Arteries, Veins and Capillaries
Arteries carry blood from the heart to organs of the body. This blood (arterial blood) has been pumped out by ventricles and puts lots of pressure on the walls of the arteries. They must be able to 'give' under the pressure and allow their walls to stretch. They must also have the ability to recoil (pull back into shape) and help to push the blood along.
Veins carry blood from organs back towards the heart. The pressure of this blood (venous blood) is much lower than that in the arteries. It puts very little pressure on the walls of the veins. Veins must be able to allow the blood to pass through easily and prevent it from flowing in the wrong direction.
Veins also have valves called 'watch-pocket valves' which prevent the backflow of blood.
Capillaries carry blood through organs, bringing the blood close to very cell in the organ. Substances are transferrd between the blood in the capillary and the cells. TO do this, capilarries must be sal