AS Level PE 8386 · Area 4 · The Respiratory System

Gaseous Exchange and Respiration

Gaseous exchange is the movement of oxygen and carbon dioxide across membranes, driven entirely by partial pressure differences. Cambridge tests it at the lungs and at the muscles, and it expects you to link the exchange to haemoglobin, myoglobin and the diffusion gradients that exercise makes steeper. Students lose marks by describing ventilation instead of diffusion, and by using respiration and breathing as though they mean the same thing.

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Syllabus coverage

  • Explain gaseous exchange at the lungs and at the muscles at rest, during exercise and during recovery, limited to the partial pressures of oxygen and carbon dioxide
  • Explain the role of the alveoli in gaseous exchange
  • Explain the role of haemoglobin and blood capillaries in gaseous exchange
  • Explain the role of myoglobin in gaseous exchange at the muscles
  • Explain how diffusion gradients affect gaseous exchange at rest, during exercise and during recovery

Part of the AS Level PE 8386 syllabus 8386.

Alveolus (air space)Capillary wallBlood capillaryOxygen diffuses from the alveolus into the blood; carbon dioxide diffuses the other way, from blood into the alveolus.
An alveolus with its surrounding blood capillaries, showing the directions of diffusion of oxygen and carbon dioxide and the partial pressures on each side of the respiratory membrane.

Gaseous exchange at the alveoli

Gaseous exchange is the movement of oxygen and carbon dioxide across a membrane, from where the partial pressure of the gas is higher to where it is lower. It happens at the alveoli, where air meets blood, and at the muscles, where blood meets tissue. Nothing is pumped across: the gases move by diffusion down their partial pressure gradients.

At the lungs, the air in the alveoli has a higher partial pressure of oxygen than the deoxygenated blood arriving in the pulmonary capillaries, so oxygen diffuses from the alveolar air into the blood. The same blood has a higher partial pressure of carbon dioxide than the alveolar air, so carbon dioxide diffuses out of the blood into the alveoli to be breathed out.

The alveoli are built for this. There are millions of them, giving a very large total surface area; their walls are one cell thick, so the diffusion distance is short; and each is surrounded by a dense network of blood capillaries, which keeps the concentration gradient steep because blood with a low partial pressure of oxygen is constantly being replaced.

Alveolar structure is a favourite 2-mark question: large surface area and a short diffusion distance, both of which speed up diffusion. Name both, and link them to the rate of diffusion rather than just describing the alveolus.

Gaseous exchange at the muscles, and the role of myoglobin

At the muscles, the partial pressures are reversed. The working muscle uses oxygen, so the partial pressure of oxygen inside the muscle is lower than in the blood arriving in the systemic capillaries, and oxygen diffuses from the blood into the muscle tissue. The muscle produces carbon dioxide as it respires, so the partial pressure of carbon dioxide in the muscle is higher than in the blood, and carbon dioxide diffuses from the muscle into the blood to be carried back to the lungs.

Myoglobin is an oxygen-carrying pigment found in skeletal muscle. It has a higher affinity for oxygen than haemoglobin, so it can take oxygen from the blood even when the partial pressure in the muscle is low, and it stores oxygen inside the muscle. When the muscle works hard and the partial pressure of oxygen falls further, myoglobin releases its stored oxygen for the muscle to use.

  • During exercise, oxygen is used faster, so the partial pressure of oxygen in the muscle falls and the diffusion gradient from blood to muscle becomes steeper, so more oxygen diffuses in
  • During exercise, more carbon dioxide is produced, so the diffusion gradient from muscle to blood becomes steeper and more carbon dioxide diffuses out
  • Myoglobin acts as a store of oxygen within the muscle and releases it when the partial pressure of oxygen falls
  • During recovery, oxygen use falls, the diffusion gradients at the muscle become less steep and exchange slows back towards resting rates

Diffusion gradients and the factors affecting the rate of diffusion

A diffusion gradient is the difference in partial pressure of a gas between two places. The bigger the difference, the faster the gas diffuses. Everything about exercise performance comes back to this: the body cannot make oxygen diffuse, it can only make the gradient steeper.

Four factors determine how fast gaseous exchange happens at the alveoli and at the muscle.

FactorHow it affects the rate of diffusionHow exercise or training changes it
Size of the diffusion gradientThe larger the difference in partial pressure between the two sides of the membrane, the faster the gas diffusesExercise raises the gradient at both the alveoli and the muscles; breathing more deeply and pumping more blood keep it high
Surface areaThe larger the surface area available for exchange, the faster the gas diffusesThe millions of alveoli provide a huge area; long-term training can increase the surface area of the capillary network around the muscles
Diffusion distanceThe shorter the distance between the two sides, the faster the gas diffusesThe one-cell-thick walls of the alveoli and capillaries give the shortest possible distance
TemperatureA rise in temperature increases the rate of diffusionThe heat produced by working muscles raises the temperature around them and speeds up exchange
If a question asks why gaseous exchange increases during exercise, never answer "because the lungs work harder". Say that the diffusion gradients at the alveoli and the muscles become steeper, so the rate of diffusion increases.

