2.6.7Unit 2 — Multicellular Organisms
Blood Vessels — SQA National 5 Biology Revision Notes & Topical Questions
5 min read 5 parts Lesson 33 of 39
Part 1 of 5Arteries
Direction
- Arteries carry blood away from the heart at high pressure to the tissues of the body.
Thick wall
- The wall is thick and muscular and contains elastic fibres that let it withstand the high pressure.
Narrow lumen
- A narrow central space helps the artery maintain the high pressure of the blood.
Oxygen content
- Arteries carry oxygenated blood, except the pulmonary artery, which carries deoxygenated blood to the lungs.
Part 2 of 5Capillaries
Exchange site
- Capillaries form extensive networks through the tissues and are where substances are exchanged with the cells.
Thin walls
- The wall is only one cell thick and has small gaps, so substances pass through quickly over a short distance.
Narrow lumen
- The lumen is so narrow that blood flows slowly, which gives more time for exchange.
Large area
- The many capillaries together give a very large surface area for diffusion.
Part 3 of 5Veins
Direction
- Veins carry blood towards the heart at low pressure.
Thin wall
- The wall is thinner than an artery wall because the blood pressure inside a vein is low.
Wide lumen
- A wide lumen offers little resistance, which makes it easier for blood to flow back to the heart.
Valves
- Valves in veins stop the blood flowing backwards.
| Feature | Artery | Capillary | Vein |
|---|---|---|---|
| Blood flow | Away from heart | To and from cells | Towards heart |
| Pressure | High | Falls through network | Low |
| Wall | Thick, muscular, elastic | One cell thick | Thin |
| Lumen | Narrow | Very narrow | Wide |
| Valves | Absent | Absent | Present |
Part 4 of 5Why the Differences Matter
Why the Differences Matter
Pressure
- Blood leaves the heart at high pressure, and the thick elastic artery wall stops the vessel bursting under that force.
Exchange
- Capillaries cannot have thick walls, because substances must cross them easily to reach the cells.
Return flow
- At low pressure blood needs help, so valves in veins prevent backflow while muscle movement pushes blood along.
Exam link
- Use the words wall, lumen, pressure and valves when comparing the three vessel types.
Part 5 of 5Exam Checklist & Tips
2.6.7 – Key Points for the Exam
- Artery A thick-walled vessel carrying blood away from the heart at high pressure.
- Capillary A one-cell-thick vessel where exchange with tissues occurs.
- Vein A thin-walled vessel with valves carrying blood towards the heart.
- Lumen The central space inside a blood vessel.
- Elastic fibres Fibres in artery walls that stretch and recoil with each pulse.
Common Mistakes
- Saying that veins have thick walls to cope with high pressure; veins carry blood at low pressure.
- Stating that arteries always carry oxygenated blood, forgetting the pulmonary artery.
- Writing that capillaries have valves; they do not.
- Describing the capillary wall as having thin cell walls; say that it is one cell thick.
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