Answers · The heart and double circulation

One beat, step by step — answer key

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

1. BQ1 · Practice

Model answer. G is a semilunar valve: its pockets lie at the foot of the aorta, where it leaves the left ventricle. F is an atrioventricular valve, between the left atrium and the left ventricle.

A full answer includes

I chose G.
I did not choose F: it lies between an atrium and a ventricle, so it is an atrioventricular valve.

Also accept

Watch for

“F, the flaps at the top of the ventricle.” — F lies between an atrium and a ventricle: it is an atrioventricular valve.
“E, the wall down the middle.” — E is the septum, a wall of muscle that never opens. Valves are flaps or pockets that open and shut.
“H, the vessel on the outside of the heart.” — H runs over the outside of the heart muscle: it is a coronary artery, not a valve.

2. BQ2 · Practice

Options as numbered on the pupil sheet: 1 — the atria contract and squeeze their remaining blood into the ventricles; 2 — the semilunar valves open and blood leaves the ventricles; 3 — the ventricles relax and the semilunar valves shut; 4 — the atrioventricular valves open and blood flows into the ventricles.

Model answer. As the ventricles contract, their pressure rises above the pressure in the arteries, so the semilunar valves open and blood leaves the ventricles. Then the ventricles relax. Their pressure falls below the arteries', so the semilunar valves shut; it goes on falling until it is below the atria's, so the atrioventricular valves open and blood flows into the ventricles. Last, the atria contract and push the last of the blood into the ventricles, which are then ready to contract again.

A full answer includes

I put the semilunar valves opening first.
I put the semilunar valves shutting before the atrioventricular valves open.
I put the atria contracting last.
I did not have the atrioventricular valves open while the semilunar valves were still open.

Also accept

semilunar valves open, blood leaves; ventricles relax, semilunar valves shut; atrioventricular valves open, ventricles fill; atria contract

Watch for

“The atrioventricular valves open before the semilunar valves shut.” — If both kinds were open together, blood could flow back from the arteries through the ventricles into the atria. The ventricles' pressure falls below the arteries' first, so the semilunar valves shut first.
“The atria contract while blood is leaving the ventricles.” — The atria contract only after the ventricles have relaxed and filled; they push the last blood in just before the ventricles contract again.

3. BQ3 · Practice

Model answer. The left ventricle has the thickest wall: it pumps blood all round the body at a high pressure.

A full answer includes

I chose the left ventricle.
I did not choose the right ventricle: it pumps blood only to the lungs.

Also accept

left ventricle

Watch for

“The right ventricle.” — The right ventricle pumps blood only to the lungs, close by, at a lower pressure; its wall is thinner.
“The left atrium.” — The atria have the thinnest walls: they push blood only into the ventricles just below them.
“The right atrium.” — The right atrium has one of the thinnest walls; it only fills the right ventricle below it.

4. BQ4 · Practice

Model answer. The septum separates the oxygenated blood in the left side of the heart from the deoxygenated blood in the right side.

A full answer includes

I chose: oxygenated blood on the left and deoxygenated blood on the right.
I did not choose the blood in the atria and the ventricles: valves lie between them.

Also accept

oxygenated blood on the left and deoxygenated blood on the right

Watch for

“The blood in the atria and the blood in the ventricles.” — Valves lie between atria and ventricles. The septum runs down the middle, between right and left.
“The blood inside the heart and the muscle of the heart's own wall.” — The wall of each chamber holds its blood in. The septum is the wall between the right and left sides.
“The arteries that leave the heart and the veins that come back to it.” — Arteries and veins join the heart at different chambers; no wall is needed between them. The septum separates the two sides.

5. BQ5 · Practice

Model answer. A. The pressure in the contracting ventricles is higher than in the atria, so the atrioventricular valves are shut, and higher than in the arteries, so the semilunar valves are open and blood flows out into the pulmonary artery and the aorta.

A full answer includes

I chose row A.
I did not choose row D: open atrioventricular valves would let blood back into the atria.

Also accept

Watch for

“Row B: atrioventricular valves open, semilunar valves shut.” — That is how the valves are while the heart relaxes and fills. While the ventricles push blood out, the semilunar valves are open and the atrioventricular valves are shut.
“Row C: both kinds shut.” — Both kinds are shut only for a moment, as the ventricles start to contract and before their pressure passes the pressure in the arteries. While blood is leaving, the semilunar valves are open.
“Row D: both kinds open.” — If the atrioventricular valves were open, the contracting ventricles would push blood back up into the atria.

