How does venous drainage work in anatomy and physiology of the human
body and what happens to the blood returning to the heart from the
body?
thanks

Veins are simply continuations of arteries.  As blood leaves the
heart, it takes the following path:
1. Blood leaves the heart (left ventricle) through the largest artery
called the aorta
2. To large and medium arteries then smaller arteries called arterioles 
3. To many tiny capillaries where oxygen-carbon dioxide exchange takes place 
4. To small veins called venules to larger veins 
5. And finally through the largest vein (the vena cava) to the heart
(right atrium).
6. Blood then goes to the lungs where gases are exchanged 
7. And then back to the heart (left atrium) where the cycle starts all
over again...
This is a over-simplification, but I hope it helps.  Here is a site
where you can read a little more
http://webschoolsolutions.com/patts/systems/heart.htm

In addition to the accurate comment already posted, I would add the
following. The blood pumps through the arteries because of the
pressure induced by the contraction of the heart. By the time the
blood has gone through the capillaries and entered the veins, all that
pressure has been dissipated. Here is where your muscles kick in. As
they contract, they compress the veins. There are many tiny valves in
the veins that prevent the blood from flowing back down in to your
feet, for instance. The muscles thus compress and push the blood past
the next valve.
You may be able to observe this as follows. Put your middle finger on
a prominent vein on the back of your hand. Then slowly move your index
finger starting from right beside the middle finger, up toward the
wrist. You will see this vein empty. Then remove your third finger,
and the vein will promptly fill. In several seconds, the vein will
fill slowly, even if you keep your third finger down. This is from
other tiny venules emptying into the larger vein.
Lack of exercise is partly responsible for some people getting swollen
ankles at the end of the day, the blood having gradually accumulated
and having enough back pressure to keep the capillaries from emtying
into the veins.