Artery or vein? — answer key

1. AQ1 · Practice

Model answer. An artery has a thick wall, with a thick layer of muscle and elastic tissue, around a narrow lumen. A vein has a thin wall around a wide lumen.

A full answer includes

• An artery has a thicker wall and a narrower lumen than a vein.

Also accept

• a thicker wall and a narrower lumen

Watch for

• “An artery has a thinner wall, so blood can be pushed through it easily.” — It is the other way round: the artery on the slide had a wall about as thick as its lumen was wide; the vein's wall was thin.

2. AQ2 · Practice

Options as numbered on the pupil sheet: 1 — vein; 2 — capillary; 3 — artery.

Model answer. P has a thick wall of muscle and elastic tissue around a narrow lumen: an artery. Q has a thin wall around a wide lumen: a vein. R has a wall one cell thick and a lumen about one red blood cell wide: a capillary (drawn far larger than its real size).

A full answer includes

• P: artery.
• Q: vein.
• R: capillary.

Also accept

• P artery; Q vein; R capillary

Watch for

• “R is an artery, because it is round and small.” — Size and shape do not decide it. R's wall is one flattened cell thick, with no muscle: it is a capillary, drawn much larger than it really is.

3. AQ3 · Practice

Model answer. Valves are found in veins. They prevent the backflow of blood, so blood in a vein can only flow towards the heart: blood that starts to flow back pushes the valve's flaps together, and the valve closes.

A full answer includes

• (1) veins; (2) backflow (blood flowing backwards); (3) towards.

Also accept

• (1) veins; (2) backflow; (3) towards

Watch for

• “Valves are found in arteries, to stop blood flowing back into the heart.” — Along their length, arteries have no valves. The valves along a vessel's length are in veins.

4. AQ4 · Practice

Model answer. Blood in an artery has just been pumped out of the heart at high pressure. The thick wall of muscle and elastic tissue withstands this pressure without bursting.

A full answer includes

• The blood in an artery is at high pressure, and a thick wall can withstand it.

Also accept

• to withstand the high pressure of the blood

Watch for

• “The thick wall stops oxygen leaking out of the blood.” — Exchange happens in capillaries, not arteries. The thick wall is about pressure.

5. AQ5 · Practice

Model answer. Between beats the elastic tissue recoils, pushing on the blood. This evens out the surges of pressure, so the blood flows more smoothly further from the heart.

A full answer includes

• It recoils (springs back) between beats.
• This evens out the surges, so the flow becomes smoother.

Also accept

• recoils; evens out the surges, smoother flow

Watch for

• “The elastic tissue pumps the blood like a second heart.” — It only stretches and springs back; it smooths the flow the heart makes. The muscle in the wall can narrow or widen the artery.

6. AQ6 · Practice

Model answer. (a) Wall = (280 µm + 320 µm) ÷ 2 = 300 µm. (b) Wall ÷ lumen = 300 µm ÷ 300 µm = 1.0; the units cancel. (c) A value near 1 means a thick wall for the size of the lumen: an artery.

Checked automatically

• (a) Wall thickness: 300 µm (within ±5 µm)
  • Common wrong answer 600 µm: You added both sides. Take their mean: (280 + 320) ÷ 2.
• (b) Wall ÷ lumen: 1.0 (within ±0.05)
  • Common wrong answer 2.0 : You used both sides added together. Use the mean wall, 300 µm.

A full answer includes

• (a) (280 + 320) ÷ 2 = 300 µm.
• (b) 300 ÷ 300 = 1.0 (no unit).
• (c) An artery: its wall is as thick as its lumen is wide.

Also accept

• (a) 300 µm; (b) 1.0; (c) artery

Watch for

• “Wall ÷ lumen is 1.0 µm.” — Both lengths are in µm, so the units cancel: the answer is a number with no unit.
• “The wall thickness is 600 µm.” — That adds both sides. The wall on each side is one wall: take the mean of the two.

7. AQ7 · Practice

Model answer. An artery has a thicker wall than a vein, with more muscle and elastic tissue, and a narrower lumen. Veins have valves along their length; arteries do not.

A full answer includes

• Any two: an artery has a thicker wall (more muscle and elastic tissue); an artery has a narrower lumen; veins have valves, arteries do not (along their length).

