1.1.8Structure and Functions in Living Organisms: Part 2

Blood Vessels: Structure and Function — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 2 Revision Notes & Topical Questions

5 min read 7 parts Lesson 9 of 24

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Part 1 of 5Three types of vessel

Arteries

Capillaries

Capillaries Capillaries are the tiny vessels inside tissues where substances are exchanged between blood and cells.

Veins

Veins Veins bring blood back to the heart at low pressure.
They are fed by small vessels called venules, which collect blood from the capillaries.

Artery

Function

Function Carry blood quickly and at high pressure from the heart to the organs.
Usually carry oxygenated blood;
- the pulmonary artery is the exception because it carries deoxygenated blood to the lungs.

Thick muscular wall with elastic fibres

Thick muscular wall with elastic fibres The strong wall withstands the high pressure without bursting.
The elastic fibres stretch as each surge of blood arrives and then recoil, which helps keep the pressure and flow going.

Narrow lumen

Narrow lumen A small space inside the vessel helps to keep the pressure high.

Vein

Function

Function Return blood slowly at low pressure to the heart.
Usually carry deoxygenated blood;
- the pulmonary vein is the exception because it brings oxygenated blood from the lungs.

Wide lumen

Wide lumen Offers little resistance, so blood can keep moving even though the pressure is low.

Valves

Valves These stop blood flowing backwards, which could otherwise happen because the pressure is so low.

Thin wall

Thin wall A high-pressure wall is not needed, so the wall has less muscle and elastic tissue than an artery.

One cell thick

One cell thick The wall is a single layer of cells, so the diffusion distance between blood and tissue cells is very short.
Oxygen, glucose and carbon dioxide can therefore pass across quickly.

Leaky walls

Leaky walls Small gaps let some plasma escape and form tissue fluid that bathes the surrounding cells.
Tissue fluid carries dissolved nutrients and oxygen to cells.

Slow, low-pressure flow

Slow, low-pressure flow Blood moves slowly through the capillary network, giving more time for exchange.
Capillaries carry both oxygenated and deoxygenated blood, depending on the point along the vessel.

Feature	Artery	Vein	Capillary
Direction of flow	Away from heart	Towards heart	Within tissues
Blood pressure	High	Low	Low
Wall	Thick, muscular, elastic	Thin	One cell thick
Lumen	Narrow	Wide	Very narrow
Valves	No	Yes	No
Speed of flow	Fast	Slow	Slow

Term	Meaning
Lumen	The central space of a blood vessel through which blood passes.
Arteriole	A small branch of an artery leading to the capillaries.
Venule	A small vessel that collects blood from capillaries and joins a vein.
Tissue fluid	Plasma that has leaked from capillaries and surrounds body cells.
Valve	A flap of tissue in a vein that prevents backflow of blood.

Saying arteries always carry oxygenated blood and veins always carry deoxygenated blood — the pulmonary artery and pulmonary vein are the exceptions.

Claiming veins have thick walls to withstand pressure; it is arteries that do, because veins carry blood at low pressure.

Writing that arteries have valves — valves are found in veins to stop backflow.

Forgetting to link each feature to its function: say that the thin capillary wall gives a short diffusion distance, not just that it is thin.

Last updated: 20 October 2026