are filled with hundreds of millions of alveoli, and their walls are one cell thick. No gas exchange happens in the model.

The rubber sheet is pulled by hand. A real diaphragm is a muscle that contracts by itself, and it flattens rather than being stretched into a cone.

Say what the model does show as well, or you lose the balance mark: it correctly demonstrates that increasing the volume of the container lowers the pressure inside it, so air is pushed into the balloons — and that the lungs are inflated from outside rather than inflating themselves.

Worked Example 4 A sensor records the pressure inside the thorax. Atmospheric pressure is 101.3 kPa. At moment X the reading is 100.6 kPa; at moment Y it is 102.0 kPa. (a) State which moment is inhalation, giving your reasoning. [2] (b) Describe what the diaphragm and both sets of intercostal muscles are doing at moment Y. [3] (c) A student says the pressure falls at X “because the air has been sucked out of the thorax”. Explain what is wrong. [2]

Step 1: compare each reading with atmospheric, not with each other

At X, 100.6 kPa is below atmospheric (101.3), so air will flow into the thorax: X is inhalation. At Y, 102.0 kPa is above atmospheric, so air flows out: Y is exhalation. Quote both figures and the word “atmospheric” — that is where the second mark is.

Step 2: moment Y is breathing out, and it is forceful

The pressure is well above atmospheric, so this is a forced exhalation. The diaphragm muscle relaxes and the diaphragm returns to its domed shape, moving up. The external intercostal muscles relax. The internal intercostal muscles contract, pulling the ribs down and in. All three points together reduce the volume of the thorax.

Step 3: fix the direction of causation

Two things are wrong. First, nothing sucks: air moves because of a pressure difference, and it is pushed by the higher pressure. Second, the student has the order backwards. Air did not leave and cause the pressure to fall — the volume of the thorax increased first, which caused the pressure to fall, and only then did air flow in. Volume change is the cause; pressure change is the consequence; air movement is the result.

(a) X, because 100.6 kPa is below atmospheric pressure so air is pushed in. (b) Diaphragm relaxes and domes upwards; external intercostals relax; internal intercostals contract, pulling the ribs down and in. (c) Air is never sucked — it is pushed by the higher pressure outside; and the volume increase causes the pressure drop, not the other way round.

Check Yourself: 11.3 Ventilation of the Lungs

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

During inhalation the diaphragm

A relaxes and moves upwards into a dome

B contracts and flattens, moving downwards

C contracts and moves upwards, squeezing the lungs

D stays still while the ribs do all the work

The diaphragm is domed when relaxed, so contracting flattens it and the floor of the thorax drops. Option C is the classic reversal, and it is worth checking yourself with the word “flatten”: a dome that flattens must move down in the middle, which makes the box bigger.

Question 2

Air enters the lungs during inhalation because

A the lungs contract and suck air down the trachea

B the alveoli expand and pull air towards them

C oxygen diffuses down the trachea into the lungs

D the pressure in the thorax falls below atmospheric pressure, so air is pushed in

The whole sub-topic is volume, then pressure, then flow. Options A and B both give the lungs a muscular power they do not have. Option C confuses ventilation with gas exchange: diffusion moves gases the last micrometre across the alveolus wall, not the whole way down the trachea.

Question 3

Which pair of events happens together during a forced exhalation?

A internal intercostals contract and the diaphragm relaxes

B external intercostals contract and the diaphragm flattens

C both sets of intercostal muscles contract together

D the diaphragm contracts and the ribs move down

The two intercostal layers are an antagonistic pair, so option C cannot happen — they would simply pull against each other. External intercostals contracting with the diaphragm flattening is the inhalation combination, and the diaphragm contracting while the ribs move down mixes one from each, which is the answer people pick when they have learned the parts but not the pattern.

Question 4

In the bell jar model of ventilation, the rubber sheet stretched across the base represents

A the pleural membranes around the lungs

B the intercostal muscles

C the diaphragm

D the floor of the abdomen

Sheet at the bottom, pulled down to increase the volume — that is the diaphragm. Option B is worth thinking about, because the intercostal muscles are precisely what the model cannot represent: the glass jar is rigid, so the model shows only the diaphragm’s share of the work.

Question 5

The pressure inside the thorax is measured as 100.8 kPa while atmospheric pressure is 101.3 kPa. At this moment

A the volume of the thorax has increased and air is flowing in

B the volume of the thorax has decreased and air is flowing out

C no air is moving, because the difference is very small

D the person is holding their breath with the glottis closed

Below atmospheric means air is pushed in, so this is inhalation. Option C is the interesting distractor: the difference really is tiny, well under 1 %, but a pressure gradient of any size will move a gas — and the smallness of it is exactly why the trachea needs cartilage rather than something as strong as bone.

Question 6

Why are the internal and external intercostal muscles described as an antagonistic pair?

