IGCSE PE 0413 · Area 1 · Anatomy & Physiology

The Respiratory System

The respiratory system gets oxygen into the blood and carbon dioxide out. Cambridge wants the pathway of air in order, the features of alveoli that make gaseous exchange efficient, how the diaphragm and intercostal muscles produce breathing, and what exercise does to tidal volume, vital capacity, residual volume and minute ventilation. Marks are most often lost on the alveoli features and on the minute ventilation calculation.

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Syllabus coverage

  • The pathway of air into the body: mouth/nasal passage, trachea, bronchi, bronchioles, alveoli
  • Identify and explain the characteristics of alveoli that enable gaseous exchange to occur
  • The function of the diaphragm and intercostal muscles in normal breathing
  • Describe and explain tidal volume, vital capacity, residual volume and minute ventilation
  • The effect of exercise on these volumes

Part of the IGCSE PE 0413 syllabus 0413 / 0995.

Nasal passageMouthTracheaBronchiLeft lungBronchiolesRight lungAlveoli: large surface area, walls one cell thick, rich blood supply and a moist surface.
The pathway of air from the mouth and nasal passage through the trachea, bronchi and bronchioles to the alveoli, with the diaphragm and intercostal muscles shown.

The pathway of air

Air follows a single route, and each part of the route has a job. Learn it in order, because a question that says "describe the pathway of air" wants the sequence, and a question that says "state" wants it as a list.

The system divides into two halves: the conducting airways, which carry the air (mouth and nasal passage, trachea, bronchi, bronchioles), and the respiratory surface, where the gas exchange actually happens (the alveoli).

StructureWhat it isFunction
Mouth / nasal passageThe opening of the airway at the headAir enters the body here. The nasal passage warms, moistens and filters the air as it passes through; the mouth is used when breathing rate increases and more air is needed quickly
TracheaThe windpipe - a wide tube running down the neck into the chestCarries air from the throat towards the lungs. Rings of cartilage keep it open so it does not collapse
BronchiThe two tubes the trachea divides into, one into each lungCarry air into each lung and divide again into smaller tubes
BronchiolesThe much smaller tubes that the bronchi branch into, spread throughout the lungsCarry air deeper into the lungs and deliver it to the alveoli
AlveoliThe tiny air sacs at the end of the bronchioles, clustered like bunches of grapesThe site of gaseous exchange - oxygen passes from the air in the alveoli into the blood, and carbon dioxide passes from the blood into the alveoli to be breathed out
The order is fixed: mouth/nasal passage → trachea → bronchi → bronchioles → alveoli. Learn the singular and plural spellings - bronchi (two tubes) and bronchioles (many small tubes) are different structures and are separately marked.

Gaseous exchange at the alveoli

Gaseous exchange means oxygen moving from the air into the blood, and carbon dioxide moving from the blood into the air. The alveoli are built to make this happen quickly.

Cambridge asks you to identify and explain the characteristics - so for each feature you must say what it is and why it speeds up exchange. A list of features with no explanation caps your mark.

CharacteristicWhy it enables gaseous exchange
There are a very large number of alveoli, giving a large surface areaMore surface area means more oxygen and carbon dioxide can diffuse across at the same time, so exchange is faster and greater in volume
The walls of the alveoli are very thin - only one cell thickA short diffusion distance means the gases do not have to travel far, so they diffuse across quickly
The alveoli are surrounded by a dense network of capillariesA good blood supply brings deoxygenated blood to the alveoli and carries oxygenated blood away, keeping the concentration gradient steep so diffusion continues
The surface of the alveoli is moistGases have to dissolve in moisture before they can diffuse, so a moist surface allows oxygen and carbon dioxide to pass into solution and cross the wall
The alveoli have a rich blood supply and constant airflow in and out during breathingBoth the oxygen and the carbon dioxide are carried away as soon as they arrive, which maintains the concentration difference between air and blood that diffusion depends on
The rule behind all five features is the same: diffusion is faster with a large surface area, a short distance and a steep concentration gradient. If you are stuck in an exam, use those three ideas to reason the answer out.

The mechanics of breathing

Two muscle groups do the work: the diaphragm, a dome-shaped sheet of muscle below the lungs, and the intercostal muscles, which sit between the ribs. They always work as a pair of opposing actions.

Inspiration (breathing in). The diaphragm contracts and flattens, moving downwards. The intercostal muscles contract, pulling the ribs upwards and outwards. The volume of the chest cavity increases, so the pressure inside the chest falls below the pressure outside, and air is drawn in.

