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Answers · Gas exchange and breathing

Gas exchange and breathing — return questions key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. AR1 · Return

Model answer. Limewater, a clear colourless solution, turns cloudy (milky white) when carbon dioxide bubbles through it. The more carbon dioxide, the sooner it turns cloudy.

A full answer includes

  • (a) I stated limewater.
  • (b) I stated that it turns from clear to cloudy (milky).
  • I did not state cobalt chloride paper.

Also accept

  • limewater; clear to cloudy

Watch for

  • “Limewater turns pink.” — Cobalt chloride paper turns pink, with water. Limewater turns cloudy with carbon dioxide.

2. AR2 · Return

Model answer. Expired air contains less oxygen (about 16% instead of 21%), more carbon dioxide (about 4% instead of 0.04%) and more water vapour than inspired air.

A full answer includes

  • (a) I wrote that expired air contains less oxygen (about 16% instead of 21%).
  • (b) I wrote that expired air contains more carbon dioxide (about 4% instead of 0.04%).
  • (c) I wrote that expired air contains more water vapour.
  • I did not write that expired air contains no oxygen.

Also accept

  • less oxygen; more carbon dioxide; more water vapour

Watch for

  • “Expired air has no oxygen and is all carbon dioxide.” — Expired air is about 16% oxygen and only 4% carbon dioxide.

3. AR3 · Return

Model answer. The lining of the alveoli is moist. Water evaporates from it into the air, which is warmed to body temperature, so expired air always leaves saturated with water vapour. Very dry cabin air brings in almost no water vapour, so each breath takes away more water from the body than a breath of moister air at home.

A full answer includes

  • I wrote that water evaporates from the moist lining of the alveoli into the air.
  • I wrote that the expired air leaves saturated with water vapour.
  • I wrote that the dry cabin air brings in very little water vapour, so each breath takes away more water.
  • I did not write that the water vapour is made in the lungs by respiration.

Also accept

  • evaporation from the moist lining; expired air saturated; dry inspired air brings little in

Watch for

  • “The extra water vapour is made by respiration in the lungs.” — It evaporates from the moist lining of the alveoli. Dry air takes more of it away.

4. LR1 · Return

Model answer. The trachea divides into two bronchi, one to each lung. Inside the lungs they branch again and again into narrower tubes, the bronchioles, which end in alveoli.

A full answer includes

  • (a) I stated the bronchi.
  • (b) I stated the bronchioles.
  • I did not state the alveoli.

Also accept

  • bronchi; bronchioles

Watch for

  • “Alveoli.” — Alveoli are the air sacs at the end. The tubes are the bronchi and the bronchioles.

5. LR2 · Return

Model answer. A gas exchange surface in humans has a large surface area, a thin surface, a good blood supply and good ventilation with air.

A full answer includes

  • I stated a large surface area.
  • I stated a thin surface.
  • I stated a good blood supply.
  • I stated good ventilation with air.
  • I did not state a thick surface.

Also accept

  • large surface area; thin surface; good blood supply; good ventilation with air

Watch for

  • “A thick surface for protection.” — The surface is thin, for a short diffusion distance.

6. LR3 · Return

Model answer. (a) The external intercostal muscles pull the ribs up and out. (b) The cartilage rings keep the trachea open when the pressure inside falls.

A full answer includes

  • (a) I stated that they pull the ribs up and out.
  • (b) I stated that it keeps the trachea open.
  • I did not state that the external intercostals pull the ribs down.

Also accept

  • (a) ribs up and out; (b) keeps trachea open

Watch for

  • “The external intercostals pull the ribs down.” — That is the internal intercostals. The external ones pull the ribs up and out.

7. LR4 · Return

Model answer. Goblet cells make mucus, which traps pathogens and particles such as dust. The cilia of ciliated cells beat and sweep the mucus up the trachea to the throat, where it is swallowed, away from the lungs.

A full answer includes

  • I wrote that goblet cells make mucus.
  • I wrote that the mucus traps pathogens and particles.
  • I wrote that the cilia on ciliated cells beat and move the mucus up to the throat.
  • I did not write that the ciliated cells make the mucus.

Also accept

  • mucus traps; cilia move it to the throat

Watch for

  • “The ciliated cells make the mucus.” — Goblet cells make mucus; ciliated cells move it.

8. VR1 · Return

Model answer. B. In forced expiration the internal intercostal muscles contract and pull the ribs down and in. The volume of the thorax decreases, so the pressure inside rises above atmospheric pressure and air is pushed out.

A full answer includes

  • I chose row B.
  • I did not choose row D: that is breathing in.

Also accept

  • B

Watch for

  • “Row A: the external intercostal muscles contract.” — The external intercostals pull the ribs up and out. Breathing out hard needs the internal ones, which pull the other way.
  • “Row C: the volume increases and the pressure falls.” — Ribs moving down and in make the thorax smaller, so the pressure rises and air leaves.
  • “Row D: external muscles, ribs up and out, volume up, pressure down.” — That is breathing in. Breathing out hard needs the opposite changes.

9. VR2 · Return

Model answer. Physical activity increases both the rate of breathing (breaths per minute) and its depth (volume per breath).

A full answer includes

  • I chose: both the rate and the depth increase.
  • I did not choose: the rate increases and the depth stays the same.

Also accept

  • both increase

Watch for

  • “The rate increases; the depth stays the same.” — The traces showed taller peaks too: the depth increases.
  • “The depth increases; the rate stays the same.” — The traces showed more peaks in 30 s too: the rate increases.
  • “Both decrease.” — Exercising muscles need more oxygen and make more carbon dioxide, so breathing increases.

10. VR3 · Return

Model answer. Exercising muscles respire faster and make more carbon dioxide, so its concentration in the blood rises. The brain detects this and sends nerve impulses to the diaphragm and the intercostal muscles, which contract more often and more strongly: breathing becomes faster and deeper.

A full answer includes

  • I wrote that the muscles respire faster and make more carbon dioxide.
  • I wrote that the carbon dioxide concentration in the blood rises.
  • I wrote that the brain detects the rise.
  • I wrote that the brain sends impulses to the diaphragm and intercostal muscles.
  • I did not write that the brain detects a lack of oxygen.

Also accept

  • more carbon dioxide in blood; brain detects; impulses to breathing muscles

Watch for

  • “The lungs decide to breathe faster.” — The brain detects the carbon dioxide and controls the breathing muscles.
  • “The brain detects a lack of oxygen.” — The brain detects the rise in carbon dioxide in the blood.