A because they contract at the same time to give a stronger movement

B because one is made of muscle and the other of cartilage

C because one contracts while the other relaxes, moving the ribs in opposite directions

D because they are found on opposite sides of the body

A muscle can only pull, never push, so any structure that has to move both ways needs two muscles pulling opposite ways — the same logic as the biceps and triceps. Option D is a misreading of the words internal and external: the two layers lie one on top of the other in the same gap between the same two ribs.

Question 7

Which of these is a fair criticism of the bell jar model?

A it wrongly shows that air enters when the volume increases

B the jar is rigid, so it cannot show the ribs moving up and out

C the balloons deflate when the sheet is pulled down

D the model shows gas exchange but not ventilation

The pressure story is the part the model gets right, so options A and C are simply wrong about what happens. Option D is backwards in an instructive way: the model shows ventilation rather well and shows no gas exchange at all, because a balloon has none of the features of an alveolus.

Question 8

A patient’s diaphragm is paralysed but their intercostal muscles still work normally. The most likely result is that

A they cannot breathe at all

B they can breathe in but not out

C they can still ventilate their lungs, but each breath is shallower than normal

D gas exchange stops but ventilation continues normally

Two mechanisms enlarge the thorax and only one has failed, so ventilation continues at a reduced volume — the depth of each breath falls, and the rate usually rises to compensate. Option D confuses the two processes again: gas exchange depends on ventilation but is not the same thing, and it would be reduced rather than stopped.

Question 9

Put these in the correct causal order for inhalation.

A air flows in → pressure falls → volume increases → muscles contract

B muscles contract → pressure falls → volume increases → air flows in

C muscles contract → volume increases → pressure falls → air flows in

D volume increases → air flows in → pressure falls → muscles contract

Volume always changes before pressure — the pressure falls because the same amount of air now occupies a bigger space. Option B swaps those two, which is the version most people write without noticing, and it costs a mark whenever the question says “explain” rather than “describe”.

Question 10

Which statement about quiet breathing out at rest is correct?

A it requires the strongest muscular effort of the whole breathing cycle

B it happens because the alveoli actively squeeze the air out

C it requires the diaphragm to contract harder than during breathing in

D it is largely passive — the muscles relax and the stretched lungs and rib cage recoil

At rest, breathing out costs almost nothing: the elastic tissue that was stretched during inhalation simply springs back. Option C is a good check on whether you have the diaphragm the right way round — if it contracted during exhalation the thorax would get bigger, and you would breathe in.

Question 11

Cartilage rings keep the trachea open. What would happen during a strong inhalation if they were absent?

A nothing, because the pressure inside and outside the trachea is always equal

B the pressure inside would be lower than outside, so the trachea would be squashed shut

C the trachea would burst outwards under the pressure of the incoming air

D mucus would no longer be produced by the lining

This is what happens when you suck too hard on a paper straw, and it is the reason the cartilage exists. Option C has the pressure gradient backwards: during inhalation the pressure inside the airway is lower than atmospheric, which is why the tube tends to collapse inwards rather than balloon outwards.

Question 12

The volume of a person’s thorax increases from 2.4 dm³ to 2.9 dm³. Which row correctly describes what happens?

A pressure falls; air flows in; external intercostals contract

B pressure rises; air flows out; internal intercostals contract

C pressure falls; air flows out; diaphragm relaxes

D pressure rises; air flows in; diaphragm contracts

A bigger box at a lower pressure, so air is pushed in and the external intercostals must be the ones contracting. Options C and D are each internally contradictory, and spotting that is a useful skill: air cannot flow out of a region whose pressure has fallen, and it cannot flow in to a region whose pressure has risen.

11.4 Exercise, Breathing Control, and Keeping the Airways Clean ▼

▶ Watch: Exercise, Breathing Rate and Clean Airways

Opens on YouTube in a new tab. Watch one, then come back and do the Check Yourself questions — watching without testing yourself feels like learning but is not.

Rate and Depth Are Two Different Measurements

Cambridge asks about the rate and the depth of breathing, and expects you to treat them as separate things.

ventilation = breathing rate × depth of each breath

Breathing rate = number of breaths per minute. At rest, about 12–16.Depth = the volume of air per breath. At rest, about 500 cm³.So at rest you move about 12 × 500 = 6000 cm³ = 6 dm³ of air per minute. During hard exercise the rate can reach 40–50 breaths per minute and the depth 2500–3000 cm³, giving well over 100 dm³ per minute — a more than tenfold increase.

Two numbers change, not one

The everyday phrase is “you breathe faster”, and a candidate who only writes that has answered half the question. During exercise you breathe faster and more deeply. If a mark scheme offers two marks for the effect of exercise on breathing, they are almost always rate and depth, one each.

This also matters when you read a graph. A trace can show more peaks per minute (rate up) or taller peaks (depth up) or both, and a data question will often show you one without the other to see whether you notice.

Investigating it yourself

Sit the person down and let them rest for 5 minutes, so that you start from a true resting value.