Expiration (breathing out). The diaphragm relaxes and returns to its dome shape, moving upwards. The intercostal muscles relax and the ribs move downwards and inwards. The volume of the chest cavity decreases, so the pressure inside rises above the pressure outside, and air is forced out.

During exercise the same muscles work harder and faster, and the surrounding muscles of the trunk also help to move the ribcage, making each breath bigger.

Inspiration (breathing in)Expiration (breathing out)
DiaphragmContracts and flattens, moving downRelaxes and rises back into its dome shape
Intercostal musclesContract, pulling the ribs up and outRelax, so the ribs move down and in
Chest cavity volumeIncreasesDecreases
Pressure inside the chestDecreases below the pressure outsideIncreases above the pressure outside
ResultAir is drawn into the lungsAir is forced out of the lungs
The chain to memorise: muscles → ribcage/diaphragm movement → change in volume → change in pressure → air moves. If a question asks you to explain breathing, each mark is one link in that chain.

The four lung volumes

Cambridge names four measurements, and they are all about how much air moves in and out of the lungs. Three are volumes; the fourth is a rate.

Learn the definitions word by word. "Vital capacity" does not mean how important your lungs are; "residual volume" is the air that never leaves.

TermDefinitionWhat it tells you
Tidal volumeThe volume of air breathed in or out in one normal breath at restHow much air moves with each ordinary breath - it is a fairly small volume when the body is resting
Vital capacityThe maximum volume of air that can be exhaled after a maximum inhalationThe greatest amount of air a performer can move in one breath, so it shows the usable capacity of the lungs
Residual volumeThe volume of air that remains in the lungs after a maximum exhalationThe air that cannot be breathed out - it stays in the lungs to keep them open and to allow some exchange to continue between breaths
Minute ventilationThe volume of air breathed in (or out) in one minuteHow much air reaches the lungs in a minute - it shows how hard the breathing system is working

Minute ventilation and the effect of exercise

Minute ventilation is the one calculation in this subtopic, and it is easy to remember because it uses the other two terms you already know:

minute ventilation = tidal volume × breathing rate

For example, a performer at rest might breathe with a tidal volume of 0.5 litres at a breathing rate of 12 breaths per minute, giving a minute ventilation of 0.5 × 12 = 6 litres per minute. During hard exercise the tidal volume might rise to about 2 litres and the breathing rate to 40 breaths per minute, giving 2 × 40 = 80 litres per minute.

Both ingredients of the calculation increase with exercise, so minute ventilation increases sharply. That is what delivers more oxygen to the working muscles and removes the extra carbon dioxide being produced.

MeasurementEffect of exerciseWhy
Tidal volumeIncreasesBreathing becomes deeper, so more air is moved in each breath to supply the extra oxygen the working muscles need and to remove the extra carbon dioxide
Vital capacityEffectively unchanged during exerciseIt is the maximum the lungs can hold, so it is a fixed capacity rather than something a single bout of exercise can increase. Regular training may improve it over the long term
Residual volumeUnchangedIt is the air that cannot be expelled from the lungs, so exercise does not change it
Minute ventilationIncreases significantlyBoth tidal volume and breathing rate rise during exercise, and minute ventilation is the product of the two, so it increases far more than either one on its own
If a question asks for the effect of exercise on the volumes, the safe answer is: tidal volume increases, breathing rate increases, therefore minute ventilation increases. Do not claim that vital capacity increases during exercise - it is a maximum, not a working measurement.

Key terms you must be able to define

Trachea
The windpipe: the tube that carries air from the throat towards the lungs, held open by rings of cartilage.
Bronchi
The two tubes into which the trachea divides, one leading into each lung.
Bronchioles
The small tubes branching from the bronchi throughout the lungs that carry air to the alveoli.
Alveoli
Tiny air sacs at the ends of the bronchioles where gaseous exchange takes place.
Gaseous exchange
The diffusion of oxygen from the air in the alveoli into the blood, and of carbon dioxide from the blood into the alveoli.
Diaphragm
The dome-shaped sheet of muscle below the lungs. It contracts and flattens for inspiration and relaxes and rises for expiration.
Intercostal muscles
The muscles between the ribs. They contract to pull the ribs up and out for inspiration and relax for expiration.
Tidal volume
The volume of air breathed in or out in one normal breath at rest.
Vital capacity
The maximum volume of air that can be exhaled after a maximum inhalation.
Residual volume
The volume of air remaining in the lungs after a maximum exhalation.
Minute ventilation
The volume of air breathed in or out in one minute. Minute ventilation = tidal volume × breathing rate.

