Answers · Gas exchange and breathing

Breathing in, breathing out — answer key

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

1. VQ7 · Practice

Model answer. The depth of breathing is the volume of air breathed in (or out) in one breath, measured in dm³ (1 dm³ = 1 litre). The rate of breathing is the number of breaths per minute.

A full answer includes

I chose: the volume of air breathed in or out in one breath.
I did not choose: the number of breaths per minute.

Also accept

volume of air in one breath

Watch for

“The number of breaths per minute.” — That is the rate of breathing. Depth is the volume of each breath.
“How far down the trachea the air goes.” — Air always reaches the alveoli. Depth is a volume, measured in dm³.
“The time taken for one breath.” — Time is measured in seconds. Depth is the volume of air in one breath.

2. VQ8 · Practice

Model answer. A minute (60 s) holds two lots of 30 s (60 ÷ 30 = 2), so the rate is 9 × 2 = 18 breaths per minute.

Checked automatically

Rate of breathing: 18 min-1 (exact value)
- Common wrong answer 9 : That is the number in 30 s. A minute is twice as long.
- Common wrong answer 4.5 : You divided by 2. A minute is longer than 30 s, so the number gets bigger.

A full answer includes

I multiplied 9 by 2.
I wrote 18 breaths per minute.
I did not divide 9 by 2.

Also accept

18 breaths per minute

Watch for

“9 breaths per minute.” — The 9 breaths were counted in only half a minute. A minute is twice as long.

3. VQ9 · Practice

Model answer. (a) There are 10 peaks, so 10 breaths in 30 s. (b) 10 × 2 = 20 breaths per minute, because a minute holds two lots of 30 s. (c) Each breath goes from 0.5 dm³ up to 1.7 dm³, so the depth is 1.7 − 0.5 = 1.2 dm³. (d) 20 breaths per minute × 1.2 dm³ = 24 dm³ per minute.

Checked automatically

(a) Breaths in 30 s: 10 (exact value)
(b) Rate of breathing: 20 min-1 (exact value)
- Common wrong answer 10 : That is the number in 30 s. Multiply by 2 for a minute.
(c) Depth of each breath: 1.2 dm³ (within ±0.05 dm³)
- Common wrong answer 1.7 dm³: That is the top of a peak. Subtract the dip, 0.5 dm³.
(d) Volume per minute: 24 dm³ per minute (within ±0.5 dm³ per minute)
- Common wrong answer 12 dm³ per minute: You used 10 breaths, the number in 30 s. Use the rate per minute, 20.

A full answer includes

(a) I counted 10 peaks.
(b) I multiplied 10 by 2.
(b) I wrote 20 breaths per minute.
(c) I subtracted 0.5 dm³ from 1.7 dm³.
(c) I wrote 1.2 dm³.
(d) I multiplied 20 by 1.2 dm³.
(d) I wrote 24 dm³ per minute.
I did not take 1.7 dm³, the top of a peak, as the depth.

Also accept

(a) 10; (b) 20 per minute; (c) 1.2 dm³; (d) 24 dm³ per minute

Watch for

“The depth is 1.7 dm³, the height of a peak.” — Depth is the change in volume in one breath: from the dip at 0.5 dm³ to the peak at 1.7 dm³.
“The volume per minute is 12 dm³.” — That uses the 10 breaths in 30 s. Use the rate per minute, 20.

4. VQ10 · Practice

Model answer. Running increased both. The rate went up from 14 to 30 breaths per minute, a little more than double, and the depth went up from 0.6 dm³ to 1.8 dm³, three times as deep (1.8 ÷ 0.6 = 3).

A full answer includes

(a) I wrote that the rate increased.
(a) I quoted 14 and 30 breaths per minute.
(b) I wrote that the depth increased.
(b) I quoted 0.6 dm³ and 1.8 dm³.
I did not write that only the rate changed.

Also accept

rate increased 14 to 30; depth increased 0.6 to 1.8 dm³

Watch for

“Only the rate changes with exercise.” — The depth changes too: here it tripled.

