Answers · Gas exchange and breathing

Lungs and gas exchange — answer key

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

1. LQ1 · Practice

Options as numbered on the pupil sheet: 1 — larynx; 2 — bronchus; 3 — lung; 4 — diaphragm; 5 — alveolus; 6 — trachea; 7 — intercostal muscle; 8 — bronchiole; 9 — capillary; 10 — rib.

Model answer. R, in the neck, is the larynx. P, the tube with rings, is the trachea. S, one of its two branches, is a bronchus, and U, a narrower branch inside the lung, is a bronchiole. V is a lung. W is a rib, and T, between two ribs, is an intercostal muscle. Q, the dome-shaped sheet under the lungs, is the diaphragm. In the magnified circle, X is an alveolus, a tiny air sac, and Y is a capillary wrapped round it.

A full answer includes

I matched P to the trachea.
I matched Q to the diaphragm.
I matched R to the larynx.
I matched S to a bronchus.
I matched T to an intercostal muscle.
I matched U to a bronchiole.
I matched V to a lung.
I matched W to a rib.
I matched X to an alveolus.
I matched Y to a capillary.
I did not match S to a bronchiole.

Also accept

P trachea; Q diaphragm; R larynx; S bronchus; T intercostal muscle; U bronchiole; V lung; W rib; X alveolus; Y capillary

Watch for

“S is a bronchiole.” — The two large branches of the trachea are the bronchi. Bronchioles, like U, are the much smaller branches inside the lungs.
“R is the trachea.” — R is the wider part at the top, in the neck: the larynx. The trachea, P, is the ringed tube below it.
“T is a rib.” — The ribs, like W, are the bones. T is the muscle between two ribs.
“X is a capillary.” — X is the round air sac: an alveolus. The capillary, Y, is the thin blood vessel wrapped round it.

2. LQ2 · Practice

Options as numbered on the pupil sheet: 1 — trachea; 2 — a bronchiole; 3 — larynx; 4 — right bronchus.

Model answer. From the throat the camera passes the larynx, goes down the trachea, turns into the right bronchus, which leads to the right lung, and then into one of its bronchioles. The alveoli at the ends are far too small for the camera to enter.

A full answer includes

I put the larynx first.
I put the trachea second.
I put the right bronchus third.
I put a bronchiole last.
I did not put the bronchiole before the bronchus.

Also accept

larynx, trachea, right bronchus, bronchiole

Watch for

“Bronchioles come before bronchi.” — The trachea divides into the two bronchi first; the bronchioles are their smaller branches inside the lungs.
“The camera reaches the trachea before the larynx.” — The larynx, the voice box, is at the top of the trachea, so the camera passes it first.

3. LQ3 · Practice

Model answer. Gases pass between the air and the blood in the alveoli, the tiny air sacs at the ends of the bronchioles, each wrapped in capillaries. The trachea, bronchi and bronchioles only carry the air there.

A full answer includes

I chose the alveoli.
I did not choose the bronchi: they only carry air.

Also accept

alveoli

Watch for

“In the trachea.” — The trachea only carries air. Its wall is thick, with cartilage, and has no thin surface for gases to cross.
“In the bronchi.” — The bronchi carry air into each lung. Their walls are too thick for gases to cross into the blood.
“In the larynx.” — The larynx is the voice box at the top of the trachea. Air only passes through it.

4. LQ4 · Practice

Model answer. A good gas exchange surface has a large surface area, a thin surface, a good blood supply and good ventilation with air. Thick walls or a few large sacs would slow diffusion down.

A full answer includes

I chose a large surface area.
I chose a thin surface.
I chose a good blood supply.
I chose good ventilation with air.
I did not choose thick walls.

Also accept

large surface area; thin surface; good blood supply; good ventilation

Watch for

“Thick walls, to make the alveoli strong.” — Thick walls would make the diffusion distance longer. The walls are one cell thick.
“A few large air sacs.” — A few large sacs have much less surface than millions of small ones holding the same air.

5. LQ5 · Practice

Options as numbered on the pupil sheet: 1 — the gases diffuse only a short distance; 2 — the air in the alveoli is replaced again and again; 3 — more gas can diffuse at the same time; 4 — oxygen is carried away as soon as it enters the blood.

Model answer. A large surface area lets more gas diffuse at once. A thin surface (two cells thick in all) makes the diffusion distance short. A good blood supply carries oxygen away as it arrives and brings carbon dioxide, so the blood's side stays low in oxygen. Ventilation replaces the air, so the air's side stays high in oxygen and low in carbon dioxide.

A full answer includes

I matched the large surface area to more gas diffusing at the same time.
I matched the thin surface to a short diffusion distance.
I matched the good blood supply to oxygen being carried away as soon as it enters the blood.
I matched good ventilation to the air in the alveoli being replaced again and again.
I did not match the thin surface to oxygen being carried away.

Also accept

area: more at once; thin: short distance; blood: keeps concentration difference; ventilation: refreshes air

Watch for

“A good blood supply makes the walls thinner.” — The blood keeps the concentrations different on the two sides; the walls are thin because they are one cell thick.
“Good ventilation brings blood to the alveoli.” — Ventilation moves air in and out. The capillaries bring the blood.

