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Cambridge IGCSE0610

Gas exchange in humans

Biology 0610 Chapter Notes

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Gas exchange in humans
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In this chapter

  1. 1Features of the gas exchange surface
  2. 2The breathing system
  3. 3Inspired and expired air
  4. 4Exercise and breathing
  5. 5Why the air changes
  6. 6Ventilation of the lungs
  7. 7Cartilage, mucus and cilia
  8. 8Carbon dioxide controls breathing

1. Features of the gas exchange surface

The gas exchange surface in humans is the alveoli. It has a large surface area and a thin surface.

It has a good blood supply and good ventilation with air. The surface is also moist and permeable to gases.

A question on features wants these features. It does not want a description of how the gases are exchanged.

Examiner insight

“Many candidates knew that features of the gas exchange surface in humans include a large surface area and a thin surface. Some candidates mistakenly believed that the gas exchange surface is thick.” “A few candidates described how gases are exchanged, rather than the features of the alveoli that enable efficient gas exchange.”

Worked example 13 marks

State three features of the gas exchange surfaces in the lungs that allow efficient gas exchange.

  1. 1

    large / increased surface area [1]

  2. 2

    thin surface / wall [1]

  3. 3

    good blood supply [1]

2. The breathing system

Air passes through the larynx, the trachea, the bronchi and the bronchioles to the alveoli. The tubes get narrower along the way: trachea, then bronchus, then bronchiole.

The alveoli are surrounded by many capillaries. Oxygen passes through the alveolus wall into a capillary.

The other parts to identify are the lungs, the ribs, the intercostal muscles and the diaphragm. Use these names: everyday names such as windpipe and voice box are not credited.

Examiner insight

“While many candidates opted for the correct answer, a common error was to place the trachea before the larynx.” “While many candidates selected the correct option, some candidates incorrectly believe that bronchioles come before bronchi.”

Worked example 11 mark

What is the sequence of structures through which a molecule of oxygen travels to pass from the air into the blood of a person?

A bronchiole, larynx, capillary, alveolus wall · B alveolus wall, capillary, bronchiole, larynx · C larynx, bronchiole, alveolus wall, capillary · D larynx, capillary, bronchiole, alveolus wall

  1. 1

    C: larynx, bronchiole, alveolus wall, capillary. [1]

3. Inspired and expired air

Inspired air is the air breathed in. Expired air is the air breathed out. Expired air contains less oxygen, more carbon dioxide and more water vapour than inspired air.

Expired air is not mainly carbon dioxide and it still contains oxygen. It is about 16% oxygen and about 4% carbon dioxide.

To investigate, pass each sample of air through limewater, the test for carbon dioxide. Limewater turns cloudy, or milky, in the presence of carbon dioxide.

Examiner insight

“Many candidates stated the exhaled air contained no oxygen, rather than less oxygen. Very few discussed the water content.” “Common errors were stating hydrogencarbonate without saying indicator and a few candidates stated calcium carbonate, the correct cation but incorrect anion.”

Worked example 13 marks

Compare the composition of expired air with inspired air. Limit your answer to oxygen, carbon dioxide and water vapour.

  1. 1

    expired air has less oxygen [1]

  2. 2

    expired air has more carbon dioxide [1]

  3. 3

    expired air has more water vapour [1]

4. Exercise and breathing

Physical activity changes breathing. Both the rate and the depth of breathing increase.

To investigate, count the breaths in a set time at rest, then again after exercise. The rate is the number of breaths per minute.

breathing rate = (number of breaths)/(time in minutes)

Examiner insight

“A few candidates only looked for a greater rate of breathing and missed the fact that the depth of breathing also increases.” “A significant number of candidates incorrectly thought that during vigorous exercise the rate of breathing increases, but that the depth of breathing decreases.”

Worked example 12 marks

State two changes to breathing during vigorous exercise.

  1. 1

    rate of breathing increases [1]

  2. 2

    depth of breathing increases [1]

5. Why the air changes

Oxygen is needed for aerobic respiration. Carbon dioxide is a waste product of respiration. So the blood arriving at the lungs has less oxygen and more carbon dioxide than the air in the alveoli.

