There are three types of blood vessels, which have different functions:
⇒ Arteries – they carry blood away from the heart and usually
carry oxygenated blood ( except from the pulmonary artery)
⇒ Veins – they carry blood to the heart and usually carry
oxygenated blood (except from the pulmonary vein)
⇒ Capillaries – they are involved in the exchange process between
When the heart pumps blood out, is leaves the heart at very high pressure,
therefore to withstand this pressure the walls of the arteries are very elastic
to ensure they can expand and deal with the extreme pressure without
The walls are made of thick muscle and elastic fibers which allows it to recoil
back into position and help maintain that pressure around the body.
Veins take blood back to the heart, because after the blood has traveled
around the body, it looses the pressure it gained from the initial contraction.
Therefore the walls of the veins do not have to have thick walls as it does
not need to deal with the pressure.
The veins do have a unique mechanism, which allows the blood to flow in the
right direction. The veins have valves to make the sure the blood travels
towards the heart.
Arteries branch into capillaries, which then divide into even more capillaries.
The capillaries are really close to the cells and are extremely thin. They also
have a very high total surface area.
All these aspects help to increase the diffusion rate into the cells.