Blood vessels

Organisation · Animal tissues, organs and organ systems · note 10 of 20

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Blood vesselsSpec 4.2.2.2

In short

Arteries carry blood away from the heart at high pressure, so they have thick walls with muscle and elastic fibres. Veins carry blood back to the heart at low pressure and have thinner walls, a wide lumen and valves to stop backflow. Capillaries have walls one cell thick, giving a short diffusion distance for exchanging substances with body cells.

The body contains three different types of blood vessel: arteries, veins and capillaries. Their structures are related to their functions.

Comparing blood vessels
	Artery	Vein	Capillary
Function	Carries blood away from the heart at high pressure	Carries blood back to the heart at low pressure	Exchanges substances with body cells
Wall	Thick, with muscle and elastic fibres	Thinner walls	One cell thick, so substances pass through easily
Lumen (inside space)	Narrow compared with the thickness of the wall	Wide	Very narrow
Valves	No	Yes, to stop backflow	No

Arteries have thick, strong walls because the blood is at high pressure. The elastic fibres let the wall stretch and spring back. The muscle strengthens the wall.

Veins have a wide lumen and valves. The blood is at low pressure, so the valves stop it flowing backwards.

Capillaries are tiny and have very thin walls. This gives a short diffusion distance, so oxygen and nutrients can pass to the cells and carbon dioxide and waste can pass back into the blood. Capillary walls are permeable.

Blood flow rate

rate of blood flow = volume of blood ÷ time

Calculating blood flow

300 cm³ of blood flows through an artery in 4 minutes. Calculate the rate of blood flow in cm³ per minute.

Rate = volume ÷ time.
Rate = 300 ÷ 4.

Answer: 75 cm³/min

Maths skill:

A rate is always something per unit time. Make sure your answer has units such as cm³/min, and that the time is in the unit you are asked for.

Cross-sections of an artery with a thick wall of muscle and elastic fibres and a narrow lumen, a vein with a thinner wall and wide lumen, and a capillary with a wall one cell thick and a very narrow lumen. — Arteries have **thick walls**, veins have **wide lumens** and capillaries have walls one cell thick. Not drawn to scale.

Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026

Frequently asked questions

Why do enzymes denature at high temperatures?

At high temperatures the shape of the enzyme's active site changes, so the substrate no longer fits and the reaction cannot be catalysed. The enzyme is denatured, and this cannot be changed back. Below the optimum, the rate rises with temperature because particles move faster and collide more often. Enzymes are not killed, because they are not alive.

How are alveoli adapted for gas exchange?

Millions of alveoli give a very large surface area for diffusion. Their walls are one cell thick, so the diffusion distance is short. A network of capillaries carries oxygen away and brings carbon dioxide, and constant ventilation brings fresh air. Both keep a steep concentration gradient, so oxygen diffuses into the blood and carbon dioxide diffuses out quickly.

Why is the left ventricle thicker than the right?

The left ventricle has a thicker muscular wall than the right because it pumps blood all the way around the body at a higher pressure. The right ventricle only pumps blood to the lungs. Both are part of the double circulatory system, in which blood passes through the heart twice on each complete circuit.

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