Rate: count the breaths in one minute by watching the chest rise, or with a hand resting on the chest. Count a whole minute, not 15 seconds × 4, so that a slow breath is not missed. Do it three times and take the mean.
Depth: measure the volume of one breath out with a spirometer, or, more simply, measure the chest with a tape measure at full breath in and at full breath out: the difference shows how deeply they are breathing.
Exercise: give a fixed piece of exercise, for example 2 minutes of step-ups onto a bench, one step every 2 seconds to a metronome. Straight after it, measure rate and depth again, then every minute until both are back to the resting values. That gives you the recovery time as well.
Keep these the same: the type, intensity and duration of the exercise; the resting time beforehand; the time between stopping and measuring. Test several people and calculate means, because fitness varies a lot between people.

Time	Breaths per minute (1st, 2nd, 3rd)	Mean	Chest expansion / cm
rest
0 min after exercise
1 min after
2 min after
3 min after
…until back to rest

A results table set out like this, with the units in the headings, earns its own mark. Expected results: both rate and depth rise during exercise. Both fall slowly afterwards, taking longer to come down than they took to go up.

Both lines rise, both plateau, and both take longer to come down than they took to go up. That asymmetry is a favourite thing to ask you to describe.

Why It Happens — and It Is Not What You Think

Ask almost anyone why you breathe harder during exercise and they will say “because you need more oxygen”. Your body does need more oxygen, but that is not the signal. The signal is carbon dioxide.

The chain, link by link — learn all of it

1. Your muscles are contracting more, so the muscle cells are respiring faster.

2. Faster respiration produces more carbon dioxide.

3. So the concentration of carbon dioxide in the blood rises.

4. This increase is detected by the brain.

5. The brain sends nerve impulses to the diaphragm and the intercostal muscles.

6. The rate and the depth of breathing both increase.

7. More carbon dioxide is removed and more oxygen taken in, so the carbon dioxide concentration falls back towards normal.

Six or seven marks are available for that chain in a Supplement question, and almost all of them are lost by starting at step 6.

Carbon dioxide, not oxygen — say it explicitly

An answer that says breathing speeds up “because the oxygen level in the blood falls” will be marked wrong, even though it sounds sensible and even though more oxygen genuinely is needed. The syllabus is unambiguous: it is the increased carbon dioxide concentration in the blood, detected by the brain.

It is also the better system if you think about it. Carbon dioxide is produced in direct proportion to how hard you are working, and its concentration changes by a large fraction as soon as you start — whereas the oxygen content of your blood barely moves during ordinary exercise, because haemoglobin is almost fully loaded either way. Carbon dioxide is simply the more sensitive signal.

Notice too that breathing does not drop back to resting the moment you stop. Ventilation stays raised for several minutes, which is exactly what the graph above shows.

Keeping the Airways Clean

You breathe in about 6 dm³ of air every minute and every bit of it carries dust, pollen, bacteria and viruses. If any of that reached the alveoli it would stay there, because an alveolus is a dead end with a wall one cell thick and no way of clearing itself. So the airways clean the air before it gets that far, using two kinds of cell working as a partnership.

The commonest error in this sub-topic is giving one cell the other’s job. Goblet cells make the mucus; cilia move it. Cilia produce nothing.

Structure	What it does	Why it matters
Goblet cells	Secrete mucus onto the surface of the airway lining	Without mucus there is nothing sticky for particles to be caught in
Mucus	Traps pathogens (bacteria and viruses) and dust particles	Stops them reaching the alveoli, which have no way to clear themselves
Ciliated cells	Their cilia beat, sweeping the mucus up the trachea towards the throat	The trapped material is swallowed and destroyed by the acid in the stomach

Applying it: when the cilia stop working (not on the syllabus as a topic of its own)

Tobacco smoke paralyses and then destroys the cilia. The goblet cells carry on making mucus — in fact they make more, because the smoke irritates the lining — but nothing is moving it. So mucus collects in the airways, which is why smokers cough (the only way left to shift it), and why the bacteria trapped in the stagnant mucus cause repeated chest infections.

Notice how the mechanism does the explaining. You are not being asked to remember that smoking is bad; you are being asked which half of the partnership has broken and what happens to the other half.

Worked Example 5 At rest a student takes 14 breaths per minute, each of 480 cm³. During exercise she takes 42 breaths per minute, each of 2200 cm³. (a) Calculate her ventilation in dm³ per minute at rest and during exercise. [3] (b) Calculate the factor by which her ventilation has increased. [1] (c) A classmate says the increase is caused by her blood running out of oxygen. Correct him. [3]

Step 1: multiply, then convert once at the end

At rest: 14 × 480 = 6720 cm³ per minute. Since 1 dm³ = 1000 cm³, that is 6.72 dm³ per minute.

During exercise: 42 × 2200 = 92 400 cm³ per minute = 92.4 dm³ per minute.