6. LQ6 · Practice

Model answer. (a) 64 m² ÷ 1.7 m² = 37.6…, so the alveoli have about 38 times the area of the skin (to 2 significant figures). Areas differ from person to person; the reading gives about 70 m² for an adult. (b) The wall of an alveolus and the wall of its capillary are each one cell thick, so oxygen and carbon dioxide diffuse only a very short distance. Through many layers of skin cells the distance is much longer and diffusion is far too slow.

Checked automatically

(a) How many times larger: 38 (2 significant figures)
- Common wrong answer 0.027 : You divided the skin by the alveoli. Divide the larger area by the smaller one.
- Common wrong answer 62 : That is the difference in m². "How many times" needs a division.

A full answer includes

(a) I divided 64 m² by 1.7 m².
(a) I wrote 38 times, to 2 significant figures.
(b) I wrote that the walls of the alveoli are one cell thick.
(b) I wrote that the gases diffuse only a short distance.
I did not divide 1.7 by 64.

Also accept

(a) 38; (b) one cell thick, so a short diffusion distance

Watch for

“The alveoli are 62 times larger.” — 64 − 1.7 = 62.3 is how many m² more. "How many times larger" needs a division: 64 ÷ 1.7.
“A large area makes the gases diffuse a shorter distance.” — Distance depends on how thin the surface is. Area sets how much can cross at once.

7. LQ7 · Practice

Model answer. Many small alveoli have a much larger surface area than a few large sacs holding the same air. With the walls broken down, the surface area is smaller, so less oxygen diffuses into the blood (and less carbon dioxide out) at the same time. During exercise the body needs more oxygen, so the person runs short of it and breathes faster and harder.

A full answer includes

I wrote that fewer, larger sacs have a smaller surface area.
I wrote that less oxygen diffuses into the blood at the same time.
I wrote that during exercise the body needs more oxygen, so the shortage is worst then.
I did not write that larger sacs hold more air, so gas exchange is better.

Also accept

smaller surface area; less oxygen diffuses into the blood at once

Watch for

“The air sacs are larger, so they hold more air and gas exchange is better.” — What matters for diffusion is the surface area, and a few large sacs have less surface than many small ones.

8. LQ8 · Practice

Options as numbered on the pupil sheet: 1 — diaphragm; 2 — external intercostal muscles; 3 — internal intercostal muscles.

Model answer. F, the layer nearer the outside of the body, is the external intercostal muscles. G, the layer nearer the lung, is the internal intercostal muscles. Their fibres slant in opposite directions.

A full answer includes

I matched F to the external intercostal muscles.
I matched G to the internal intercostal muscles.
I did not match either letter to the diaphragm.

Also accept

F external intercostal muscles; G internal intercostal muscles

Watch for

“F is the internal intercostal muscles.” — F is the outer layer, nearer the skin: the external intercostals. The internal layer lies inside it, nearer the lung.
“G is the diaphragm.” — The diaphragm is the sheet of muscle under the lungs. G lies between two ribs: an intercostal muscle.

9. LQ9 · Practice

Model answer. (a) Both sets lie between neighbouring ribs. The external intercostal muscles are on the outside and the internal intercostal muscles inside them, with fibres slanting the opposite way. (b) A muscle can only pull when it contracts; it cannot push. So one set (external) is needed to pull the ribs up and out, and the other (internal) to pull them down and in. They work against each other.

A full answer includes

(a) I wrote that both sets lie between the ribs.
(a) I wrote that the internal muscles are the inner layer.
(b) I wrote that a muscle can only pull when it contracts.
(b) I wrote that one set pulls the ribs up and out and the other pulls them down and in.
I did not write that one set pushes the ribs.

Also accept

(a) both between ribs, internal is the inner layer; (b) muscles only pull, so a pair moves the ribs both ways

Watch for

“One set pushes the ribs out and pulls them back in.” — Muscles cannot push. Each set pulls the ribs one way.

10. LQ10 · Practice

Model answer. The C-shaped rings of cartilage hold the trachea open, so it does not collapse when the pressure inside it falls as you breathe in. Air can always pass.

A full answer includes

I chose: it keeps the trachea open.
I did not choose: it traps dust and bacteria.

Also accept

keeps the trachea open; stops it collapsing

Watch for

“It traps dust and bacteria.” — Mucus traps dust. Cartilage is a firm tissue that holds the trachea open.
“It makes mucus.” — Goblet cells in the lining make mucus. The cartilage lies deeper in the wall and only supports it.
“It moves the ribs.” — The intercostal muscles move the ribs. The cartilage is in the trachea and does not move anything.

11. LQ11 · Practice

Model answer. When you breathe in, the pressure in the airways falls below the pressure outside them. A soft trachea would be pushed in and would flatten, like a soft rubber tube on a pump, and air could not reach the lungs. The cartilage rings are firm enough to stop this.

