Question

Veins have a much lower blood pressure than arteries. What prevents backflow of blood in veins?

Answer 1

one way valves made of folding of endothelium to the lumen of the vein

try this:
inspect the veins at the back of your hand. block one of them with your index finger. "squeeze" the blood out of it in a sliding motion toward your arm (while mainting the pressure with your index finger). with some luck and good veins u;ll see backflow of blood until a valve. part of the vein will stay collaped and part will be filled.