The circulatory system is essentially a closed series of pipes throughout the body.
Every pipe system requires some sort of pump to keep the fluid moving.
Arteries carry blood away from the heart to the body.
They are very thick since they are under a high pressure carrying blood from the heart.
Although the artery its self is very thick, the actual passage way for the blood is small.
Veins carry the blood from the body back to the heart.
Since they are carrying the blood back, they are under less pressure than the arteries. This means the walls aren’t as thick as Arteries.
The actual space for the blood following through is much larger than arteries.
Another important feature, is that they have valves to prevent back flow. These valves only open one way and stop blood from flowing backwards.
Capillaries are tiny. In fact they are so small that red blood cells have to go single file through them.
This is where the majority of diffusion occurs. They are found throughout the muscles and lungs.
The pressure here, is very low.
Bellow is a cross section of a vein, artery and capillary. Notice the difference in size and thickness.
Bellow is a diagram of how Arteries, Capillaries and Veins interact with each other.