💪 Thick muscular and elastic walls
The thick wall is strong enough to stand the high pressure without bursting. The elastic part stretches as each surge of blood arrives, then springs back. This helps keep the blood moving along.
Transport Systems » Arteries, Veins and Capillaries
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 2.68
- The three types of blood vessel and what each one does
- How the structure of each vessel fits its function
- How to compare arteries, veins and capillaries in a table
- How to tell the three apart in a cross-section diagram
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Unlock This CourseThree vessels, three jobs
Your blood travels around your body inside tubes called blood vessels. There are three types, and each one is built for a different job. You will see the same idea again and again in biology: structure relates to function. That means the way something is built helps it do its job well.
Think of a road network. A motorway, a country lane and a front path all carry traffic, but they are built differently because they do different jobs. Blood vessels are the same.
Key terms:
- Artery: a blood vessel that carries blood away from the heart.
- Vein: a blood vessel that carries blood back to the heart.
- Capillary: a tiny blood vessel where substances pass between the blood and the cells.
- Lumen: the hollow space in the middle of a vessel, where the blood flows.
- Valve: a flap inside a vessel that lets blood go one way only.
Arteries: built to take the pressure
Each beat of the heart pushes blood out hard. So blood in an artery is under high pressure. Arteries carry this blood away from the heart.
🔧 Narrow lumen
The small space in the middle helps keep the pressure high, so blood is pushed quickly away from the heart.
You can feel an artery working. Press two fingers on your wrist and you can feel a pulse as each surge of blood stretches the artery wall.
Veins: carrying blood back
By the time blood reaches the veins, its pressure is low. Veins carry blood back to the heart.
💧 Thin walls and a wide lumen
The pressure is low, so the wall does not need to be thick. The wide lumen lets blood flow through easily, even with little pressure pushing it.
🚪 Valves
Low pressure means blood could slip backwards. Valves are one-way flaps that close if blood starts to flow the wrong way. This stops backflow.
Valves in the heart are covered in the lesson Heart Structure and Function. Here, remember that veins have them too.
Capillaries: where the swap happens
Capillaries are the tiniest vessels. They branch out until they run close to almost every cell in your body. This is where oxygen and food leave the blood and where waste, such as carbon dioxide, enters it.
🔬 Wall one cell thick
Substances only have a very short distance to diffuse through.
🔍 Tiny lumen
The vessel is very narrow, so it can reach in between the cells.
🎯 Close to every cell
No cell is far from a capillary, so diffusion is fast.
You will meet capillaries again around the air sacs in the lungs (Alveoli Adaptations for Gas Exchange) and in the villi of the gut (Small Intestine Absorption Adaptations). The same wall one cell thick helps there too.
Compare the three vessels
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Direction of blood | Away from the heart | Back to the heart | Between arteries and veins, next to cells |
| Blood pressure | High | Low | Lower than in arteries |
| Wall | Thick, muscular and elastic | Thin | One cell thick |
| Lumen | Narrow | Wide | Tiny |
| Valves | No | Yes | No |
Spotting them in a cross-section
A cross-section is a slice cut straight across a vessel, so you see it as a ring. To identify it, look at the wall and the hole in the middle.
- Artery: a thick wall around a small, round lumen.
- Vein: a thin wall around a large lumen, often a floppier, less round shape.
- Capillary: far smaller than the others, with a wall just one cell thick. You need a microscope to see it.
Quick trick
Look at the wall first. Thick wall means artery. Thin wall and big hole means vein. A very small tube with a wall one cell thick means capillary.
Worked example
A vessel carries blood away from the heart. Its wall is thick and its lumen is narrow. Step 1: blood moving away from the heart means an artery. Step 2: the thick wall matches the high pressure. Step 3: the narrow lumen helps keep that pressure high. So the vessel is an artery, and its structure fits its function.
Common mistakes
Many students say arteries carry oxygenated blood and veins carry deoxygenated blood. That is not the rule to use here, because the oxygen content does not decide which vessel it is. Use direction instead: arteries take blood away from the heart and veins bring it back. Another mistake is saying that arteries have valves. It is veins that have them. Finally, do not say the capillary wall is thin. Say it is one cell thick.
Exam-style question
(a) Describe two ways the structure of an artery is related to its function. [2 marks]
(b) Explain how the structure of a capillary allows substances to reach the cells quickly. [2 marks]
(c) State why veins have valves. [1 mark]
Model answer
(a) The wall is thick, muscular and elastic (1), so it can withstand the high pressure of blood leaving the heart (1). Or: the lumen is narrow (1), which helps keep the pressure high (1). [2 marks]
(b) The wall is one cell thick (1), so substances have a short distance to diffuse. They also run close to every cell (1). [2 marks]
(c) To stop blood flowing backwards (1). [1 mark]
Exam tip
In "explain" questions, always link the feature to the job. Do not just write "thick wall". Write "thick wall, so it can withstand high pressure".