Oxygen transport: haemoglobin, association and dissociation

Oxygen is not very soluble in plasma, so almost all of it is carried by haemoglobin inside the red blood cells. Haemoglobin can bond with oxygen to form oxyhaemoglobin, and this reaction is reversible.

At the lungs, the partial pressure of oxygen is high, so haemoglobin associates with oxygen to form oxyhaemoglobin in the red blood cells. At the muscles, the partial pressure of oxygen is lower, so oxyhaemoglobin dissociates and releases its oxygen, which diffuses into the muscle tissue.

The relationship between the partial pressure of oxygen and how much oxygen is carried is shown by the oxyhaemoglobin dissociation curve. The curve is not a straight line: it is flatter at high partial pressures, which is what allows haemoglobin to load oxygen fully in the lungs, and steep at lower partial pressures, which is what allows it to unload a large amount of oxygen at the muscles. During exercise the curve shifts so that oxygen is released more readily — the increased carbon dioxide, lower pH and higher temperature at the working muscle reduce haemoglobin's affinity for oxygen, so more oxygen is unloaded exactly where it is needed.

Two words carry the marks: associate at the lungs and dissociate at the muscles. Writing "haemoglobin takes oxygen to the muscles" is a description, not an explanation, and it rarely scores.

Carbon dioxide transport

Carbon dioxide is produced by the muscles as they respire, and it diffuses into the blood down its diffusion gradient. It is carried back to the lungs in three ways: most of it dissolves in the plasma and is carried as bicarbonate ions, some of it binds directly to haemoglobin to form carbaminohaemoglobin, and a small amount dissolves unchanged in the plasma.

At the lungs the partial pressure of carbon dioxide in the blood is higher than in the alveolar air, so carbon dioxide diffuses out of the blood into the alveoli and is expired. Removing carbon dioxide matters as much as supplying oxygen: carbon dioxide dissolves to form an acid, so if it is not removed the pH of the blood falls.

Respiration is not breathing

The two words are used interchangeably in everyday speech, and Cambridge uses them precisely. Confusing them in an answer can cost marks in a question that asks about either.

Breathing is the mechanical process of moving air into and out of the lungs. It is muscular and controlled by the respiratory control centre, and it is sometimes called ventilation. Respiration is the chemical process that happens inside the muscle cells, breaking down glucose to release the energy the muscle uses to contract. Respiration produces carbon dioxide as a waste product; breathing only supplies the oxygen for respiration and removes the carbon dioxide respiration produces.

BreathingRespiration
The mechanical movement of air into and out of the lungsThe chemical breakdown of glucose to release energy inside the muscle cells
Muscular and controlled by the respiratory control centre in the medulla oblongataHappens continuously in the tissues and is not controlled by the nervous system
Supplies oxygen to the alveoli and removes carbon dioxide from themUses oxygen and produces carbon dioxide as a waste product
Can be increased voluntarily, for example by taking a deep breathCannot be increased voluntarily; it is increased by the demand of the muscles for energy
A question about "why breathing rate increases" is asking about receptors and the respiratory control centre. A question about "why the muscles need more oxygen" is asking about respiration. Read which word the question uses.

Key terms you must be able to define

Gaseous exchange
The movement of oxygen and carbon dioxide across a membrane from where the partial pressure of the gas is higher to where it is lower, by diffusion.
Partial pressure
The pressure exerted by a single gas within a mixture of gases, such as the pressure exerted by oxygen within the air in the alveoli.
Diffusion gradient
The difference in partial pressure of a gas between two places; the steeper the gradient, the faster the gas diffuses.
Alveoli
The tiny air sacs of the lungs, which have a very large total surface area and walls one cell thick and are surrounded by blood capillaries, giving the ideal conditions for gaseous exchange.
Haemoglobin
The oxygen-carrying pigment in red blood cells that bonds reversibly with oxygen to form oxyhaemoglobin.
Oxyhaemoglobin
The compound formed when haemoglobin associates with oxygen at the lungs; it dissociates at the muscles to release oxygen.
Association
The bonding of oxygen to haemoglobin, which takes place at the lungs where the partial pressure of oxygen is high.
Dissociation
The release of oxygen from oxyhaemoglobin, which takes place at the muscles where the partial pressure of oxygen is lower.
Myoglobin
An oxygen-carrying pigment in skeletal muscle with a higher affinity for oxygen than haemoglobin, which stores oxygen in the muscle and releases it when the partial pressure of oxygen falls.
Bicarbonate ions
The form in which most carbon dioxide is carried in the plasma, having dissolved in the blood, on its way back to the lungs.
Breathing
The mechanical process of moving air into and out of the lungs, controlled by the respiratory control centre.
Respiration
The chemical process inside the muscle cells in which glucose is broken down to release energy, producing carbon dioxide as a waste product.