A full answer includes

I predicted that the trachea would flatten (collapse).
I wrote that the pressure inside it falls below the pressure outside when you breathe in.
I wrote that the air flow would stop.
I did not write that air pushes the trachea open.

Also accept

collapses; pressure inside falls when breathing in

Watch for

“Nothing would happen, because air pushes the trachea open.” — When you breathe in, the pressure inside falls, so the outside pushes the wall in.

12. LQ12 · Practice

Options as numbered on the pupil sheet: 1 — cilia; 2 — mucus; 3 — ciliated cell; 4 — goblet cell.

Model answer. J, the cup-shaped cell full of droplets, is a goblet cell, which makes mucus. L, the sticky layer on top, is mucus, with dust and bacteria trapped in it. K, the tiny hair-like structures, are cilia, and M, a cell carrying them, is a ciliated cell.

A full answer includes

I matched J to a goblet cell.
I matched K to cilia.
I matched L to mucus.
I matched M to a ciliated cell.
I did not match L to cilia.

Also accept

J goblet cell; K cilia; L mucus; M ciliated cell

Watch for

“L is the cilia.” — L is the sticky layer that traps dust: mucus. The cilia are the tiny hairs under it.
“M is a goblet cell.” — M carries cilia, so it is a ciliated cell. The goblet cell, J, is cup-shaped and full of droplets.

13. LQ13 · Practice

Model answer. Goblet cells make mucus, which traps pathogens (such as bacteria) and dust. The cilia on ciliated cells beat and move the mucus up the trachea to the throat, where it is swallowed.

A full answer includes

(1) I wrote mucus.
(2) I wrote pathogens (or bacteria).
(3) I wrote cilia.
(4) I wrote up.
I did not write that the cilia trap the dust.

Also accept

mucus; pathogens; cilia; up

Watch for

“The cilia trap the dust.” — The mucus traps the dust; the cilia move the mucus along.
“The mucus is moved down into the lungs.” — It is moved up, away from the lungs, to the throat.

14. LT1 · Apply

Model answer. The trachea divides into two bronchi, one to each lung. With the right bronchus blocked, air still passes down the trachea into the left bronchus, its bronchioles and the alveoli of the left lung. The right lung's bronchioles and alveoli get no fresh air, so gas exchange there slows down.

A full answer includes

I stated that air can still reach the left bronchus, its bronchioles and the alveoli of the left lung.
I wrote that the right bronchus leads only to the right lung.
I wrote that the alveoli of the right lung get no fresh air.
I did not write that no air can reach either lung.

Also accept

left lung still reached; right lung not, because each bronchus leads to one lung

Watch for

“No air can reach either lung.” — The trachea divides before the block, so the left bronchus is still open.

15. LT2 · Apply

Model answer. The many filaments give a large surface area, so more oxygen diffuses at once. Thin filaments make the diffusion distance short. The capillaries carry oxygen away as it arrives, keeping the concentration difference. Water flowing over the gills keeps bringing more oxygen, like ventilation with air in the lungs.

A full answer includes

I wrote that many filaments give a large surface area.
I wrote that thin filaments give a short diffusion distance.
I wrote that the capillaries carry oxygen away, keeping a concentration difference.
I wrote that the flowing water brings fresh oxygen all the time, like ventilation.
I did not write that the gills suck oxygen in.

Also accept

surface area; thin; blood supply; ventilation with water

Watch for

“Gills work by sucking oxygen in.” — Oxygen diffuses into the blood, as in the alveoli; the features only speed up diffusion.

16. LT3 · Apply

Model answer. (a) Like the biceps and triceps, the external and internal intercostals pull in opposite directions, because a muscle can only pull: the external muscles pull the ribs up and out, the internal muscles down and in. When one set contracts, the other relaxes. (b) The C-shaped rings of cartilage in the trachea: like the wire, they keep the tube open when the pressure inside it falls.

A full answer includes

(a) I wrote that the external muscles pull the ribs up and out.
(a) I wrote that the internal muscles pull the ribs down and in.
(a) I wrote that when one set contracts, the other relaxes.
(b) I identified the cartilage rings in the trachea.
(b) I wrote that they keep the trachea open.
I did not write that both sets contract together.

Also accept

(a) antagonistic pair, opposite pulls; (b) cartilage keeps the trachea open

Watch for

“Both sets of intercostals contract together to move the ribs.” — They work against each other: when one set contracts, the other relaxes.

17. LT4 · Apply

Model answer. Mucus still traps pathogens, but without beating cilia it is not swept up the trachea to the throat to be swallowed. The mucus, with the trapped bacteria and viruses, stays in the airways and builds up, so the pathogens can multiply and reach the lungs, causing infections.

A full answer includes

I wrote that the mucus is not moved up to the throat.
I wrote that pathogens trapped in it stay in the airways.
I wrote that the pathogens can reach the lungs and cause infections.
I did not write that they make no mucus.

Also accept

mucus not moved away; trapped pathogens stay and cause infection

Watch for

“They make no mucus, so nothing is trapped.” — The goblet cells still make mucus. The problem is that it is not moved away.