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2.7.2Additional lessons

Identifying Blood Vessels — IB Biology SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 35 of 142

📚 IB Biology SL Hub / Topical Revision
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Part 1 of 5Structure and Function

Each vessel has a wall structure that suits the pressure of the blood it carries.

Artery
  • Artery Thick wall with muscle and elastic fibres and a narrow lumen;
    • it carries blood away from the heart at high pressure.
Vein
  • Vein Thin wall with little muscle and a wide lumen;
    • it returns blood to the heart at low pressure and contains valves.
Capillary
  • Capillary Wall of a single endothelial cell and a lumen just wide enough for one red blood cell;
    • this is where exchange happens.
Elastic fibres
  • Elastic fibres Artery walls stretch with each heartbeat and recoil afterwards, which smooths and maintains the flow of blood.
Valves
  • Valves Pocket-shaped flaps in veins close if blood tries to flow backwards, so flow towards the heart is one-way.
FeatureArteryVeinCapillary
Wall thicknessThickThinOne cell thick
LumenNarrowWideVery narrow
PressureHighLowFalls along length
ValvesAbsentPresentAbsent
DirectionAway from heartTo heartBetween arteries and veins

Part 2 of 5Identifying Vessels in Sections

1
Look first at the wall: a thick wall with a clear layer of muscle and elastic tissue points to an artery.
2
Check the lumen shape: arteries keep a rounded lumen, whereas the thin wall of a vein often collapses to an irregular shape.
3
Compare the size of the lumen with the thickness of the wall; a small lumen inside a thick wall is an artery.
4
If the wall is only one cell thick and a red blood cell fills the lumen, the vessel is a capillary.
5
Remember that arteries and veins usually lie side by side, so the thicker one is the artery.

Part 3 of 5Measuring and Magnifying

Measuring and Magnifying
Magnification
  • Magnification Magnification equals the image size divided by the actual size, measured in the same units.
Scale bar
  • Scale bar A scale bar on a micrograph lets you calculate the real size of a structure by proportion.
Drawing
  • Drawing Draw clear, unshaded lines and label the lumen, wall and any layers you can see.
Pulmonary vessels
  • Pulmonary vessels The pulmonary artery carries deoxygenated blood, so an artery is not always oxygenated blood.

Part 4 of 5Worked Example

Question

  • A micrograph shows an artery with a diameter of 45 mm.
  • The real diameter is 3 mm.
  • Calculate the magnification, and the real width of a wall that measures 12 mm on the image.

  1. Magnification = image size ÷ actual size = 45 mm ÷ 3 mm = 15.
  2. Actual size = image size ÷ magnification = 12 mm ÷ 15 = 0.8 mm.

Answer

  • Magnification ×15;
    • real wall thickness 0.8 mm.

Part 5 of 5Exam Checklist & Tips

2.7.2 – Key Points for the Exam

  • Lumen The central space inside a blood vessel through which blood flows.
  • Endothelium The single inner layer of flat cells lining all blood vessels.
  • Elastic fibres Fibres in artery walls that stretch and recoil to maintain blood pressure.
  • Capillary bed A network of capillaries where exchange with tissue fluid occurs.

Common Mistakes

  • Stating that all arteries carry oxygenated blood; the pulmonary artery carries deoxygenated blood.
  • Describing a vein as having a narrow lumen; veins have a wide lumen compared with arteries.
  • Writing that capillaries have valves or muscle; they have neither.
  • Forgetting units when calculating magnification, which should cancel to give a ratio.

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