Applied to your sport

Cambridge requires you to apply this knowledge to physical activities. Use these as models, then write your own for the sport you play.

Swimming

A competitive swimmer cannot breathe freely while the face is in the water, so each breath is taken at the point of turning the head. Being able to fill the lungs deeply raises tidal volume and increases minute ventilation during the swim.

Athletics

As a cross-country runner climbs a hill, both tidal volume and breathing rate rise, so minute ventilation increases sharply to deliver more oxygen to the working leg muscles and remove the extra carbon dioxide.

Basketball

During a fast break the intercostal muscles contract harder to pull the ribs up and out and the diaphragm contracts and flattens, drawing in a deeper breath to fuel the sprint down the court.

Cycling

A cyclist in a time trial settles into a deep, steady breathing rhythm. Tidal volume is high but a residual volume of air always remains in the lungs, so the alveoli do not collapse between breaths.

Flashcards

Click a card to flip it. Mark honestly — the tracker uses this.

Q1List the pathway of air into the body, in order.
Mouth / nasal passage → trachea → bronchi → bronchioles → alveoli.
Q2Which structure is the site of gaseous exchange?
The alveoli - oxygen diffuses from the air into the blood and carbon dioxide diffuses from the blood into the air.
Q3Give two characteristics of alveoli and explain why each helps gaseous exchange.
A large surface area from the huge number of alveoli, so more gas can diffuse at once; and very thin walls one cell thick, giving a short diffusion distance so gases cross quickly.
Q4Why is the moist surface of the alveoli important?
Gases must dissolve in moisture before they can diffuse, so the moist surface allows oxygen and carbon dioxide to pass into solution and cross the alveolar wall.
Q5What does the diaphragm do when you breathe in?
It contracts and flattens, moving downwards. This increases the volume of the chest cavity, lowering the pressure inside so air is drawn in.
Q6What do the intercostal muscles do when you breathe in?
They contract, pulling the ribs upwards and outwards, which helps to increase the volume of the chest cavity.
Q7What happens to cause air to be forced out when you breathe out?
The diaphragm relaxes and rises into its dome shape and the intercostal muscles relax, so the ribs move down and in. Chest volume decreases and pressure inside rises, forcing air out.
Q8Define tidal volume.
The volume of air breathed in or out in one normal breath at rest.
Q9Define vital capacity.
The maximum volume of air that can be exhaled after a maximum inhalation.
Q10Define residual volume.
The volume of air remaining in the lungs after a maximum exhalation.
Q11What is the equation for minute ventilation, and what is its unit?
Minute ventilation = tidal volume × breathing rate. It is measured in litres per minute.
Q12How does exercise affect tidal volume and minute ventilation?
Tidal volume increases because breathing becomes deeper, and breathing rate also increases, so minute ventilation increases significantly.
0 of 12 marked as known

Exam-style questions

8 questions, 23 marks. Write your answer, then reveal the mark scheme and tick each point you actually made. Answering honestly here is what makes the rest of the site useful.

StateAO11 mark
State the volume of air remaining in the lungs after a maximum exhalation.

Mark scheme — tick each point you actually made

Examiner tip: One mark, one term. Do not explain it - the command word is 'state'.
IdentifyAO12 marks
Identify the two structures that air passes through between the trachea and the alveoli.

Mark scheme — tick each point you actually made

Examiner tip: Keep the spelling and the order right. Bronchi come first, then bronchioles - naming them the other way round suggests you do not know the pathway.
DescribeAO12 marks
Describe the function of the diaphragm and the intercostal muscles during inspiration.

Mark scheme — tick each point you actually made

Examiner tip: Describe the muscle action and the effect on the chest cavity. Naming the muscles without saying what they do scores nothing.
ExplainAO22 marks
Explain how the structure of the alveoli allows efficient gaseous exchange.

Mark scheme — tick each point you actually made

Examiner tip: Two marks means two features, each with a reason. A list of features with no explanation caps you at the first mark.
ExplainAO23 marks
A swimmer has a tidal volume of 0.6 litres and a breathing rate of 15 breaths per minute at rest. During a race the tidal volume rises to 2 litres and the breathing rate to 40 breaths per minute. Calculate the minute ventilation at rest and during the race, and explain why it changes.

