Arteries and veins
The arteries and the veins differ because the blood in them is under very different pressure.
Arteries
Carry blood away from the heart, under high pressure.
Thick, muscular, elastic walls.
No valves (except where they leave the heart).
Mostly oxygen-rich blood; the pulmonary artery is the exception.
Veins
Carry blood back to the heart, under low pressure.
Thin walls with a wide space inside.
Valves stop blood flowing backwards.
Mostly oxygen-poor blood; the pulmonary veins are the exception.
Capillaries have walls only one cell thick, so substances pass easily between the blood and the cells.
Figure to be added