Step 2: a factor is a division, not a subtraction

92.4 ÷ 6.72 = 13.75, so her ventilation has increased about 13.8 times — not by 13.8 %, and not by 85.7 dm³ unless the question asked for the increase rather than the factor. Read which one is wanted.

Step 3: replace the wrong cause with the right chain

The signal is not a fall in oxygen. Her muscles are contracting more, so her muscle cells are respiring faster and producing more carbon dioxide. The concentration of carbon dioxide in her blood rises, this is detected by her brain, and the brain sends impulses to the diaphragm and intercostal muscles that increase both the rate and the depth of her breathing.

You can add the reason it is a better system: carbon dioxide concentration changes sharply and in proportion to the work being done, whereas the oxygen content of the blood changes very little during ordinary exercise.

(a) 6.72 dm³/min at rest and 92.4 dm³/min during exercise. (b) About 13.8 times. (c) The trigger is a rise in the carbon dioxide concentration of the blood, detected by the brain, which increases the rate and depth of breathing — not a fall in blood oxygen.

Worked Example 6 A researcher measures breathing in two people. Person 1 breathes 30 times a minute, each breath 1000 cm³. Person 2 breathes 15 times a minute, each breath 2000 cm³. About 150 cm³ of every breath stays in the trachea and bronchi, where no gas exchange takes place. (a) Show that both people ventilate the same total volume per minute. [1] (b) Explain which of them gets more air to their alveoli each minute. [3]

Step 1: the totals really are equal

Person 1: 30 × 1000 = 30 000 cm³ per minute. Person 2: 15 × 2000 = 30 000 cm³ per minute. Identical, which is the whole point of the question.

Step 2: subtract the wasted air per breath, not per minute

Person 1 wastes 150 cm³ on each of 30 breaths, so 30 × 150 = 4500 cm³ never reaches an alveolus. Useful air = 30 000 − 4500 = 25 500 cm³.

Person 2 wastes 150 cm³ on each of only 15 breaths, so 15 × 150 = 2250 cm³. Useful air = 30 000 − 2250 = 27 750 cm³.

Step 3: state the general principle, because that is the third mark

Person 2 gets more air to their alveoli. The same volume of air moved in fewer, deeper breaths wastes less on the airways, because the air left in the trachea and bronchi is a fixed volume per breath. This is exactly why the body increases the depth of breathing during exercise and not just the rate — deep breathing is more efficient.

(a) Both = 30 000 cm³ per minute. (b) Person 2: only 2250 cm³ is wasted on the airways rather than 4500 cm³, so 27 750 cm³ reaches the alveoli against 25 500 cm³. Fewer, deeper breaths waste less.

Check Yourself: 11.4 Exercise, Breathing Control and Clean Airways

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

During exercise the rate and depth of breathing increase. The change is triggered by

A a fall in the concentration of oxygen in the blood, detected by the lungs

B a rise in the concentration of carbon dioxide in the blood, detected by the brain

C a rise in body temperature, detected by the skin

D a fall in the concentration of glucose in the blood, detected by the liver

This is the single most important sentence in 11.4 and option A is the answer almost everyone gives. Carbon dioxide is the signal because it is produced in direct proportion to how hard the muscles are working, whereas the oxygen content of blood barely changes during ordinary exercise.

Question 2

A person takes 15 breaths per minute, each of 500 cm³. What volume of air do they ventilate per minute?

A 0.75 dm³

B 7.5 dm³

C 75 dm³

D 750 dm³

15 × 500 = 7500 cm³, and 1000 cm³ = 1 dm³, so 7.5 dm³. Every wrong option here is the right multiplication with the conversion done by the wrong power of ten — which is exactly how marks are lost in this calculation. Do the multiplication in cm³, then convert once.

Question 3

Which cells produce the mucus that lines the trachea?

A goblet cells

B the cells lining the alveoli

C ciliated cells

D white blood cells in the capillaries of the trachea

Goblet cells make it; ciliated cells move it. Naming the ciliated cells is the standard confusion and it costs a mark every time, because a question that asks about the partnership expects you to give each cell its own job. The two cell types sit side by side in the same lining, which is probably why they get muddled.

Question 4

In which direction do the cilia move the mucus, and what happens to it?

A down towards the alveoli, where it is absorbed into the blood

B down towards the alveoli, where white blood cells destroy the trapped bacteria

C up towards the throat, where it is breathed out as a fine spray

D up towards the throat, where it is swallowed

Up and out, then swallowed — the acid in the stomach finishes the job. Options A and B both send the mucus the wrong way, which would defeat the entire point: the alveoli are a dead end with no way of clearing themselves, and everything about this system exists to keep particles away from them.

Question 5

Goblet cells and ciliated cells work together to protect the gas exchange system. Which describes what each does?

