you're so vein

Arteries and veins are the major vessels that carry blood to your body parts. Arteries carry blood away from the heart, while veins carry blood back to your heart. Although these vessels seem to be identical mirror images of each other, they are certainly not. There are profound differences between arteries and veins.

Arteries are more highly pressurized the veins, since they are at the beginning of the circuit and carry blood being pumped right out of the heart. Like a garden hose with one of the spray triggers on the end: the water pressure is greatest right at the opening of the trigger, and less the farther away from the opening you get. To account for this pressure, the walls of the arteries have a layer of muscle that contract in rhythm with your heart to absorb the pressure waves (so when you feel your pulse in your wrist, it is this muscular contraction you are feeling rather than the pressure wave from your heart).

Veins are not muscularized and so do not assist in pumping blood back to the heart. On top of not having any muscular getty-up, the blood they carry is under little pressure on its way back to the heart. And furthermore, blood has to fight gravity on its way back to the heart, seeing as most of the body is below the position of the heart. So what makes blood "go" in the veins?

Skeletal muscle contraction squeezes it back through the veins. Walking, running, playing hackysack (that’s what the kids are doing these days, right?), any motor activity squeezes the blood like toothpaste through your veins. But there is still the low pressure issue, so there must be a mechanism in place to keep backflow from occurring- especially in the legs, where gravity is pulling it back down towards the feet. Once again, nature has got it covered.

Left: valve open, blood moves forward
Right: valve closed, blood is stationary

Your veins have little one-way valves in them that open when blood is squeezed forward, and shut when pressure drops and gravity starts to pull it back down. These valves are passive, meaning they require no expenditure of energy and no innervation. They run off of gravity and blood pressure generated by skeletal muscle contraction. An elegant solution. Unless, these valves fail. 

If the valves become worn out and do not close completely, backflow of blood occurs. This pooling of blood generates little out-pockets along the veins. Fairly benign, but unsightly. We call these "varicose veins." 

Two questions for you:

1. When you sleep and are not moving, how is blood squeezed back through your veins? Hint: Latin word for "partition"

2. Say you were in a severe car accident. The powers that be give you the option of having either an artery or a vein severed. Which would you choose and why?