Answers · The heart and double circulation

Twice through the heart — answer key

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

1. DQ1 · Practice

Options as numbered on the pupil sheet: 1 — the vena cava; 2 — the aorta; 3 — the left atrium; 4 — the left ventricle; 5 — the right atrium; 6 — the right ventricle; 7 — the capillaries of an organ.

Model answer. From the pulmonary vein the cell enters the left atrium and passes into the left ventricle, which pumps it into the aorta. Arteries carry it to the capillaries of an organ, where it gives up oxygen. Veins bring it back to the vena cava, which empties into the right atrium; it passes into the right ventricle, which pumps it to the lungs again.

A full answer includes

I put the left atrium first.
I put the left ventricle second.
I put the aorta before the capillaries of an organ.
I put the vena cava after the capillaries.
I put the right atrium after the vena cava.
I put the right ventricle last.
I did not send the cell from the left side of the heart straight to the right side.

Also accept

left atrium, left ventricle, aorta, capillaries of an organ, vena cava, right atrium, right ventricle

Watch for

“The cell goes from the left ventricle to the right ventricle through the septum.” — The septum has no opening. The cell must go round the body first, and come back in the vena cava.
“The vena cava empties into the left atrium.” — The vena cava brings blood from the body to the right atrium. The left atrium receives blood from the lungs.

2. DQ2 · Practice

Model answer. The heart is the only pump, so the blood is at its highest pressure just after it leaves the heart, in the vessel to the gills. It loses pressure in the narrow capillaries of the gills, and more in the capillaries of the body's organs, and it comes back to the heart at a low pressure.

A full answer includes

I chose the vessel carrying blood from the heart to the gills.
I did not choose the vessel from the gills to the body: the blood there has lost pressure in the gill capillaries.

Also accept

the vessel carrying blood from the heart to the gills

Watch for

“The vessel carrying blood from the gills to the body.” — The blood there has taken in oxygen, but oxygen does not raise the pressure. It has already lost pressure in the narrow gill capillaries.
“The capillaries inside the organs of the body.” — Capillaries are narrow, but the blood loses pressure as it is pushed through them. Blood reaching the body's capillaries has already passed through the gills.
“The vein carrying blood from the body to the heart.” — This blood is about to be pumped, but it has not been pumped yet: it has lost most of its pressure on the way round.

3. DQ3 · Practice

Model answer. In a double circulation, blood that has lost pressure in the lungs comes back to the heart and is pumped again by the left ventricle, so it reaches the body's organs quickly at a high pressure.

A full answer includes

I chose: blood is pumped a second time after the lungs, so it reaches the body at a high pressure.
I did not choose that the blood passes through the lungs twice.

Also accept

blood from the lungs is pumped again: reaches the body quickly at high pressure

Watch for

“The blood passes through the lungs twice, so it picks up twice as much oxygen.” — The blood passes through the heart twice, not the lungs. The gain is that it is pumped again after the lungs.
“The heart only has to beat half as often, because each beat sends blood twice as far.” — The number of beats does not halve. Both sides of the heart contract in every beat; the gain is the second pumping after the lungs.
“Each side of the heart pumps half of the blood, so each side does less work.” — Each side pumps all of the blood: on every trip, all of it passes through the right side and then the left side. The gain is that the blood is pumped again after the lungs.

4. DQ4 · Practice

Model answer. D. A fish's heart has one atrium and one ventricle. It pumps blood to the gills, and from there the blood flows straight on to the body and back to the heart, so it passes through the heart once on each trip: a single circulation.

A full answer includes

I chose row D.
I did not choose row C: that is a mammal's heart and circulation.

Also accept

Watch for

“Row A: two atria and two ventricles, once through the heart.” — A fish's heart has only one atrium and one ventricle. Two of each make a heart with a right and a left side, like a mammal's.
“Row B: one atrium and one ventricle, twice through the heart.” — The chambers are right, but the fish's blood does not come back to the heart after the gills. It flows straight on to the body, so it passes through the heart once.
“Row C: two atria and two ventricles, twice through the heart.” — That is a mammal's heart and double circulation, not a fish's.