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.

Cross-country running

As a runner climbs a hill, oxygen is used faster at the quadriceps, so the partial pressure of oxygen in the muscle falls. The diffusion gradient from blood to muscle becomes steeper, so more oxygen diffuses into the muscle while more carbon dioxide diffuses the other way.

Competitive swimming

The pressure of the water on the chest works the respiratory muscles harder, but gaseous exchange still depends on the gradients. A swimmer with a large lung capacity can take a bigger breath at each turn, keeping the alveolar partial pressure of oxygen high and the gradient steep.

Track and field athletics

In the final 200 m of an 800 m race, the muscles are producing carbon dioxide faster than it can be removed. The increased carbon dioxide, lower pH and higher muscle temperature reduce haemoglobin's affinity for oxygen, so more oxygen is unloaded where the demand is greatest.

Rugby union

A prop pushing in a scrum works hard while breathing is restricted by the effort of the push. Myoglobin in the muscle releases its stored oxygen to cover the gap while the next breath arrives, which is one reason a strong player can sustain a long push.

Badminton

Between rallies, breathing rate stays high for a short time even though the rally has ended. This clears the carbon dioxide that built up in the blood so the pH returns to normal before the next rally begins.

Flashcards

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

Q1What drives gaseous exchange?
Diffusion down a partial pressure gradient. Gas moves from where its partial pressure is higher to where it is lower.
Q2In which direction does oxygen diffuse at the alveoli, and why?
From the alveolar air into the blood, because the partial pressure of oxygen is higher in the alveolar air than in the deoxygenated blood arriving in the pulmonary capillaries.
Q3Why is carbon dioxide able to diffuse out of the blood at the lungs?
Because the partial pressure of carbon dioxide in the blood is higher than in the alveolar air, so it diffuses into the alveoli and is expired.
Q4Give two features of the alveoli that speed up gaseous exchange.
A very large total surface area, and walls one cell thick so the diffusion distance is short. They are also surrounded by a dense capillary network that keeps the gradient steep.
Q5What is the role of haemoglobin in oxygen transport?
It bonds reversibly with oxygen in the red blood cells to form oxyhaemoglobin at the lungs, and releases the oxygen at the muscles where the partial pressure of oxygen is lower.
Q6Define association and dissociation in relation to oxygen transport.
Association is haemoglobin bonding with oxygen at the lungs, where the partial pressure of oxygen is high. Dissociation is oxyhaemoglobin releasing oxygen at the muscles, where the partial pressure of oxygen is lower.
Q7What is myoglobin and where is it found?
An oxygen-carrying pigment found in skeletal muscle. It has a higher affinity for oxygen than haemoglobin, stores oxygen in the muscle, and releases it when the partial pressure of oxygen falls.
Q8Name the four factors affecting the rate of diffusion.
The size of the diffusion gradient, the surface area available, the diffusion distance (thickness of the membrane) and temperature.
Q9How is most carbon dioxide carried back to the lungs?
As bicarbonate ions in the plasma, having dissolved in the blood. Some is bound to haemoglobin and a small amount dissolves unchanged in the plasma.
Q10Define breathing.
The mechanical process of moving air into and out of the lungs, controlled by the respiratory control centre in the medulla oblongata.
Q11Define respiration.
The chemical breakdown of glucose inside the muscle cells to release energy, producing carbon dioxide as a waste product.
Q12Why does the diffusion gradient at the muscles get steeper during exercise?
The working muscle uses oxygen faster and produces carbon dioxide faster, so the partial pressure of oxygen in the muscle falls and the partial pressure of carbon dioxide rises.
0 of 12 marked as known

Exam-style questions

8 questions, 20 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 what is meant by the term diffusion gradient.

Mark scheme — tick each point you actually made

Examiner tip: One mark, one idea. The examiner is looking for the idea of a difference in partial pressure, not just the word diffusion.
StateAO12 marks
State the direction in which oxygen and carbon dioxide diffuse at the alveoli.

Mark scheme — tick each point you actually made

Examiner tip: Give both gases and both directions. The two are usually awarded separately, so leaving one out halves your marks.
DescribeAO12 marks
Describe how the structure of the alveoli helps gaseous exchange to take place.

Mark scheme — tick each point you actually made

Examiner tip: Every structural feature needs its consequence. "The alveoli are thin" earns nothing; "the wall is one cell thick, so the diffusion distance is short" earns the mark.
ExplainAO23 marks
Explain how gaseous exchange at the muscles changes when a performer begins to run.

