Blood is pumped from the heart in the arteries. It is returned to the heart in the veins.
The capillaries connect the two types of blood vessel and molecules are exchanged between the blood and the cells across their walls.
|Always carry blood away from the heart||Always carry blood to the heart|
|Carry oxygenated blood, except for the pulmonary artery||Always carry deoxygenated blood, except for the pulmonary vein|
|Carry blood under high pressure||Carry blood under low or negative pressure|
|Have thick muscular and elastic walls to pump and accommodate blood||Have thin walls - have less muscular tissue than arteries|
|A type of supporting tissue called connective tissue provides strength||Have less connective tissue than arteries|
|The channel in the blood vessel that carries blood - the lumen - is narrow||Have a wide lumen|
Veins contain valves which prevent the backflow of blood.
In order to control blood flow through the vessels, the smooth muscle surrounding the arteries can constrict which causes vasoconstriction or they can relax which causes vasodilation.