A goblet cells secrete mucus that traps pathogens; cilia beat to move the mucus away

B ciliated cells secrete mucus that traps pathogens; goblet cells move the mucus away

C goblet cells engulf pathogens; cilia beat to draw air down into the alveoli

D goblet cells secrete mucus that warms the air; cilia beat to trap dust particles

Goblet cells make the mucus, and the mucus traps pathogens and dust. The cilia make nothing: they beat and sweep the mucus up towards the throat, where it is swallowed. Giving one cell the other’s job is the commonest error here. Engulfing pathogens is what phagocytes do, not goblet cells, and cilia move mucus, not air.

Question 6

Two people ventilate the same total volume of air per minute, but person X takes fewer, deeper breaths than person Y. Compared with Y, person X will

A get less air to the alveoli, because each breath spends longer in the airways

B get exactly the same amount of air to the alveoli, since the totals are equal

C get more air to the alveoli, because less is wasted filling the airways

D take in more oxygen but also lose more water vapour, so the two effects cancel out

A fixed volume of every breath stays in the trachea and bronchi where no exchange happens, so the fewer breaths you take, the less of that waste you pay for. This is why the body deepens breathing during exercise instead of just speeding it up. Option B is the answer that ignores the fixed cost per breath.

Question 7

Why does breathing stay faster and deeper for several minutes after exercise has stopped?

A because the muscles keep contracting for several minutes after exercise

B because the brain takes several minutes to notice that exercise has stopped

C because the carbon dioxide concentration in the blood is still raised and takes time to return to normal

D because the alveoli remain stretched and cannot return to their normal size quickly

The control system responds to the carbon dioxide concentration, and that concentration is still above normal when you stop — so the raised breathing continues until it has been cleared. Option B misunderstands what the brain is detecting: it is not watching what you are doing, it is monitoring a chemical in the blood.

Question 8

Two students investigate the effect of exercise on breathing rate. Which is the most important variable to control?

A the colour of the clothes each person is wearing

B the intensity and duration of the exercise each person does

C the time of day at which the readings are taken

D whether the readings are recorded on paper or on a computer

A controlled variable is one that would change the result if you let it vary, and the amount of work done is the obvious one — someone who runs harder will breathe harder whatever else is true. Option C is the tempting choice because it sounds scientific, but it has a far smaller effect than exercise intensity and would only matter if the two people were tested at very different times.

Question 9

A graph shows a breathing trace during rest and then during exercise. Which change on the trace shows an increase in the depth of breathing?

A the peaks become closer together

B the peaks become taller

C the whole trace shifts upwards without changing shape

D the trace becomes a straight horizontal line

Taller peaks mean a bigger volume of air per breath, which is what depth means; closer peaks mean more breaths per minute, which is rate. A challenge paper will often show you one changing without the other precisely to see whether you can read the difference off the trace rather than assuming both.

Question 10

A person breathes in and out of a sealed plastic bag for a minute. Their breathing becomes much faster and deeper. The best explanation is that

A the bag prevents the diaphragm from contracting fully

B the air in the bag becomes colder, which stimulates the brain

C the carbon dioxide in the bag builds up, so the concentration in the blood rises and is detected by the brain

D the oxygen in the bag runs out completely within a few breaths

Rebreathing is the neatest demonstration that the trigger is carbon dioxide: the person is not exercising at all, yet their breathing changes dramatically, and the only thing that has altered is the composition of the air they are inhaling. Option D also gets the numbers wrong — expired air is still 16 % oxygen, so the bag is far from empty of it.

Question 11

Which sequence correctly describes what happens when a person starts to exercise?

A muscles respire faster → more carbon dioxide in the blood → detected by the brain → breathing rate and depth increase

B breathing rate increases → more oxygen in the blood → muscles respire faster → carbon dioxide rises

C oxygen falls in the blood → detected by the lungs → the diaphragm contracts harder → more oxygen is absorbed

D carbon dioxide rises in the alveoli → detected by the alveolus walls → more air is drawn in

The chain starts in the muscle, not in the lungs. Option B reverses cause and effect, which is worth noticing because it is how the process feels from the inside. Options C and D both put the detector in the wrong place: it is the brain that detects the carbon dioxide, and it detects it in the blood.

Question 12

Why is the mucus and cilia system found in the trachea and bronchi but not in the alveoli?

A because the alveoli are too small for bacteria to enter

B because the alveoli are kept sterile by the mucus made further up

C because there is no air movement in the alveoli to carry particles in

D because a layer of mucus over an alveolus would increase the diffusion distance and block gas exchange

This is a design trade-off, and it is the sort of reasoning a challenge paper rewards. The alveolus is thin on purpose; covering it in mucus would add to the diffusion distance and destroy the very feature that makes it work. So the cleaning has to be finished before the air arrives, which is exactly why damage to the cilia matters so much.

