To understand why varicose veins form, we must first understand how normal veins work.  First, there are two types of vessels in the body: arteries and veins.  Arteries transport blood pumped by the heart throughout the body.  Veins serve to return blood from the body tissues back to the heart.  In the legs, the arteries have a pretty easy job, because the heart pushes the blood down the leg under pressure, and gravity helps.
Unfortunately, the veins have a bit of a disadvantage.  Aside from being much more delicate and having thinner walls than arteries, they have to get the blood up the leg, against gravity, without much help from the heart.  In order to accomplish this task, the veins have one-way valves.  As leg muscles contract, they squeeze the veins, pushing blood through the valves, and gradually the blood makes it way back up the leg.

There are two systems of veins in the leg, the superficial system – just below the skin surface, and the deep system.  These form a complex network of veins which communicate and serve to transport blood from the legs back to the heart.

In people with varicose veins, the valves have failed.  Without properly functioning valves, the veins in the legs have to endure constant increased back-pressure.  That abnormally high pressure leads to more failing valves.  The thin vein wall was not designed for high pressures, so the veins dilate.  The veins in the superficial vein system, being just under the skin surface, start to bulge, forming what we call varicose veins.  This condition is called Venous Insufficiency.

As the veins become bigger, they start to become noticeable first as prominent veins under the skin, and then as larger, rope-like veins that stick out. These large veins often contain pooled blood that is moving very slowly, and blood clots can occur as a result. Such blood clots can cause irritation in the veins, causing a condition called thrombophlebitis. Sometimes the blood clots can extend into the deep venous system, causing DVT, or deep venous thrombosis, a condition which can lead to a fatal pulmonary embolus (meaning a blood clot comes loose and gets lodged in the blood supply of the lungs).

Some patients develop leg ulcers because of the high pressure in the veins. This condition is called venous hypertension or venous congestion. Because the tissues are congested with low oxygen venous blood, a small wound may not heal properly. If untreated, such wounds can grow into large ulcers, and the larger they get the more difficult they become to heal. This can lead to amputations. Many physicians do not understand this process and may try to heal the wounds with wraps and medications, often with poor results. Correction of the underlying vein problem can lead to dramatic, rapid healing of these ulcers.