5. DQ5 · Practice

Model answer. Blood takes in oxygen in the lungs. Vessel 1 (the pulmonary vein) brings it from the lungs to the left side of the heart, and vessel 4 (the aorta) carries it from the left side of the heart to the body. Vessel 2 (the pulmonary artery) carries deoxygenated blood to the lungs, and vessel 3 (the vena cava) brings deoxygenated blood back from the body.

A full answer includes

I chose 1.
I chose 4.
I did not choose 2: it carries blood from the heart to the lungs, before the blood takes in oxygen.

Also accept

1 and 4

Watch for

“2 and 4, because both carry blood away from the heart.” — Blood leaving the heart is not always oxygenated. Vessel 2 carries blood from the heart to the lungs, where it has still to take in oxygen.
“3, because it carries blood back to the heart to be pumped.” — Vessel 3 brings blood back from the body, where the cells have taken much of its oxygen: it is deoxygenated.
“Only 1, the vessel from the lungs.” — Vessel 1 carries oxygenated blood, but so does vessel 4: the left side of the heart pumps that same blood on to the body.

6. DQ6 · Practice

Model answer. The blood that returns to a fish's heart comes from the body, where the cells have taken much of its oxygen, so it is deoxygenated. If the heart pumped it straight to the body, the cells would get little oxygen. Pumped to the gills first, it takes in oxygen from the water, and then flows on to the body's organs, where the cells use the oxygen for respiration.

A full answer includes

I wrote that the blood entering the fish's heart has come from the body, so it is deoxygenated.
I wrote that the blood takes in oxygen in the gills.
I wrote that the blood then reaches the body's organs carrying oxygen for the cells' respiration.
I did not write that the gills need the blood's oxygen.

Also accept

the blood entering the heart is deoxygenated (from the body); it takes in oxygen in the gills; then the body gets oxygenated blood for respiration

Watch for

“The gills need the oxygen in the blood first.” — The gills are where the blood takes in oxygen from the water. The blood arrives there deoxygenated.
“The blood must be cooled in the gills before it goes to the body.” — The gills are where gases are exchanged with the water: the blood takes in oxygen and gives out carbon dioxide.

7. DQ7 · Practice

Model answer. In both animals the blood loses pressure in the narrow capillaries of the gills or the lungs: the carp's falls from 4.0 to 2.6 kPa, the rabbit's from 3.0 to 0.9 kPa. The carp's blood then flows straight on to the body, still at 2.6 kPa. The rabbit's blood returns to the heart, and its left ventricle pumps it again, so it reaches the body at 13 kPa.

A full answer includes

I wrote that in both animals the blood loses pressure in the capillaries of the gills or the lungs.
I used the carp's data: its blood falls from 4.0 to 2.6 kPa in the gills.
I used the rabbit's data: its blood falls from 3.0 to 0.9 kPa in the lungs.
I wrote that the carp's blood goes straight on from the gills to the body, at 2.6 kPa.
I wrote that the rabbit's blood goes back to the heart.
I wrote that the rabbit's left ventricle pumps it again, to 13 kPa.
I did not write that the rabbit's blood loses less pressure in its lungs.

Also accept

pressure falls in the gill or lung capillaries in both; the carp's blood goes straight to the body (2.6 kPa), the rabbit's is pumped again by the left ventricle (13 kPa)

Watch for

“The rabbit's blood loses less pressure in its lungs.” — The data show the opposite: the rabbit's blood falls by 2.1 kPa in the lungs, the carp's by 1.4 kPa in the gills. The rabbit's blood is at a high pressure again because it is pumped again.
“The rabbit's heart is bigger, so it pumps harder.” — The rabbit's right ventricle makes a lower pressure (3.0 kPa) than the carp's heart (4.0 kPa). The difference is the second pumping after the lungs.

8. DQ8 · Practice

Model answer. (a) Loss = 4.8 kPa − 3.2 kPa = 1.6 kPa. (b) 1.6 kPa ÷ 4.8 kPa × 100 = 33.3… % = 33 % to 2 significant figures. A third of the pressure is lost in the gills, and the trout's blood is not pumped again before it goes to the body.

Checked automatically

(a) Pressure lost in the gills: 1.6 kPa (2 significant figures)
(b) Loss as a percentage of the pressure leaving the heart: 33 % (2 significant figures)
- Common wrong answer 67 %: That is the share of the pressure left after the gills. The question asks for the share lost.
- Common wrong answer 50 %: You divided by the pressure after the gills. Divide by the pressure leaving the heart.