6. BQ6 · Practice

Model answer. (a) Point 3. There the right ventricle's line falls below the pulmonary artery's (2.0 kPa at 0.70 s). Blood in the artery starts to flow back towards the relaxing ventricle and fills the valve's pockets, so the semilunar valve shuts. (b) Between points 1 and 2 (0.40 to 0.45 s) the right ventricle is contracting. Its pressure is above the right atrium's, so the atrioventricular valve is shut, but still below the pulmonary artery's, so the semilunar valve is shut too. With both valves shut, the contracting muscle squeezes blood that cannot go anywhere: the pressure rises quickly, but no blood moves until point 2, when the ventricle's pressure passes the artery's (1.5 kPa) and the semilunar valve opens.

A full answer includes

(a) I chose point 3.
(a) I wrote that at point 3 the right ventricle's pressure falls below the pulmonary artery's.
(a) I wrote that blood starting to flow back from the artery fills the valve's pockets, so the valve shuts.
(b) I wrote that the right ventricle's pressure is above the right atrium's, so the valve between them is shut.
(b) I wrote that the right ventricle's pressure is still below the pulmonary artery's, so the semilunar valve is shut.
(b) I wrote that with both valves shut, the contracting muscle raises the pressure, but no blood can move.
I did not write that a valve contracts or relaxes.

Also accept

(a) point 3: the right ventricle's pressure falls below the pulmonary artery's; (b) both valves shut: the ventricle's pressure is above the atrium's but below the pulmonary artery's

Watch for

“The valve contracts to shut.” — Valves have no muscle. The pressure difference across a valve opens and shuts it.
“(a) Point 4, where the ventricle's pressure is lowest.” — At point 4 the ventricle's line falls below the atrium's, so that is where the atrioventricular valve opens. The semilunar valve shut earlier, where the ventricle's line fell below the artery's.
“(b) Blood is already leaving the ventricle, because its pressure is rising.” — Blood can leave only through the semilunar valve, and that opens only when the ventricle's pressure passes the artery's. Until then it stays shut.

7. BQ7 · Practice

Model answer. Blood from the veins flows through the atria into the relaxed ventricles by itself; the atria's contraction only adds the last part, with a short push into the ventricles just below. So when the atria only quiver, the ventricles still fill well enough. A short push at a low pressure needs little muscle, so the walls of the atria are thin. The ventricles must pump all the blood out of the heart, to the lungs and all round the body, which needs a higher pressure, so they have much more muscle. When they only quiver, no blood leaves the heart.

A full answer includes

I wrote that most of the blood flows into the ventricles without the atria contracting.
I wrote that the atria give only a short push, into the ventricles just below them.
I wrote that this needs only a low pressure, so the atria need little muscle.
I wrote that the ventricles pump all the blood out of the heart, to the lungs and round the body.
I wrote that this needs a higher pressure, so the ventricles need more muscle.
I did not write that the atria hold less blood.

Also accept

most filling happens without the atria; the atria give a short, low-pressure push into the ventricles (little muscle); the ventricles pump all the blood out to the lungs and body at a higher pressure (more muscle)

Watch for

“The atria hold less blood, so they need thinner walls.” — The thickness matches the pressure each chamber makes, not how much it holds.
“The atria are not muscle, so they cannot contract.” — The atria are muscle and do contract; in atrial fibrillation they only quiver. Their push is small because the blood has only a short way to go.

8. BQ8 · Practice

Model answer. U. An atrioventricular valve lies between an atrium and the ventricle below it, and T and U are the two that do. The heart is seen from the front, so its left side is on the right of the page: U is the atrioventricular valve on the left side. V and W sit at the foot of the two arteries, where they leave the ventricles, so they are semilunar valves.

A full answer includes

I chose U.
I did not choose T: it lies between an atrium and a ventricle, but on the heart's right side.

Also accept

Watch for

“T, the valve between the two spaces on the left of the page.” — T is an atrioventricular valve, but the heart is seen from the front, so the left of the page is the heart's right side.
“V, the pockets at the foot of the arching vessel.” — V sits where an artery leaves a ventricle, so it is a semilunar valve, not an atrioventricular one.
“W, the pockets at the foot of the front vessel.” — W is at the foot of the artery that leaves the lower space on the left of the page: it is a semilunar valve.

9. BQ9 · Practice

Model answer. Oxygenated blood from the lungs enters the left side of the heart in the pulmonary vein, and the left ventricle pumps it into the aorta. Deoxygenated blood from the body enters the right side in the vena cava. The septum is a solid wall between the two sides, so the two kinds of blood do not mix, and the aorta's blood carries as much oxygen as the blood that left the lungs. The body's cells get as much oxygen as possible for respiration.

