IB Biology HLTransport in Animals & PlantsPaper 1 & 2~9 min read
Identifying Blood Vessels
A micrograph question gives you a grey blob with a hole in the middle and asks what it is. You do not need to recognise tissue layers by name. You need one comparison: how thick is the wall compared with the lumen?
📚 What you need to know
Arteries have walls that are much thicker and stronger than those of veins, because of more collagen, more elastic fibres and a thicker layer of smooth muscle.
The lumen of an artery is narrow compared with the wall, which helps maintain blood pressure.
Veins have much thinner walls, because they do not withstand high pressure.
The lumen of a vein is much wider compared with the wall.
A wide lumen reduces friction between blood and the endothelium, so blood returns to the heart at an adequate speed.
Blood flows more slowly in veins, but the wide lumen means the volume delivered per unit time is the same.
You are not required to name the tissue layers (tunica intima, media and externa) – just compare wall and lumen.
The one comparison that decides it
Do not measure the wall on its own. A large vein can have a thicker wall than a small artery in absolute terms. What never changes is the ratio.
The test
wall thick and lumen narrow → artery wall thin and lumen wide → vein
Shape is a second clue. The thick muscular wall keeps an artery circular even when the tissue is cut and mounted; a vein flops.
Why the vein is built that way
It is tempting to describe a vein as an artery that has gone soft. It is not – the wide lumen does real work.
A wide lumen means less of the blood is in contact with the endothelium, so there is less friction slowing it down.
Blood in a vein moves more slowly than in an artery, but because the tube is so much wider, the volume of blood delivered per unit of time is the same. Nothing backs up.
A thick muscular wall would be pointless. There is no high pressure to resist, and building it would waste energy and space.
If a question asks you to “suggest” why a vein has a wide lumen, the friction and volume points are what lift you above the crowd. Most students only write “because pressure is low”, which describes the situation rather than explaining the adaptation.
Working through a micrograph
Capillaries are usually obvious from size alone: if a single red blood cell fills the tube, it is a capillary.
Measuring on the image
Some questions ask you to back up your answer with a measurement. Use a ruler on the printed image and give a ratio:
Two calculations worth practising
wall : lumen ratio = wall thickness ÷ lumen diameter actual size = image size ÷ magnification
Watch your units. Measure the image in millimetres, then convert the answer to micrometres by multiplying by 1000. A capillary lumen is around 7 µm, a small artery lumen a few hundred micrometres.
Worked examples
WE 1
Identify the vessel
On a micrograph, vessel X has a wall 12 mm thick and a lumen 10 mm across. Vessel Y has a wall 2 mm thick and a lumen 26 mm across. Identify each vessel and justify your answer. (4 marks)
Step 1: ratio for X
12 ÷ 10 = 1.2 : 1, so the wall is thicker than the lumen is wide.
Step 2: identify X
A thick wall with a narrow lumen means X is an artery; the thick muscle and elastic tissue withstand high pressure.
Step 3: ratio for Y
2 ÷ 26 = 0.08 : 1, so the wall is very thin compared with the lumen.
Step 4: identify Y
Y is a vein; it carries blood at low pressure so it needs no thick muscular wall.
X is an artery, Y is a veinthe marks are for the justification, so always quote the numbers you measured
WE 2
Actual size from a micrograph
The lumen of a vessel measures 30 mm on a micrograph taken at a magnification of ×80. Calculate the actual diameter of the lumen in micrometres. (3 marks)
Step 1: the equationactual size = image size ÷ magnificationStep 2: substitute
30 ÷ 80 = 0.375 mm
Step 3: convert
0.375 × 1000 = 375
375 µmcheck the answer is sensible: hundreds of micrometres fits a small vessel, 375 mm would be absurd
WE 3
Explaining the wide lumen
Suggest why a wide lumen is an advantage in a vein. (3 marks)
Point 1: friction
A wide lumen means less of the blood touches the endothelial lining, so there is less friction resisting flow.
Point 2: speed
Blood in veins moves slowly because the pressure is low, so reducing resistance helps it return to the heart at an adequate speed.
Point 3: volume
A wider tube carries more blood at once, so the volume returned per unit of time still matches the volume leaving the heart.
Less friction, same volume delivered per unit time“suggest” questions reward reasoning, so give the consequence as well as the feature
💡 Exam tips
Always answer with a comparison: “the wall is thick relative to the lumen”.
Give a reason with your identification. “It is an artery” alone is usually one mark out of two.
Use the shape as backup evidence: arteries stay round, veins collapse.
Learn actual size = image size ÷ magnification and be able to rearrange it.
Convert mm to µm by ×1000, and µm to mm by ÷1000.
Do not waste time naming tissue layers – the specification does not ask for them.
⚠ Common mistakes
Judging by absolute size. A big vein can have a thicker wall than a tiny artery; the ratio is what counts.
Assuming red-stained means artery. Stain colour tells you nothing about the vessel type.
Forgetting to convert units in a magnification calculation.
Dividing the wrong way round. Actual size is smaller than the image, so you divide by magnification.
Saying veins have thin walls because they are less important. They are thin because there is no high pressure to withstand.
Measuring the wall on only one side when the section is cut at an angle. Take a typical thickness.
Up next: Measuring Pulse Rate – using an artery you can feel through your skin to measure what the heart is doing.
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