11.5 Exam Technique and the Vocabulary That Scores ▼

The Eight Sentences That Cost the Most Marks

What people write	Why it scores nothing	What to write instead
“Respiration takes place in the lungs.”	Respiration is a chemical reaction inside cells. The lungs do gas exchange.	“Gas exchange takes place at the alveoli; respiration takes place in every cell.”
“The lungs suck in air.”	Nothing can pull on a gas, and the lungs contain no muscle.	“The pressure in the thorax falls below atmospheric, so air is pushed in.”
“The diaphragm moves up when you breathe in.”	Exactly backwards. Contracting flattens it, so it moves down.	“The diaphragm muscle contracts and flattens, moving downwards.”
“Expired air contains no oxygen.”	It contains 16 % — over three quarters of what went in.	“Oxygen falls from 21 % to 16 %.”
“You breathe faster because your oxygen runs low.”	The trigger is a rise in carbon dioxide, detected by the brain.	“More carbon dioxide in the blood is detected by the brain, which increases rate and depth.”
“The limewater went white.”	Limewater does not gain a colour; it loses its transparency.	“The limewater turned cloudy (milky).”
“The cilia make mucus.”	Goblet cells make it. Cilia only move it.	“Goblet cells secrete mucus; the cilia beat and sweep it upwards.”
“A big surface area means more oxygen is available.”	Nothing makes oxygen available; area affects the rate of diffusion.	“A large surface area means more diffusion can occur at the same time.”

Reading the Command Word

Four commands, four different answers

State or name — one short fact, no reasoning. “State the function of cartilage in the trachea.” Answer: it holds the trachea open. Do not write a paragraph; you will not earn more and you may contradict yourself.

Describe — say what happens, in order, with figures if there are figures. “Describe what happens to breathing rate during the first four minutes” wants the shape of the line and the numbers, not the cause.

Explain — say why. Every explain answer in this topic should contain one of: concentration gradient, pressure gradient, diffusion distance, or surface area. If none of those four words appears, you are probably describing.

Suggest — you are being asked to apply what you know to something unfamiliar, and there is usually more than one acceptable answer. Say what you think and give the biological reason; the reason is the mark.

How to Attack a Topic 11 Data Question

Six steps, in this order

1. Read the axes and the units first. Breaths per minute and cm³ per breath look similar on a graph and mean completely different things. A dual-axis graph will have one line on each scale.

2. Note what the person is doing and when. Rest, exercise, recovery. Mark the boundaries on the paper.

3. Quote figures whenever you describe. “It rises” is one mark at best; “it rises from 14 to 45 breaths per minute between 2 and 8 minutes” is the full answer.

4. Do the multiplication if you are given rate and depth. Ventilation per minute is nearly always the number the question is really about.

5. Check the percentage trap. A percentage change is taken of the original value, and a huge percentage change in carbon dioxide is still a small absolute change because it starts at 0.04 %.

6. Never claim more than the data show. A limewater result shows there is more carbon dioxide; it does not show how much, and it does not show where the carbon dioxide came from.

Three Scenarios to Test Yourself On

A diver breathes from a cylinder at the surface. Their breathing rate is 16 breaths per minute and each breath is 500 cm³. After swimming hard for five minutes their rate is 34 and each breath is 1400 cm³. A friend says the cylinder must be supplying oxygen faster to make this happen.

Calculate the change in ventilation, and explain what is actually driving it.

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Do the arithmetic first

At the surface: 16 × 500 = 8000 cm³ = 8 dm³ per minute. After swimming: 34 × 1400 = 47 600 cm³ = 47.6 dm³ per minute. That is an increase by a factor of 5.95, close to six times.

Fix the direction of causation

The cylinder is passive. It does not decide anything; it supplies air whenever the diver breathes in. The diver is breathing harder, so more air is being drawn from the cylinder — the increased supply is the consequence, not the cause. Getting cause and effect the right way round is very often the mark in these questions.

Give the real chain

The diver’s muscles are contracting hard, so the muscle cells are respiring faster and producing more carbon dioxide. The carbon dioxide concentration in the blood rises, this is detected by the brain, and the brain increases both the rate and the depth of breathing — which is exactly what the two figures show, since both have gone up.

A student writes: “When you breathe in, the intercostal muscles contract and the ribs move up. The lungs get bigger and suck the air in. The diaphragm relaxes and moves down so there is more room.”

There are three separate errors. Find all three and correct each.

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Error one: which intercostal muscles

“The intercostal muscles contract” is too vague at Extended level, because there are two sets and they do opposite things. It should be the external intercostal muscles that contract, while the internal intercostal muscles relax. Naming both layers is Supplement material and Cambridge asks for it by name.

Error two: the lungs do not suck

The lungs contain no muscle and cannot enlarge themselves, and nothing can pull on a gas. The correct chain is: the volume of the thorax increases, so the pressure inside falls below atmospheric pressure, so air is pushed in from outside. The lungs are stretched by the thorax, not the other way round.

Error three: the diaphragm is the wrong way round

The diaphragm does move down during inhalation, so the student has the direction right but the mechanism wrong. It moves down because its muscle contracts, which makes the dome flatten. A relaxed diaphragm is domed and sits high. Half-right here scores nothing, because the mark is for “contracts and flattens”.