5. VQ11 · Practice

Model answer. A fair test changes only the exercise: the same person, the same counting time (30 s each), and the count taken straight after the exercise, before the breathing falls back to the resting rate. Counting the pulse measures the heart rate, not the breathing.

A full answer includes

I chose the plan that counts Sam's breaths for 30 s at rest and for 30 s straight after skipping.
I did not choose the plan that uses a different person after the skipping.

Also accept

Sam, 30 s at rest, then 30 s straight after 2 minutes of skipping

Watch for

“Count Sam at rest, then Ali after the skipping.” — A different person brings a second difference: Ali's resting rate may not be the same as Sam's.
“Count 10 minutes after the skipping.” — By then the breathing has fallen back towards the resting rate, so the effect of the exercise is missed.
“Count Sam's pulse.” — The pulse is the heart rate. The rate of breathing is the number of breaths.

6. VQ1 · Practice

Options as numbered on the pupil sheet: 1 — the trachea and bronchi; 2 — the walls of the thorax; 3 — the diaphragm; 4 — the lungs.

Model answer. The bell jar stands for the walls of the thorax (the ribs), the rubber sheet for the diaphragm, the balloons for the lungs and the Y-shaped tube for the trachea and the two bronchi.

A full answer includes

I matched the bell jar to the walls of the thorax.
I matched the rubber sheet to the diaphragm.
I matched the balloons to the lungs.
I matched the Y-shaped tube to the trachea and bronchi.
I did not match the rubber sheet to the walls of the thorax.

Also accept

jar thorax walls; sheet diaphragm; balloons lungs; tube trachea and bronchi

Watch for

“The rubber sheet stands for the ribs.” — The sheet is under the "lungs" and moves down and up: it is the diaphragm.
“The Y-shaped tube stands for the lungs.” — The tube carries air to the balloons, as the trachea and bronchi carry air to the lungs.

7. VQ2 · Practice

Options as numbered on the pupil sheet: 1 — The volume of the thorax increases.; 2 — The pressure inside the thorax falls below atmospheric pressure.; 3 — Air flows into the lungs.; 4 — The diaphragm and the external intercostal muscles contract..

Model answer. The diaphragm contracts and flattens, and the external intercostal muscles contract, pulling the ribs up and out. This increases the volume of the thorax, so the pressure inside falls below atmospheric pressure, and air flows in through the trachea into the lungs.

A full answer includes

I put the muscles contracting first.
I put the volume of the thorax increasing second.
I put the pressure falling below atmospheric pressure third.
I put air flowing into the lungs last.
I did not put air flowing in before the volume increases.

Also accept

muscles contract; volume increases; pressure falls; air flows in

Watch for

“Air flows in first and pushes the chest out.” — The chest gets bigger first; air flows in because the pressure has fallen.
“The pressure falls before the volume changes.” — The pressure falls because the volume has increased.

8. VQ3 · Practice

Model answer. At rest, the diaphragm relaxes and moves up into its dome, and the external intercostal muscles relax, so the ribs move down and in. The volume of the thorax decreases, the pressure inside rises above atmospheric pressure, and air flows out.

A full answer includes

(1) I wrote relaxes.
(2) I wrote down and in.
(3) I wrote decreases.
(4) I wrote rises.
(5) I wrote out.
I did not write that the diaphragm contracts.

Also accept

relaxes; down and in; decreases; rises; out

Watch for

“The diaphragm contracts to move up.” — At rest the diaphragm relaxes and is pushed back up into its dome.

9. VQ4 · Practice

Model answer. (a) The balloons fill much less than before, or hardly at all. (b) Pulling the sheet down still makes the volume larger, but air from the room flows in through the crack, so the pressure around the balloons does not fall far below atmospheric pressure. With almost no pressure difference, little air flows down the tube into the balloons. In the body, the space around the lungs is sealed for the same reason.

A full answer includes

(a) I predicted that the balloons hardly fill.
(b) I wrote that air enters the jar through the crack.
(b) I wrote that the pressure around the balloons stays close to atmospheric pressure.
(b) I wrote that without a lower pressure, little air flows into the balloons.
I did not write that the balloons fill as usual because the volume still increases.

