The structure and function of blood vessels

Exchange and transport in animals · Blood and blood vessels · note 2 of 2

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In short

There are three types of blood vessel. Arteries carry blood away from the heart at high pressure and have thick, muscular walls with elastic fibres. Veins carry blood back to the heart at low pressure, with a wide lumen and valves to stop backflow. Capillaries have walls one cell thick, giving a short diffusion distance for exchange.

There are three types of blood vessel. Each is adapted to its job.

Arteries, veins and capillaries
	Artery	Vein	Capillary
Direction of blood	Away from the heart	Towards the heart	Between arteries and veins, through the tissues
Blood pressure	High	Low	Falls as blood passes through
Wall	Thick, muscular, with elastic fibres	Thinner wall	One cell thick
Lumen	Narrow	Wide	Very narrow
Valves	No	Yes	No
Function	Carry blood at high pressure to the organs	Carry blood back to the heart	Exchange substances with cells

Arteries have thick, muscular walls and elastic fibres. The wall can stand the high pressure and stretches and springs back with each heartbeat.
Veins carry blood at low pressure. They have a wide lumen to let blood flow easily, and valves to stop the blood flowing backwards.
Capillaries have walls only one cell thick, giving a short diffusion distance. They form a huge network so there is a large surface area for exchange, and they are narrow so they can reach every cell.

Exam tip:

Do not say that arteries always carry oxygenated blood and veins always carry deoxygenated blood. The pulmonary artery and pulmonary vein are the exceptions.

Cross-sections of an artery with a thick wall of muscle and elastic fibres and a narrow lumen, a vein with a thin wall and a wide lumen, and a capillary with a wall one cell thick and a very narrow lumen, plus a vein cut lengthways showing a valve that lets blood flow only towards the heart. — Arteries have thick walls, veins have a wide lumen and valves, capillaries have walls one cell thick. (The capillary is drawn much larger than its real size.)

Quick check

Which part of the blood carries oxygen, and what is the name of the substance that binds to it?
Show answer
Red blood cells; haemoglobin.
What is the job of phagocytes?
Show answer
They engulf and digest pathogens.
Which blood vessels have valves?
Show answer
Veins.
Why are capillary walls only one cell thick?
Show answer
To give a short diffusion distance for exchange of substances.

Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026

Frequently asked questions

How are red blood cells adapted to carry oxygen?

Red blood cells have a biconcave disc shape, which gives a large surface area for absorbing oxygen. They are full of haemoglobin, which binds to oxygen to form oxyhaemoglobin, so each cell can carry a lot of oxygen. They have no nucleus, which leaves more room for haemoglobin. Their job is to carry oxygen.

How do white blood cells fight infection?

There are two types of white blood cell. Phagocytes are flexible, so they can change shape to surround pathogens, engulf them and digest them, which is called phagocytosis. Lymphocytes make and release antibodies into the blood. The antibodies attach to specific pathogens and help to destroy them.

Why do arteries have thick elastic walls?

Arteries have thick, muscular walls with elastic fibres because they carry blood away from the heart at high pressure. The thick wall can stand the high pressure, and the elastic fibres let it stretch and spring back with each heartbeat. Arteries also have a narrow lumen and, unlike veins, no valves.

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