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Circulation
In I/GCSE Biology, do you remember about circulatory system of human body?
- At the centre of your blood system is your heart and the rest is made up of tubes called blood vessels.
- 3 types of blood vessels: arteries, veins and capillaries.
- ARTERIES carry blood away from the heart
- VEINS carry blood back to the heart
- When they get to an organ, arteries branch many times -> capillaries
- Capillaries join up to form veins.
- Oxygen, food and carbon dioxide pass in and out of the blood in the capillaries.
- Veins have valves to keep the blood flowing in the right direction.

Arteries, veins and capillaries
In I/GCSE Biology, please remember the features of each blood vessel!
Arteries:
- When the heart contracts, blood is forced into your arteries
- Each beat of the heart pumps the blood along under high pressure.
- Elastic-like walls -> stretch to take the blood
- When the walls contract and bounce back they force the blood along -> felt as a pulse.
- Thick, muscly walls
Veins:
- Wider than arteries
- Thinner walls
- Lower pressure -> blood flows slower
- Blood controlled by valves to prevent blood flowing backwards.
Capillaries:
- Very narrow.
- Blood flows through them slowly to give time for gas exchange to occur.
- Only one cell thick.
- CO2 passes into capillaries.

Double Circulation
In I/GCSE Biology, it's also important to understand what double circulation is.
RIGHT side of the heart
- Pumps blood and then back to the heart again.
- In the lungs, the blood picks up oxygen.
- Carries oxygenated blood.
- CO2 passes into the blood from body cells
LEFT side of the heart
- Pumps blood , then back to the heart.
- The blood gives up its oxygen to its body cells
- Carries deoxygenated blood
- CO2 passes into the blood from body cell
William Harvey made discoveries about the circulatory system.
Look at the diagram showing the double circulation below!

References:
- "Capillaries Illustrations & Vectors", https://thumbs.dreamstime.com/b/blood-vessels-human-body-capillaries-156698780.jpg
- "Blood vessels – Grade 9 Understanding for IGCSE Biology 2.68", https://pmgbiology.files.wordpress.com/2015/02/3289630_orig.jpg
- "What is the Function of Veins in the Circulatory System?", https://www.firstaidforfree.com/wp-content/uploads/2020/05/Pulmonary-Vein-Diagram.jpg
"Biology 3 - Blood and Circulation", https://getrevising.co.uk/resources/biology_3_blood_and_circulation
And we're done with this topic! Well Done!
Drafted by Alyssa (Biology)