Capillaries — answer key

1. CQ1 · Practice

Model answer. A capillary's wall is a single layer of thin, flattened cells, one cell thick, and its lumen is about the width of one red blood cell, about 8 µm.

A full answer includes

• A wall one cell thick and a lumen about the width of one red blood cell.

Also accept

• a wall one cell thick and a lumen about one red blood cell wide

Watch for

• “A capillary has a thin layer of muscle in its wall.” — A capillary's wall has no muscle: it is a single layer of flattened cells.

2. CQ2 · Practice

Options as numbered on the pupil sheet: 1 — from the cells to the blood; 2 — from the blood to the cells.

Model answer. Oxygen and glucose pass out of the blood, through the capillary walls, to the cells, which use them in respiration. Carbon dioxide, made by respiration, passes from the cells into the blood.

A full answer includes

• Oxygen and glucose pass from the blood to the cells.
• Carbon dioxide passes from the cells to the blood.

Also accept

• oxygen and glucose out to the cells; carbon dioxide into the blood

Watch for

• “Red blood cells pass out of the capillaries to the cells.” — The red blood cells stay inside the capillaries. Oxygen leaves them and diffuses through the wall.

3. CQ3 · Practice

Model answer. A wall one cell thick means substances have only a very short distance to diffuse between the blood and the cells, so exchange is fast.

A full answer includes

• It gives a short diffusion distance, so substances are exchanged quickly.

Also accept

• short diffusion distance, so exchange is fast

Watch for

• “So that red blood cells can squeeze out between the cells of the wall.” — Red blood cells stay inside the capillary. The thin wall is for fast diffusion.

4. CQ4 · Practice

Model answer. A capillary's wall is one cell thick, with no muscle or elastic tissue, while an artery's wall is thick, with muscle and elastic tissue. A capillary's lumen is much narrower, about the width of one red blood cell.

A full answer includes

• Any two: the capillary's wall is one cell thick, the artery's is thick; the capillary has no muscle or elastic tissue; the capillary's lumen is much narrower, about one red blood cell wide.

Also accept

• two of: wall one cell thick vs thick; no muscle or elastic tissue; much narrower lumen

Watch for

• “A capillary is a small artery with a thinner wall.” — A capillary is a different kind of vessel: its wall is a single layer of cells, with no muscle at all.

5. CQ5 · Practice

Model answer. Capillaries carry blood from the arteries to the veins, close to every cell of a tissue, and they are where substances are exchanged between the blood and the cells: oxygen and glucose pass out, carbon dioxide passes in.

A full answer includes

• They carry blood close to every cell, between the arteries and the veins.
• They allow substances (such as oxygen, glucose and carbon dioxide) to be exchanged between the blood and the cells.

Also accept

• carry blood close to every cell (artery to vein); exchange of substances with the cells

Watch for

• “Capillaries pump blood through the tissues.” — Capillaries have no muscle and cannot pump. The heart moves the blood; capillaries carry it close to the cells and let substances be exchanged.

6. CQ6 · Practice

Model answer. 65 µm ÷ 0.5 µm = 130. Even the thin wall of a vein is 130 times thicker than a capillary's wall, far too thick for oxygen to diffuse through quickly enough to supply cells. Exchange happens through the walls of capillaries.

Checked automatically

• Times further: 130 (within ±2)
  • Common wrong answer 32.5 : Check the division: 65 ÷ 0.5 is the same as 65 × 2.

A full answer includes

• 65 ÷ 0.5 = 130 times.

Also accept

• 130 times

Watch for

• “0.0077 times.” — You divided the capillary wall by the vein wall. Divide the thicker wall by the thinner one.

7. CQ7 · Practice

Model answer. The lumen is about the width of one red blood cell, so the red blood cells pass through in single file, each pressed against the wall. The air sac is just the other side of two thin walls, each one cell thick, so oxygen diffuses only a very short distance from the air into each red blood cell.

A full answer includes

• The red blood cells pass in single file, each pressed close to the capillary wall, next to the air sac.
• So oxygen has only a short distance to diffuse from the air into each red blood cell.

Also accept

• single file, close to the wall next to the air sac; short diffusion distance into the red blood cells

Watch for

• “The narrow lumen squeezes oxygen into the red blood cells.” — Oxygen moves by diffusion. The narrow lumen keeps each red blood cell close to the wall, so the distance is short.

8. CT1 · Apply

Model answer. 8 ÷ 1800 is less than 0.5 % of the field, so at ×100 the whole vessel is a tiny dot and its wall cannot be made out. A vessel just wide enough for one red blood cell is probably a capillary. At ×400 the field is 450 µm across, so the vessel appears four times wider: look for a very thin wall with no muscle. An artery, however small, has muscle in its wall.

A full answer includes

• 8 µm is less than half a percent of an 1800 µm field, so at ×100 the vessel is a tiny dot and its wall cannot be seen.
• A vessel just wide enough for one red blood cell is probably a capillary: use ×400 and look for a very thin wall with no muscle.

Also accept

• wall too small to see at ×100 (under 0.5 % of the field); probably a capillary; use ×400, look for a very thin wall, no muscle

Watch for

• “The vessel must be an artery, because it has blood in it.” — Every kind of vessel carries blood. A vessel only one red blood cell wide is probably a capillary: check its wall at ×400.

9. CT2 · Apply

Model answer. The outermost layer of the skin has no capillaries, so a shallow cut does not bleed. Its living cells near the bottom of the layer get oxygen and glucose by diffusion from the capillaries in the deeper layer, which is why they are only a few cells away from it.

A full answer includes

• The outer layer (the dead or outermost cells) has no capillaries.
• Substances diffuse to its cells from capillaries in the layer below.

Also accept

• outer layer has no capillaries; diffusion from capillaries just below

Watch for

• “The outer cells get oxygen straight from the red blood cells, which leave the capillaries.” — Red blood cells stay in the capillaries; substances diffuse from the blood to the cells.

10. CT3 · Apply

Model answer. Without capillaries inside it, the cells in the middle of the lump are far from the blood. Diffusion is quick only over very short distances, so they get too little oxygen and glucose. New capillaries, with walls one cell thick, bring blood closer to every cell: the diffusion distance is short and the surface area for exchange is larger, so the cells get oxygen and glucose and lose carbon dioxide, and the tumour can keep growing.

A full answer includes

• Cells far from a capillary cannot get enough oxygen and glucose by diffusion, because diffusion over long distances is too slow.
• New capillaries bring blood closer to every cell: a short diffusion distance and a larger surface area for exchange (and carbon dioxide is removed).

Also accept

• too far for diffusion without capillaries; new capillaries: short distance, larger surface area

Watch for

• “New capillaries pump blood into the tumour, which makes its cells divide faster.” — Capillaries do not pump. They bring blood close to the cells, so substances can diffuse in and out.