What is the difference between an artery and a vein?

Arteries are thick walled vessels which carry blood from the heart to the periphery.  Veins are thin walled vessels which carry blood from the periphery back to the heart.  There is a third kind of vessel, called lymphatic vessels which are tiny, extremely delicate vessels that carry proteins and lymphatic fluid from the periphery back to the heart.

What are the important veins of the legs?

There are three different kinds of veins in the legs.  The deep veins run down the center of the leg (dark blue in the picture).  In the calf there are three pairs of deep veins.  The superficial veins are closer to the skin.  Two have names: the long (or greater) saphenous vein, and the short (or lesser) saphenous vein.  There are short connecting veins between the superficial and deep veins.  These are called perforaters because they penetrate the fascia of the leg.  There are numerous other veins which are quite variable in their distribution.

The Heart

The heart pumps oxygenated blood returning from the lung out to the periphery of the body.

How does blood get back from the periphery to the heart?

The muscles of the foot and the calf constitute the "venous pump."  When you walk using your calf muscles and compressing the veins in your feet, the blood is propelled upward toward the heart.  If you sit motionless for a long period of time, such as riding in a car or plane, blood accumulates in your feet, ankles and lower legs causing swelling.

A One Way Street

As you walk blood is pumped up the veins by the calf muscles.  Meanwhile gravity iis trying to pull the blood back down your veins.  Venous valves are located throughout the veins of your legs to prevent the backward or downward flow of blood (reflux).  If these valves are damaged or defective, reflux occurs; that means that the blood flows the wrong way down the vein, creating an increased pressure within the venous system. This is the cause of venous hypertension (high blood pressure in the veins) and the fundamental cause of all the problems associated with varicose veins or chronic venous insufficiency.