Mark scheme — tick each point you actually made

Examiner tip: Show the equation and the units. A calculated answer with no unit or no working can lose the mark even when the number is right.
ExplainAO14 marks
Explain how gaseous exchange occurs at the alveoli.

Mark scheme — tick each point you actually made

Examiner tip: Four marks means four stages: blood arriving, oxygen in, carbon dioxide out, gases removed. Write it as a sequence.
CompareAO14 marks
Compare tidal volume and vital capacity, and state the effect of exercise on each.

Mark scheme — tick each point you actually made

Examiner tip: The second half of this question is where marks are won. Many students wrongly say exercise increases vital capacity.
EvaluateAO25 marks
A cyclist is training for a long-distance event and asks whether they should focus on increasing their tidal volume or their vital capacity. Evaluate which change would have the greater effect on their performance during the ride.

Mark scheme — tick each point you actually made

Examiner tip: An evaluate answer needs a judgement. Give both sides, then say which matters more and why - do not sit on the fence.

Exam technique for this topic

  • Learn the pathway of air in order and keep the plural/singular forms correct - bronchi and bronchioles are separate structures and are separately credited.
  • "Explain the characteristics of the alveoli" means every feature needs a reason. State the feature, then say how it speeds up diffusion.
  • For breathing questions, write the cause and the effect: muscle action, then the change in volume and pressure, then air moving.
  • For minute ventilation, show the calculation as tidal volume × breathing rate and give the unit (litres per minute). A bare number with no unit may be penalised.
  • Do not claim vital capacity increases during exercise. It is a maximum lung capacity, not a working measurement.

Why students lose marks here

A student wroteVital capacity is the amount of air you breathe in and out normally.
Why it loses marksThat is tidal volume. Vital capacity is the maximum volume of air that can be exhaled after a maximum inhalation.
The answer that scoresVital capacity is the maximum volume of air that can be exhaled after a maximum inhalation. Tidal volume, by contrast, is the volume of air breathed in or out in one normal breath at rest.
A student wroteWhen you breathe in, the diaphragm relaxes and moves up.
Why it loses marksThis reverses the action. The diaphragm contracts and flattens, moving downwards on inspiration, which is what increases the volume of the chest cavity and draws air in.
The answer that scoresOn inspiration the diaphragm contracts and flattens, moving downwards, and the intercostal muscles contract to pull the ribs up and out. This increases the volume of the chest cavity, which lowers the pressure inside below the pressure outside, so air is drawn into the lungs. On expiration the diaphragm relaxes and rises back into its dome shape while the intercostal muscles relax, decreasing the volume and forcing air out.
A student wroteMinute ventilation = tidal volume × vital capacity.
Why it loses marksThis uses the wrong second term. Minute ventilation is calculated using the breathing rate, not vital capacity.
The answer that scoresMinute ventilation = tidal volume × breathing rate. For example, a tidal volume of 0.5 litres at a breathing rate of 12 breaths per minute gives a minute ventilation of 6 litres per minute. During exercise both figures rise, so minute ventilation increases sharply.
A student wroteThe alveoli have thick walls so they are strong enough to hold the air.
Why it loses marksThis is the opposite of the truth and misses the point of the feature. Alveolar walls are very thin - one cell thick - precisely to shorten the diffusion distance.
The answer that scoresThe walls of the alveoli are only one cell thick, giving a very short diffusion distance, so oxygen and carbon dioxide can diffuse across quickly. Combined with the large surface area provided by the huge number of alveoli and the dense capillary network that maintains the concentration gradient, this makes gaseous exchange efficient.
A student wroteExercise increases vital capacity, so minute ventilation goes up.
Why it loses marksVital capacity is a maximum lung capacity and does not change during a single bout of exercise. The rise in minute ventilation comes from increases in tidal volume and breathing rate.
The answer that scoresDuring exercise, tidal volume increases (breathing becomes deeper) and breathing rate increases, so minute ventilation - tidal volume × breathing rate - increases significantly. Vital capacity and residual volume are effectively unchanged by exercise; whatever increase in vital capacity occurs comes from long-term training, not from a single session.

Where this fits in IGCSE PE 0413

This page is one topic of Cambridge IGCSE PE 0413 / 0995. IGCSE PE 0413 is the course home: every area and every topic in Cambridge's own order, the papers you sit and what each one tests, and the exam-practice tools. The full topic list, with each topic linked to its revision page, is on the IGCSE PE 0413 syllabus page.