Also accept

• two of: thicker wall; narrower lumen; valves only in veins

Watch for

• “Arteries carry oxygenated blood and veins deoxygenated blood.” — That is not a difference in structure, and it is not always true: the pulmonary artery carries deoxygenated blood.

8. AQ8 · Practice

Model answer. In the arteries the pressure is high, about 10.5 to 16 kPa, so it keeps pushing the blood away from the heart and no valves are needed. By the veins it has fallen below about 2 kPa. At such a low pressure the blood could flow backwards, so veins have valves to prevent backflow.

A full answer includes

• The pressure in the arteries is high (about 10 to 16 kPa) and keeps the blood moving away from the heart.
• In the veins the pressure is very low (under about 2 kPa), so blood could flow backwards; valves prevent this backflow.

Also accept

• high pressure in arteries keeps blood moving; very low pressure in veins, valves prevent backflow

Watch for

• “The pressure in the veins is so high that valves are needed to slow the blood down.” — Read the graph: the veins have the lowest pressure of all. Valves stop low-pressure blood flowing back.

9. AQ9 · Practice

Model answer. The blood in a vein is at low pressure, so a thick wall is not needed: the wall is thin. The wide lumen lets the blood flow back to the heart easily, with little to slow it down. Valves prevent the low-pressure blood from flowing backwards; the squeezing of nearby muscles pushes it towards the heart.

A full answer includes

• Any two: a thin wall, because the low pressure does not need a thick wall to withstand it; a wide lumen, so the blood flows back easily, with little to slow it down; valves, which prevent backflow of the low-pressure blood.

Also accept

• two of: thin wall (no high pressure to withstand); wide lumen (blood flows easily); valves (prevent backflow)

Watch for

• “A vein has a thin wall so that substances can diffuse through it.” — A vein's wall is still far too thick for exchange; exchange happens in capillaries. The thin wall fits the low pressure.

10. AT1 · Apply

Model answer. Disagree. The shape of a vessel on a slide can change when the section is cut and mounted, so it is not good evidence. Decide from the wall: the artery's wall is thick compared with its lumen, with a thick layer of muscle and wavy elastic fibres; the vein's wall is thin around a wide lumen (and a flattened one is more likely to be the vein).

A full answer includes

• Disagree: shape is not good evidence, because a vessel can be squashed when the slide is made.
• Look at the wall: the artery has the thicker wall for the size of its lumen, with a thick layer of muscle and elastic tissue.

Also accept

• disagree; shape is not reliable; artery has the thicker wall for its lumen, muscle and elastic tissue

Watch for

• “The round one must be the vein, because veins are wider.” — A vein has a wider lumen, but roundness is not the test: use the thickness of the wall compared with the lumen.

11. AT2 · Apply

Model answer. The aorta takes blood straight from the left ventricle, so it has the largest surges of pressure, 16 kPa at each beat. More elastic tissue lets it stretch with each surge and recoil between beats, so the pressure is evened out and the flow becomes smoother further on.

A full answer includes

• The aorta receives blood straight from the left ventricle, where the pressure surges are largest.
• Its elastic tissue stretches with each surge and recoils between beats, evening out the pressure and smoothing the flow.

Also accept

• largest pressure surges straight from the left ventricle; stretches and recoils, smooths the flow

Watch for

• “The aorta needs elastic tissue to stretch so it can hold all the blood in the body.” — The point is the surges: the elastic wall stretches with each beat and recoils between beats, smoothing the flow.

12. AT3 · Apply

Model answer. The blood in the leg veins is at low pressure and must travel upwards. Each time the leg muscles tighten, they squeeze the veins between them; the valves let the squeezed blood go only one way, towards the heart, and close to prevent it flowing back down. Standing completely still, there is no squeezing, so blood collects in the legs.

A full answer includes

• When the leg muscles contract they squeeze the veins between them.
• The valves let the blood go only towards the heart, so each squeeze pushes blood up towards the heart.

Also accept

• muscles squeeze the veins; valves make the blood go only towards the heart

Watch for

• “Tightening the leg muscles makes the heart pump harder, which pushes the blood up the leg veins.” — By the time blood reaches the veins, its pressure from the heart is very low. The squeezing of the leg muscles, with the valves, moves it.