Also accept

(a) balloons hardly fill; (b) air enters through the crack, so the pressure does not fall

Watch for

“The balloons fill as usual, because the volume still increases.” — Air flows into the balloons only if the pressure around them falls. Air through the crack stops it falling.
“The balloons burst, because more air comes in.” — The air from the crack goes into the jar, around the balloons, not into them.

10. VQ5 · Practice

Model answer. In forced expiration, the internal intercostal muscles contract and pull the ribs down and in further and faster; the muscles of the abdomen push the diaphragm up. At rest, breathing out happens as the muscles relax.

A full answer includes

I chose the internal intercostal muscles.
I did not choose the external intercostal muscles.

Also accept

internal intercostal muscles

Watch for

“The external intercostal muscles.” — They pull the ribs up and out, for breathing in.
“The diaphragm alone.” — The diaphragm is under the lungs and does not pull on the ribs. When it relaxes it moves up.
“No muscles at all.” — That is true of breathing out at rest. Breathing out hard needs muscles to contract.

11. VQ6 · Practice

Model answer. (a) The ribs, which the intercostal muscles move. (b) More air enters the balloons. Moving the sides out as well as pulling the sheet down makes the volume inside the bottle increase more, so the pressure falls further below atmospheric pressure, and more air flows in before the pressures are equal again. In the chest, the ribs and the diaphragm move together in the same way, for a deeper breath. The bottle is still only a model: the real diaphragm is a dome of muscle that flattens when it contracts, not a sheet pulled by hand.

A full answer includes

(a) I stated the ribs.
(b) I predicted that more air enters the balloons.
(b) I wrote that the volume inside the bottle increases more.
(b) I wrote that the pressure falls further below atmospheric pressure.
I did not write that the moving sides push air into the balloons.

Also accept

(a) ribs; (b) more air: a bigger volume increase, so a bigger fall in pressure

Watch for

“The moving sides push air into the balloons.” — Nothing pushes air in. A larger volume makes the pressure fall further, and air from outside flows in.
“The same amount of air enters, because the sheet moves the same distance.” — The sides add to the increase in volume, so the pressure falls further and more air flows in.

12. VQ12 · Practice

Options as numbered on the pupil sheet: 1 — Breathing becomes faster and deeper.; 2 — Nerve impulses go to the diaphragm and intercostal muscles.; 3 — The carbon dioxide concentration in the blood rises.; 4 — Muscles respire faster.; 5 — The brain detects the rise..

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

A full answer includes

I put the muscles respiring faster first.
I put the carbon dioxide in the blood rising second.
I put the brain detecting the rise third.
I put nerve impulses to the breathing muscles fourth.
I put faster and deeper breathing last.
I did not put the brain first.

Also accept

muscles respire; carbon dioxide rises; brain detects; impulses; faster and deeper

Watch for

“The brain detects a lack of oxygen in the muscles.” — The brain detects the rise in carbon dioxide in the blood.
“The brain acts first and then the muscles respire faster.” — The brain responds to a change in the blood, so the muscles' respiration comes first.

13. VQ13 · Practice

Model answer. Respiring muscles make more carbon dioxide, and its concentration in the blood rises. The brain detects this rise and makes breathing faster and deeper.

A full answer includes

I chose a rise in the carbon dioxide concentration in the blood.
I did not choose a fall in the oxygen concentration in the leg muscles.

Also accept

rise in carbon dioxide in the blood

Watch for

“A fall in the carbon dioxide concentration in the blood.” — Exercising muscles make more carbon dioxide, so its concentration rises, not falls.
“A fall in the oxygen concentration in the leg muscles.” — The trigger is the rise in carbon dioxide in the blood, which the brain detects.
“A rise in the temperature of the blood in the lungs.” — The brain responds to the carbon dioxide in the blood, not to the temperature in the lungs.

14. VT1 · Apply

Model answer. The statement gets the cause the wrong way round. The ribs move out because the external intercostal muscles contract and pull them up and out (while the diaphragm flattens). The volume of the thorax increases, the pressure inside falls below atmospheric pressure, and only then does air flow in.