Oxygen moves from a higher concentration in the alveoli to a lower concentration in the blood by diffusion. Carbon dioxide moves from the blood into the alveoli by diffusion.

So expired air has less oxygen and more carbon dioxide than inspired air. Blood leaving the lungs has more oxygen and less carbon dioxide than blood arriving.

Examiner insight

“Many candidates understood that oxygen moves from a region of higher concentration in the alveoli to a region of lower concentration in the blood by diffusion. Some candidates incorrectly selected active transport or osmosis.” “Some candidates incorrectly believed that the air entering the alveolus from the bronchiole only contains oxygen.”

Worked example 11 mark

Which row shows how the concentrations of carbon dioxide and oxygen change in the blood as it passes through the lungs?

carbon dioxideoxygen
Aincreasedecrease
Bincreaseincrease
Cdecreasedecrease
Ddecreaseincrease
  1. 1

    Carbon dioxide diffuses out of the blood into the alveoli, so it decreases.

  2. 2

    Oxygen diffuses from the alveoli into the blood, so it increases.

  3. 3

    D [1]

6. Ventilation of the lungs

There are two sets of intercostal muscles, the external and the internal. Always say which set.

In inspiration the external intercostal muscles and the diaphragm contract and the internal intercostal muscles relax. The ribs move up and out, the volume of the thorax increases and the pressure decreases. Air flows in.

In forced expiration the internal intercostal muscles contract. The ribs move down and in, the volume of the thorax decreases and the pressure increases. Write volume of the thorax, not volume of the lungs.

Examiner insight

“Candidates often did not state which set of intercostal muscles (external or internal) were involved in decreasing air pressure in the lungs and in forced expiration and so did not gain credit for their answers in rows 1 and 3.” “Some candidates incorrectly believed that the pressure in the lungs increases at the start of inspiration. The increased volume of the thorax decreases the pressure in the lungs allowing air to flow in from outside.”

Worked example 11 mark

The graph shows the changes in volume of the thorax when a human is inspiring and expiring.

Which row shows the regions on the graph when the internal and external intercostal muscles are contracting?

internal intercostal muscles contractexternal intercostal muscles contract
AP to QR to S
BQ to RS to T
CR to SQ to R
DS to TP to Q
  1. 1

    Internal intercostal muscles contract in forced expiration. The volume of the thorax decreases: R to S.

  2. 2

    External intercostal muscles contract in inspiration. The volume of the thorax increases: Q to R.

  3. 3

    C [1]

7. Cartilage, mucus and cilia

Cartilage in the trachea prevents the trachea from collapsing, so the airway stays open. Support or protection alone is too vague.

Goblet cells secrete mucus. The mucus traps pathogens and particles.

Ciliated cells move the mucus, so small particles do not enter the alveoli. The cilia move the mucus that holds the bacteria. They do not trap bacteria.

Examiner insight

“Most candidates correctly selected option B, but some incorrectly thought that ciliated cells produce mucus.” “A common misconception was that cilia trap bacteria, rather than move bacteria trapped in mucus.”

Worked example 12 marks

The gas exchange system contains specialised cells called ciliated cells. State one location of ciliated cells in the gas exchange system and describe their function.

  1. 1

    location: trachea / bronchi [1]

  2. 2

    function: move mucus / stop small particles entering the alveoli [1]

8. Carbon dioxide controls breathing

During physical activity the carbon dioxide concentration in the blood increases. This is detected by the brain.

This leads to an increased rate and greater depth of breathing. The stimulus is carbon dioxide, not oxygen.

Examiner insight

“A number of candidates incorrectly thought that a decrease in oxygen in the blood is the stimulus for an increase in breathing rate during exercise, rather than an increase in carbon dioxide in the blood.”

Worked example 11 mark

What causes a bigger volume of air to be inspired during one breath when exercising?

A A decreased carbon dioxide concentration in the blood is detected by the brain, which leads to an increased contraction of the internal intercostal muscles.

B A decreased carbon dioxide concentration in the blood is detected by the brain, which leads to an increased contraction of the external intercostal muscles.

