The road home
Veins carry blood back to your heart. By the time blood gets into a vein, it has dropped off most of its oxygen and picked up waste.
Tiny veins join to make bigger veins, like creeks running into a river. The biggest veins of all, the vena cava, pour into the heart.
At any moment, most of your blood is in your veins, slowly making its way home.
Thin walls, little valves
Blood in veins moves gently, without the big push it had in the arteries. So veins do not need thick walls. Their walls are thinner and softer.
Blood in your legs has to travel uphill to get back to your heart. To help, many veins have tiny valves inside, little one-way flaps. They let blood move up but snap shut if it starts to slide back down.
Your muscles help
When you walk, run or wiggle your toes, the muscles in your legs squeeze the veins between them. Each squeeze pushes blood up toward your heart, and the valves keep it from falling back.
Some people call your calf muscles your second heart, because they help pump blood up from your legs.
Breathing helps too. Each breath in helps pull blood up into your chest.
That is one reason it feels good to stand up and move after sitting still for a long time.
Why veins look blue
Myth: blood in your veins is blue until it touches air. Truth: blood is always red. Blood in veins is dark red, and blood in arteries is bright red.
So why do veins look blue or green through your skin? It is a trick of light. Red light goes deep into your skin, and the blood in the vein soaks a lot of it up. Bluer light bounces back to your eyes more easily.
So less red light comes back from where the vein is, and it looks bluish next to the skin around it.
Try it: look at the inside of your wrist in bright light. Can you see thin lines? Those are veins near the surface.