A full answer includes

I wrote that the external intercostal muscles pull the ribs up and out first.
I wrote that the volume of the thorax increases.
I wrote that the pressure inside falls below atmospheric pressure.
I wrote that air flows in only after that.
I did not write that the lungs push the ribs out.

Also accept

order wrong; muscles move ribs first; volume up, pressure down, then air flows in

Watch for

“The pupil is right: the lungs inflate and push the ribs.” — The lungs have no muscles that can pull them open. The chest gets bigger first and air follows.

15. VT2 · Apply

Model answer. (a) A: 18 × 1.4 = 25.2 dm³ per minute. B: 28 × 0.9 = 25.2 dm³ per minute. (b) Rate = volume per minute ÷ depth = 25.2 ÷ 2.1 = 12 breaths per minute. (c) A and B move the same volume of air each minute, but in different ways: A breathes more slowly and more deeply, B faster and less deeply. Rate alone does not tell you how much air is breathed.

Checked automatically

(a) Pupil A: 25.2 dm³ per minute (within ±0.25 dm³ per minute)
(a) Pupil B: 25.2 dm³ per minute (within ±0.25 dm³ per minute)
(b) Pupil C's rate: 12 min-1 (exact value)
- Common wrong answer 53 : You multiplied by the depth. Rate = volume per minute ÷ depth.

A full answer includes

(a) I multiplied 18 by 1.4 dm³ for A.
(a) I multiplied 28 by 0.9 dm³ for B.
(a) I wrote 25.2 dm³ per minute for each.
(b) I divided 25.2 dm³ by 2.1 dm³.
(b) I wrote 12 breaths per minute.
(c) I wrote that A breathes more slowly but more deeply than B, for the same volume.
I did not write that B breathes more air because B breathes faster.

Also accept

(a) 25.2 dm³ per minute each; (b) 12 breaths per minute; (c) same volume, A slower and deeper, B faster and shallower

Watch for

“B breathes more air because B breathes faster.” — B's breaths are shallower. Multiply rate by depth: both give 25.2 dm³.

16. VT3 · Apply

Model answer. To scale a count up to a minute, multiply by the number of counting periods in a minute. A minute holds six lots of 10 s (60 ÷ 10 = 6), so the rate is 7 × 6 = 42 breaths per minute. The pupil multiplied by 10, the length of the count, instead.

Checked automatically

Correct rate: 42 min-1 (exact value)
- Common wrong answer 70 : That multiplies by the counting time. Work out how many lots of 10 s are in a minute.
- Common wrong answer 14 : That multiplies by 2, which is for a 30 s count. Use 60 ÷ 10.

A full answer includes

I identified that the pupil multiplied by 10, the length of the count.
I wrote that a minute holds six lots of 10 s (60 ÷ 10 = 6).
I multiplied 7 by 6.
I wrote 42 breaths per minute.
I did not multiply by 2, as for a 30 s count.

Also accept

multiply by 60 ÷ 10 = 6; 42 breaths per minute

Watch for

“Multiply by 2, as for a 30 s count.” — The 2 comes from 60 ÷ 30. For a 10 s count it is 60 ÷ 10.

17. VT4 · Apply

Model answer. Breathing becomes faster and deeper, even without exercise. With so much carbon dioxide in the air, less diffuses out of the blood in the alveoli, so the carbon dioxide concentration in the blood rises. The brain detects the rise and sends nerve impulses to the diaphragm and intercostal muscles, which contract more often and more strongly.

A full answer includes

(a) I predicted that breathing becomes faster and deeper.
(b) I wrote that less carbon dioxide diffuses out of the blood.
(b) I wrote that the carbon dioxide concentration in the blood rises.
(b) I wrote that the brain detects the rise.
(b) I wrote that the brain sends impulses to the diaphragm and intercostal muscles.
I did not write that nothing changes because there is still plenty of oxygen.

Also accept

faster and deeper; carbon dioxide in blood rises; brain detects; impulses to breathing muscles

Watch for

“Nothing changes, because there is still plenty of oxygen.” — The brain responds to carbon dioxide in the blood, not to oxygen, so breathing still speeds up.