Arterys:
Your arteries carry blood away from your heart to the organs of your body. This is usually oxygentated blood so it is bright red. They stretch as the blood is forced through themand go back into shape afterwards. You can feel this as a pulse where the arteries run close to the surface (like your wrist).
Veins:
The veins carry blood towards your heart - it is usually low in oxygen and so is a deep purply-red colour. They do not have a pulse, but they have valves to prevent the back flow of blood as it moves from various parts of the body back to the heart.
Cappilaries:
You blood flows through a huge netwrok of cappilaries. Cappilaries are narrow with vey thin walls so the substances needed by your body cells (oxygen and glucose) can easily pass out of your blood and into your cells by diffusion.
A. Because the walls are thin enough.
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