Compare artery vein and capillary structure
Use wall thickness, lumen size and valves to distinguish typical blood vessels.
Section B9.3 · Blood vessels
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall artery and vein flow directions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is a vessel lumen?
What you will learn
- Distinguish the wall from the lumen
- A capillary is an exchange vessel
Distinguish the wall from the lumen
The lumen is the space through which blood moves; the wall is the tissue surrounding it. In a comparison of typical arteries and veins of similar overall size, an artery has a thicker wall and a relatively narrower lumen. A vein has a thinner wall and a relatively wider lumen. Do not compare an enormous artery with a tiny vein and assume the same absolute diameter relationship.
The original cross-sections show wall tissue as rings around white lumens. Compare the ring and its opening separately. A thick wall is not a large lumen, and a vessel drawn large on a page does not establish its actual size without a scale.
A capillary is an exchange vessel
A capillary has a very narrow lumen and a wall consisting of a single layer of endothelial cells. Its wall is much thinner than the multilayered walls of arteries and veins. Red blood cells often pass through narrow capillaries in single file; the cell size and lumen in this drawing are schematic.
Many veins, especially in limbs, contain valves that prevent backflow. Typical arteries and capillaries do not have valves along their length. The heart has valves at its arterial exits, so distinguish a heart valve at a vessel entrance from valves distributed within the vessel itself. Not every vein contains valves.
Sources for this lesson
Pause and explain
Which combination describes a typical capillary?
Worked example
A vessel has a thin wall, wide lumen and internal flaps. Identify the typical vessel and justify it.
Show the worked solution
- Separate the surrounding tissue from its open lumen.
- The thin wall, wide lumen and flaps acting as valves support vein identification.
Answer A vein; its relatively thin wall, wide lumen and valves distinguish it from a typical artery or capillary.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Identify A, B and C in the cross-sections: A has a thick ring, B a thinner ring with a wider opening, C a single-cell-thick wall.
Give me a hint
Compare wall and lumen separately.
Compare my reasoning
- A has the typical arterial wall.
- B matches a vein; C matches a capillary.
A artery; B vein; C capillary.
Look for these in your work
- Assign all three
- Use structural evidence
Compare a typical artery with a vein of similar overall size using wall and lumen.
Give me a hint
State both relative comparisons.
Compare my reasoning
- Arterial wall is thicker than the venous wall.
- Arterial lumen is relatively narrower than the venous lumen.
Artery: thicker wall and relatively narrower lumen; vein: thinner wall and relatively wider lumen.
Look for these in your work
- Compare wall thickness
- Compare relative lumen diameter
A vein in a diagram has no visible valve. Does that prove it is an artery?
Give me a hint
A missing flap in one view is not a full structural description.
Compare my reasoning
- Not all veins contain valves and a section may miss a valve.
- Use direction of blood flow and the other wall/lumen evidence.
No; absence of a visible valve alone does not establish artery identity. Check flow direction and other structural evidence.
Look for these in your work
- Recognise the limitation
- Propose useful additional evidence
Common mistakes
- Calling the hollow lumen the wall.
- Claiming every vein contains valves.
What can you explain now?
Which combination describes a typical capillary?
Compare with the explanation
One-cell-thick wall and very narrow lumen
Both features distinguish an exchange vessel.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Sketch three lumens and add their walls, then explain the valve qualification.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.