A class bubbles expired air through 25 cm³ of limewater in tube B, which turns cloudy after 4 breaths. Room air is drawn through 10 cm³ of limewater in tube A, which is still clear after 40 breaths. The class concludes that expired air contains more than ten times as much carbon dioxide as inspired air.

Evaluate that conclusion.

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What the data do support

Tube B turned cloudy after 4 breaths and tube A had not turned cloudy after 40. That is a clear qualitative result: expired air contains much more carbon dioxide than inspired air. Say that first — an evaluation that only attacks is an incomplete evaluation.

The flaw that breaks the number

The volumes of limewater were not the same: 25 cm³ against 10 cm³. The tube with more limewater turned cloudy sooner, which if anything makes the difference look smaller than it really is — but either way the comparison is not fair, and no ratio can be calculated from it.

There is a second problem with the number itself: tube A never turned cloudy at all, so 40 breaths is a minimum, not a measurement. You cannot form a ratio from a number you did not actually reach.

Reach a judgement and say how to fix it

The conclusion that expired air contains more carbon dioxide is sound; the claim of “more than ten times” is not supported by this experiment. To improve it: use the same volume and concentration of limewater in both tubes, continue bubbling through tube A until it does turn cloudy so that both numbers are real, repeat with several people, and use a carbon dioxide sensor if a genuine quantitative answer is wanted.

Notice that the true figure is about a hundredfold, so the class was under-claiming. That does not rescue the conclusion — a claim can be wrong in method even when the number happens to point the right way.

The Night-Before Checklist

Can you say all of these without looking?

The four features of a gas exchange surface, in Cambridge’s words. Why a large organism needs one at all, in terms of surface area to volume and diffusion distance. The order air travels: nose or mouth, larynx, trachea, bronchi, bronchioles, alveoli. The function of cartilage in the trachea, and why bronchioles do not have any. Which structures ventilate and which one carries out gas exchange. The three words — ventilation, gas exchange, respiration — with a definition and a location for each. Why good blood supply and good ventilation are the same argument from two sides. The diffusion distance and why it is about 1 µm. The composition table: oxygen 21 to 16, carbon dioxide 0.04 to 4, nitrogen 78 unchanged, water vapour up, temperature up. The reason for each of those four changes. The limewater test, the word cloudy, the apparatus with two tubes and one-way valves, and three controls. What the limewater experiment does not show. Inhalation: both sets of intercostals, the ribs, the diaphragm, the volume, the pressure, the direction of flow — and the same six for exhalation. Why the internal and external intercostals are an antagonistic pair. Why quiet exhalation is passive. The bell jar model, what each part represents and three reasons it is a poor model. Breathing rate and depth as two separate measurements, with resting values. Ventilation equals rate times depth, and the conversion between cm³ and dm³. The seven-link control chain from contracting muscles to falling carbon dioxide. Carbon dioxide, not oxygen, and why it is the better signal. Goblet cells, mucus, ciliated cells, the direction of travel and what happens at the throat. What happens when one half of the goblet cell and cilia partnership fails.

That list is the whole topic. If you can say it out loud in about five minutes, you are ready.

Check Yourself: 11.5 Exam Technique

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

Which answer would score both marks for “Explain why air enters the lungs during inhalation. [2]”?

A Because the lungs expand and suck the air in through the trachea

B Because the volume of the thorax increases, so the pressure inside falls below atmospheric and air is pushed in

C Because the body needs oxygen for respiration

D Because the diaphragm and the intercostal muscles contract

The two marks are the volume change and the pressure change; option B supplies both and gets the order right. Option D is true and is where most answers stop — it names the cause but never reaches the mechanism, so it collects one mark at most. Option C answers a “why do we breathe” question that was not asked.

Question 2

A question says “Describe the changes in breathing rate shown in Fig. 3.1. [3]”. The best approach is to

A explain the role of carbon dioxide in controlling breathing

B state the shape of the trace in stages, quoting values and times from the axes

C state simply that the rate increases and then decreases

D calculate the total volume of air breathed over the whole graph

Describe means say what happens, with figures; three marks usually means three stages, and each stage is worth a number. Option A is the answer to explain, and writing it here wastes time you will need later. Option C is the same answer with the marks stripped out of it.

Question 3

Which phrase should appear in almost every “explain” answer about the alveolus?

A “so that the body gets the oxygen it needs”

B “because the lungs are working harder”

C “so that respiration can happen in the lungs”

D “maintaining the concentration gradient”

Gradient, distance, area — those are the words that turn a description into an explanation in this topic. Option A restates the purpose without any mechanism, which is the commonest way of writing four lines for nothing. Option C repeats the respiration error into the bargain.