A full answer includes

(a) I subtracted: 4.8 kPa − 3.2 kPa.
(a) I gave 1.6 kPa.
(b) I divided the loss by the pressure leaving the heart: 1.6 ÷ 4.8 × 100.
(b) I gave 33 %, to 2 significant figures.
I did not divide by 3.2 kPa.

Also accept

(a) 1.6 kPa; (b) 33 %

Watch for

“The percentage is 67 %.” — 67 % is the share of the pressure that is left. The question asks for the share that is lost.

9. DQ9 · Practice

Model answer. The capillaries of the lungs have very thin walls, right next to the air spaces, so that gases diffuse quickly. A pressure as high as the left ventricle's would damage these thin walls and push fluid out of the capillaries into the air spaces, which would make gas exchange harder. The right ventricle's lower pressure protects them; the left ventricle then pumps the blood again, at a high pressure, to the body.

A full answer includes

I wrote that the walls of the lung capillaries are very thin.
I wrote that a high pressure would damage the lung capillaries.
I wrote that fluid would be pushed out of the capillaries into the air spaces.
I did not write that the lungs need less blood than the body.

Also accept

high pressure would damage the thin lung capillaries and push fluid into the air spaces

Watch for

“The lungs need less blood than the body, so a weaker pump is enough.” — All the blood that goes round the body also passes through the lungs on every trip: both ventricles pump the same volume, about 70 cm³ a beat at rest. The lower pressure protects the lung capillaries; it is not because they need less blood.
“A higher pressure would let the blood take up more oxygen.” — Oxygen enters the blood by diffusion across thin walls. A higher pressure would not add oxygen, but it would damage those walls.

10. DT1 · Apply

Model answer. Blood from the arm returns in veins to the vena cava and the right atrium. The right ventricle pumps it to the lungs (the pulmonary circulation). It returns to the left atrium, and the left ventricle pumps it into the aorta and on to the brain (the systemic circulation). In a double circulation, blood from the body always goes through the lungs before it reaches the body again.

A full answer includes

I wrote that the medicine goes from the arm vein to the vena cava and the right atrium.
I wrote that the right ventricle pumps it to the lungs.
I wrote that it returns from the lungs to the left side of the heart.
I wrote that the left ventricle pumps it into the aorta, which carries it to the brain.
I did not write that the lungs are nearer to the arm.

Also accept

vein → right side of heart → lungs → left side → aorta → brain

Watch for

“The lungs are closer to the arm than the brain.” — Distance does not decide. Blood from any vein goes to the right side of the heart, then to the lungs, before the left side sends it to the body.

11. DT2 · Apply

Model answer. In a double circulation, oxygenated blood from the lungs is pumped again at a high pressure, so it reaches the body's cells quickly. The cells receive oxygen and glucose fast and can respire quickly; respiration releases energy, some of it as heat, which keeps the bird warm.

A full answer includes

I wrote that blood is pumped again after the lungs, so it reaches the body at a high pressure.
I wrote that oxygen and glucose reach the cells quickly.
I wrote that the cells can respire fast.
I wrote that respiration releases heat, which keeps the bird warm.
I did not write that the heart warms the blood.

Also accept

high pressure, fast supply of oxygen and glucose → fast respiration → heat

Watch for

“The double circulation makes the blood warmer because it passes through the heart twice.” — The heart does not heat the blood by passing it through. The heat comes from fast respiration in the cells.

12. DT3 · Apply

Model answer. In a single circulation the heart receives blood only from the body, where the cells have taken much of its oxygen, and pumps it to the gills. The oxygenated blood from the gills flows straight on to the body without returning to the heart, so the heart only ever holds deoxygenated blood.

A full answer includes

I wrote that blood enters the fish's heart from the body, after the body's cells have taken its oxygen.
I wrote that the heart pumps this blood to the gills.
I wrote that oxygenated blood from the gills goes straight to the body, not back to the heart.
I did not write that the heart takes the oxygen out of the blood.

Also accept

heart receives blood from the body only; oxygenated blood from the gills goes straight to the body

Watch for

“The fish's heart takes the oxygen out of the blood.” — The body's cells use the oxygen. The heart holds deoxygenated blood because of the route, not because it removes oxygen.