Mark scheme — tick each point you actually made

Examiner tip: The phrase that keeps appearing in mark schemes is steeper diffusion gradient. Use it, and say which way the gas moves as a result.
ExplainAO23 marks
Explain the role of haemoglobin in supplying oxygen to a footballer's muscles during a match.

Mark scheme — tick each point you actually made

Examiner tip: Structure your answer as a journey: lungs, transport, muscles. Each stage is usually worth a mark, and the words associates and dissociates are the ones examiners look for.
ExplainAO23 marks
Explain how myoglobin helps a rugby player to keep working during a long passage of play.

Mark scheme — tick each point you actually made

Examiner tip: The mark comes from the higher affinity and from the release when the partial pressure falls. Simply writing "myoglobin stores oxygen" is usually worth one mark at most.
ExplainAO24 marks
Explain why the rate of gaseous exchange at the alveoli increases during exercise of increasing intensity.

Mark scheme — tick each point you actually made

Examiner tip: Deal with both gases and both gradients. "The lungs work harder" is not an explanation of exchange and will not score.
CompareAO12 marks
Compare breathing with respiration.

Mark scheme — tick each point you actually made

Examiner tip: Two marks, two definitions. Add the distinguishing feature — mechanical against chemical — so the comparison is explicit rather than two separate sentences.

Exam technique for this topic

  • Always name the gas, the direction and the gradient: "oxygen diffuses from the alveoli into the blood because the partial pressure of oxygen is higher in the alveolar air than in the deoxygenated blood".
  • Never use "breathing" when the question asks about respiration. Write "respiration" only for the chemical release of energy in the muscle cells, and "breathing" only for the movement of air.
  • For a question on why exchange increases during exercise, use the phrase steeper diffusion gradient at both the alveoli and the muscles, and say what each gradient does.
  • When describing the alveoli, give the structural feature and its consequence: large surface area and one-cell-thick walls mean a faster rate of diffusion. A feature with no consequence scores nothing.
  • Learn the two directions of haemoglobin's job as a pair: association at the lungs, dissociation at the muscles. They are usually worth one mark each.

Why students lose marks here

A student wroteDuring exercise the lungs pump more oxygen into the blood.
Why it loses marksNothing pumps gas across a membrane. Oxygen moves by diffusion, driven by the partial pressure difference, so a description of ventilation is not an explanation of exchange.
The answer that scoresDuring exercise the working muscles use oxygen faster, so the partial pressure of oxygen in the blood arriving at the muscles falls and the partial pressure of oxygen in the alveolar air stays high. The diffusion gradient from the alveoli into the blood is therefore steeper, so oxygen diffuses into the blood more quickly and is carried to the muscles by haemoglobin.
A student wroteHaemoglobin takes oxygen to the muscles and drops it off.
Why it loses marksThis is a description, not an explanation, and the key terms are missing. The examiner wants association at the lungs and dissociation at the muscles.
The answer that scoresAt the lungs the high partial pressure of oxygen causes haemoglobin to associate with oxygen, forming oxyhaemoglobin in the red blood cells. At the muscles the partial pressure of oxygen is lower, so oxyhaemoglobin dissociates and releases oxygen, which diffuses into the muscle tissue.
A student wroteMyoglobin carries oxygen from the lungs to the muscles.
Why it loses marksMyoglobin does not travel in the blood. Oxygen is transported from the lungs to the muscles by haemoglobin in the red blood cells.
The answer that scoresMyoglobin is found inside skeletal muscle. It has a higher affinity for oxygen than haemoglobin, so it stores oxygen in the muscle and releases it when the partial pressure of oxygen in the muscle falls during hard exercise.
A student wroteRespiration is when you breathe in and out, and it speeds up during exercise.
Why it loses marksThis confuses respiration with breathing. Respiration is the chemical process inside the muscle cells; breathing is the mechanical movement of air, and it is the one that speeds up.
The answer that scoresBreathing is the mechanical movement of air into and out of the lungs, controlled by the respiratory control centre, and its rate increases during exercise. Respiration is the chemical breakdown of glucose inside the muscle cells to release energy, producing carbon dioxide as a waste product.
A student wroteCarbon dioxide is carried back to the lungs by the haemoglobin.
Why it loses marksOnly a small proportion of carbon dioxide travels bound to haemoglobin. Most of it is carried as bicarbonate ions after dissolving in the plasma.
The answer that scoresCarbon dioxide is carried back to the lungs mainly as bicarbonate ions in the plasma, with some bound to haemoglobin and a small amount dissolved unchanged in the plasma. At the lungs it diffuses out of the blood into the alveoli down its partial pressure gradient and is expired.