A full answer includes

I wrote that blood from the pulmonary vein enters the left side of the heart.
I wrote that the left ventricle pumps this blood into the aorta.
I wrote that blood from the vena cava enters the right side of the heart.
I wrote that the septum keeps the two sides apart, so the two kinds of blood do not mix.
I wrote that the body's cells then get as much oxygen as possible for respiration.
I did not write that the septum adds oxygen to the blood.

Also accept

pulmonary vein → left side → aorta; vena cava → right side; the septum keeps the two apart, so the blood sent to the body is fully oxygenated and the cells get the most oxygen

Watch for

“The septum adds oxygen to the blood in the left side.” — Blood takes in oxygen only in the lungs. The septum only keeps oxygenated blood apart from deoxygenated blood.
“The septum stops blood flowing back into the atria.” — That is what the atrioventricular valves do. The septum separates the right and left sides.

10. BQ10 · Practice

Model answer. B. A valve at the foot of an artery is a semilunar valve. When the right ventricle relaxes, its pressure falls below the pressure in the pulmonary artery; blood starts to flow back and fills the valve's pockets, so the valve shuts and stops blood flowing back into the right ventricle. The valve between the right atrium and the right ventricle, which shuts when the ventricle contracts, is the right atrioventricular valve (row D).

A full answer includes

I chose row B.
I did not choose row D: that is the valve between the right atrium and the right ventricle.

Also accept

Watch for

“Row A: it is an atrioventricular valve.” — An atrioventricular valve lies between an atrium and a ventricle. This valve is at the foot of an artery.
“Row C: it shuts when the right ventricle contracts.” — If it were shut while the ventricle contracts, no blood could leave for the lungs. It shuts when the pressure in the artery is higher.
“Row D: it stops blood flowing back into the right atrium.” — That describes the valve between the right atrium and the right ventricle, not the one at the pulmonary artery.

11. BT1 · Apply

Model answer. Chamber 3 is the left ventricle: its wall, 11 mm, is the thickest. Chambers 2 and 4 (1.6 mm and 1.9 mm) are the atria, and chamber 1 (4.5 mm) is the right ventricle. The left ventricle pumps blood all round the body at the highest pressure, so it needs the most muscle, which contracts with the most force.

A full answer includes

(a) I chose chamber 3.
(b) I wrote that chamber 3 has the thickest wall, 11 mm.
(b) I wrote that the left ventricle pumps blood all round the body, at the highest pressure.
(b) I wrote that a thicker wall has more muscle, which contracts with more force.
I did not choose chamber 1: it is a ventricle, but the right one.

Also accept

(a) chamber 3; (b) thickest wall: pumps blood round the whole body at the highest pressure

Watch for

“Chamber 1, because a ventricle's wall is a few millimetres thick.” — Chamber 1 is a ventricle, but only the right one: the left ventricle's wall is by far the thickest.

12. BT2 · Apply

Model answer. When the ventricle relaxes, its pressure falls below the aorta's. A healthy semilunar valve shuts, but a leaky one lets some blood flow back into the left ventricle. That blood must be pumped out again, so the heart works harder to send the same volume to the body.

A full answer includes

I wrote that the pressure in the aorta is higher than in the relaxing ventricle.
I wrote that some blood flows back from the aorta into the left ventricle.
I wrote that the heart must pump this blood again, so it works harder.
I did not write that the blood flows back into the left atrium.

Also accept

blood flows back from the aorta into the left ventricle; the heart must pump it again (works harder)

Watch for

“The blood flows back into the left atrium.” — The leaky valve is between the aorta and the ventricle, so the blood flows back into the ventricle; the atrioventricular valve is still working.

13. BT3 · Apply

Model answer. With only one ventricle, there is no septum to keep the two kinds of blood apart, so some oxygenated and deoxygenated blood mix. The blood sent to the body is not fully oxygenated, so the cells get less oxygen.

A full answer includes

I wrote that there is no septum between the ventricles.
I wrote that some oxygenated and deoxygenated blood mix in the one ventricle.
I wrote that the blood pumped to the body carries less oxygen.
I wrote that the cells get less oxygen for respiration.
I did not write that the frog's blood gets no oxygen.

Also accept

blood mixes (no septum between ventricles); less oxygen reaches the body's cells

Watch for

“The frog's blood cannot get any oxygen.” — The frog's blood does get oxygen in the lungs; the problem is that some of it mixes with deoxygenated blood in the one ventricle.