C An increased carbon dioxide concentration in the blood is detected by the brain, which leads to an increased contraction of the internal intercostal muscles.

D An increased carbon dioxide concentration in the blood is detected by the brain, which leads to an increased contraction of the external intercostal muscles.

  1. 1

    Exercise increases the carbon dioxide concentration in the blood, so it is C or D.

  2. 2

    A bigger breath in needs the external intercostal muscles, which contract in inspiration.

  3. 3

    D [1]

Recap

  • Gas exchange surface: large surface area, thin, good blood supply, good ventilation with air.
  • Air passes through the larynx, trachea, bronchi and bronchioles to the alveoli.
  • Expired air has less oxygen (about 16%), more carbon dioxide (about 4%) and more water vapour than inspired air.
  • Limewater is the test for carbon dioxide. It turns cloudy, or milky.
  • Exercise increases both the rate and the depth of breathing.
  • Extended: oxygen and carbon dioxide move between the alveoli and the blood by diffusion.
  • Extended: inspiration needs the external intercostal muscles and the diaphragm to contract. The thorax volume increases and the pressure decreases.
  • Extended: cartilage prevents the trachea from collapsing. Mucus traps particles and pathogens. Cilia move the mucus.
  • Extended: more carbon dioxide in the blood is detected by the brain, giving faster and deeper breathing.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State three features of the gas exchange surface in the lungs.3 marks
  2. Put these parts of the breathing system in the order that air passes through them from the nose: bronchi, alveoli, larynx, bronchioles, trachea.2 marks
  3. A student writes: 'Expired air is mainly carbon dioxide and contains no oxygen.' (a) State the approximate percentage of oxygen in expired air. (b) State the approximate percentage of carbon dioxide in expired air. (c) State how the water vapour in expired air compares with inspired air.3 marks
  4. A student bubbles inspired air through limewater in tube A. She bubbles the same volume of expired air through limewater in tube B. (a) Name the gas that limewater tests for. (b) State what she sees when the limewater gives a positive result. (c) Explain why the limewater in tube B changes first.3 marks
  5. A student counts her breaths for 3 minutes at rest and again for 3 minutes after running. Calculate the increase in her breathing rate, in breaths per minute. | | breaths counted | time / min | | --- | --- | --- | | at rest | 45 | 3 | | after running | 96 | 3 |3 marks
  6. During inspiration: (a) name the set of intercostal muscles that contracts (b) state the direction the ribs move (c) state what happens to the pressure in the lungs.3 marks
  7. Explain how the internal intercostal muscles help to push air out of the lungs in forced expiration.4 marks
  8. Explain why blood leaving the lungs has more oxygen and less carbon dioxide than blood arriving at the lungs.4 marks
  9. A volunteer sits at rest and breathes three gas mixtures in turn. Every mixture contains 21% oxygen. Only the carbon dioxide concentration differs. The volume of air breathed in one minute is the breathing rate multiplied by the volume of one breath. (a) Calculate the volume of air breathed in one minute with 0.04% carbon dioxide and with 6.0% carbon dioxide. (b) Calculate how many times greater the second volume is than the first. (c) Use the data to decide which increases by the larger factor between 0.04% and 6.0%: the rate or the depth of breathing. (d) Explain why the breathing changes although the volunteer is at rest. (e) Name the set of intercostal muscles that contracts more strongly to give the bigger breath in. | carbon dioxide in inspired air / % | breathing rate / breaths per minute | volume of one breath / cm³ | | --- | --- | --- | | 0.04 | 15 | 500 | | 3.0 | 17 | 1000 | | 6.0 | 25 | 1800 |9 marks
  10. In one lung disease the cilia stop moving and mucus collects in the bronchioles. The walls between alveoli also break down, leaving fewer, larger air spaces. (a) Explain why a person with this disease gets more lung infections. (b) The mucus narrows the bronchioles. State the feature of the gas exchange surface that this reduces. (c) State the feature of the gas exchange surface that is reduced when the walls between alveoli break down. Explain the effect on the oxygen concentration of the blood leaving the lungs. (d) A student writes that cartilage 'protects the trachea'. State the function of cartilage in the trachea.8 marks

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