Structure and function of blood vessels

Blood is transported in arteries, veins and capillaries.

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.

Arteries carry blood away from the heart

A cross-section of the blood vessels in the body
ArteriesVeins
Always carry blood away from the heartAlways carry blood to the heart
Carry oxygenated blood, except for the pulmonary arteryAlways carry deoxygenated blood, except for the pulmonary vein
Carry blood under high pressureCarry blood under low or negative pressure
Have thick muscular and elastic walls to pump and accommodate bloodHave thin walls - have less muscular tissue than arteries
A type of supporting tissue called connective tissue provides strengthHave less connective tissue than arteries
The channel in the blood vessel that carries blood - the lumen - is narrowHave a wide lumen

Veins contain valves which prevent the backflow of blood.

Diagram showing the blood flow through veinsCross-section through valve

Controlling blood flow

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.