Once the blood has passed through the capillaries of the body and oxygen and nutrients have been transferred, and waste has been collected, blood from the capillaries flows on to the smallest form of veins which are called venules. If we continue our analogy of the tree we now see the tiny branches of the venules carrying the blood back up to the heart through branches that gradually become thick and thicker.
At this point much of the force of the hearts 'pump' has been used up so the pressure in veins is a lot lower than in arteries. Much of the time, blood in the veins also has to move against gravity. In order to cope with this, veins containing valves that stop blood from flowing backwards.