Question 4

Carbon dioxide rises from 0.04 % to 4.0 %. A student calculates the percentage increase as 99 %. Their error was to

A divide the increase by the final value instead of the original value

B subtract instead of dividing

C use percentages when they should have used a ratio

D forget to multiply by 100

3.96 ÷ 4.00 × 100 = 99 %, which is where that answer comes from; the correct calculation is 3.96 ÷ 0.04 × 100 = 9900 %. A percentage change is always taken of the starting value, and this is worth checking every single time because the two answers look equally plausible on the page.

Question 5

“Suggest why a person with damaged cilia in their airways is more likely to get chest infections. [2]” The word suggest tells you that

A the answer is a single fact to be recalled from the syllabus

B no marks are given for biological reasoning, only for the conclusion

C you should give as many possible answers as you can think of

D you must apply what you know to this situation, and the biological reason is where the marks are

Suggest means apply, not recall, and the reasoning is the mark: mucus is no longer swept up, so it collects, and the bacteria trapped in it stay in the airways and multiply. Option C is a genuinely bad habit — a scattergun of answers usually contains a wrong one, and a wrong statement can cancel a right one.

Question 6

Which statement about a limewater experiment is a fair conclusion?

A Expired air contains exactly 100 times as much carbon dioxide as inspired air

B Respiration takes place in the alveoli of the lungs

C Expired air contains more carbon dioxide than inspired air

D Oxygen is absorbed from the air in the alveoli

Limewater detects carbon dioxide and nothing else, so it can support only a statement about carbon dioxide — and only a comparison, not a figure. Option A claims a number the experiment never measured; option D is true biology but is not evidenced by this test at all, and claiming it would lose the mark.

Question 7

Which answer would score full marks for “State the function of the cartilage in the trachea. [1]”?

A It is flexible, and it is arranged in C-shaped rings which are incomplete at the back

B It holds the trachea open so that it does not collapse

C It protects the lungs from infection

D It helps to warm and moisten the air on its way to the alveoli

One mark, one idea, no decoration. Option A is a good example of a very common way to score zero: everything in it is true, but none of it answers the word function — it describes the cartilage instead. When a question says state, give the fact and stop.

Question 8

A four-mark question asks for the features that adapt the alveoli for gas exchange. The safest structure for the answer is

A four separate points, each naming a feature and saying what it achieves

B one long paragraph explaining the single most important feature in depth

C a list of eight features in the hope that four of them are right

D a description of how air reaches the alveoli, in order

Four marks almost always means four distinct points, and pairing each feature with its consequence is what separates a four out of four from a two. Option C is riskier than it looks: padding wastes time, and an incorrect statement mixed in with correct ones can cost you the mark it contradicts.

Question 9

Which of these describes rather than explains?

A Oxygen diffuses into the blood because its concentration is higher in the alveolus than in the blood

B Breathing rate rose from 14 to 42 breaths per minute over the first four minutes

C The wall is one cell thick, so the diffusion distance is short and diffusion is fast

D The ribs move up and out, so the volume of the thorax increases

Option B says what happened, with figures, and offers no cause — that is a description, and it is exactly right for a describe question and worth nothing on an explain. The other three all contain a “because” or a “so” that links a structure to a consequence, which is what explaining means.

Question 10

In an evaluation question about a flawed experiment, the strongest answer

A lists every possible criticism and concludes that the experiment is worthless

B says the results are correct because the biology behind them is correct

C says what the data do support, names the specific flaw and what it makes uncertain, then gives a judgement

D suggests repeating the experiment without saying what was wrong with it

Evaluate means weigh both sides and then decide, so an answer that only attacks is as incomplete as one that only defends. Option B is a subtle trap worth naming: knowing the right answer in advance does not make a badly controlled experiment into evidence for it.

Question 11

Which sentence uses all three of ventilation, gas exchange and respiration correctly?

A Ventilation brings fresh air to the alveoli, gas exchange moves oxygen into the blood by diffusion, and respiration uses that oxygen in the cells

B Respiration brings air into the lungs so that gas exchange can ventilate the blood

C Ventilation happens at the alveolus, gas exchange in the thorax, and respiration in the trachea

D Gas exchange moves air in and out of the lungs, and respiration is another name for ventilation

The sentence about fresh air reaching the alveoli, oxygen diffusing into the blood and that oxygen being used in the cells is the whole topic in one sentence, and it is worth learning as a sentence: three processes, three places, in the order oxygen actually travels. If a question ever asks you to distinguish the three, this is the answer, and being able to write it is worth more than any single fact in Topic 11.

Question 12

A graph of breathing rate against time is shown, but the question asks how much air the person breathed per minute. The correct response is to

A read the breathing rate off the graph and give that as the answer

B assume each breath is 500 cm³ and multiply

C state that the depth of each breath is also needed, and that it is not shown on this graph

D estimate the area under the curve

Ventilation is rate multiplied by depth, and a rate-only graph gives you just one of the two — so the honest answer names what is missing. Option B is the one that catches good students: 500 cm³ is a typical resting value and assuming it during exercise, when depth may be five times greater, would be badly wrong.