Varicose veins are large, often winding dilated veins that often stick out from under the skin and occur mostly in the legs. They occur because the valves in the veins that prevent blood from flowing backward (away from the heart rather than towards it) are not working properly. In some people this is a hereditary problem, in others the problem may be acquired (multiple pregnancies, large babies, pelvic surgeries, obstructions to the veins of the abdomen, tumors, and injuries to the legs, hip or knee replacements, to name just a few causes). People who spend most of the day on their feet may develop worse symptoms from these veins then people who do not.
Superficial "spider veins" that discolor the legs (called telangiectasia) and may to some people be unsightly, are not varicose veins, and do not cause blood clots, thrombophlebitis, or pulmonary emboli.
Thrombophlebitis is an inflammatory condition in which the vein has been injured and as a result of this injury accumulates clots within it. When it is superficial (or close to the skin) and in a small vein in the lower leg it can usually be treated with elevation of the affected leg to promote proper blood flow back to the heart, and anti-inflammatory drugs. Superficial thrombophlebitis does not generally cause blood clots that go to the lungs. As opposed to the superficial problem, deep veins in the leg can also become inflamed and a sourced of clotting. This is especially dangerous when it occurs above the knee or in the pelvis. Clots in these areas tend to be larger, and if they break off can cause catastrophic problems. When a clot or "thrombus" breaks off from the vein in the upper thigh or pelvis, it migrates through the veins to the right side of the heart and into the pulmonary artery. We call this a "pulmonary embolus."
Blood clots and particularly pulmonary emboli are more common in smokers, obesity, hypertension, and high estrogen states (pregnancy, especially shortly after childbirth; birth control pills; hormone replacement therapy at menopause). People with varicose veins, certain cancers, and hereditary or acquired clotting disorders are also more likely to suffer from pulmonary emboli.
According to recent statistics more than 600,000 people in the US have pulmonary emboli each year, and more than 60,000 die. Most of the deaths occur in elderly people who have been hospitalized or are bed bound. Unfortunately, many people otherwise considered healthy may also develop clots. The risk is elevated if you have been exposed to increased doses of estrogen (taking birth control pills or had a recent pregnancy. The risk of such events increases also with long trips in the seated position (car, bus, train or plane) especially if you have had injuries to the veins in your legs or pelvis in the past. Presumably, getting up and walking around or moving your legs to restore proper return of blood to the heart may reduce this. It hasn’t been proven scientifically yet, but there is good reason for believing it. We used to see more blood clots after hip fractures or surgery to the hip or knee. We were able to reduce the number of clots and pulmonary emboli initially by doing foot exercises that started right after surgery. Since then we have found that using tight stockings (we call them anti embolism stockings) and mechanical devices that squeeze the calf further reduce the chance of clotting in the leg veins. Of course, in the hospital we use a lot of blood thinners too. In people at very high risk of blood clots the doctor may decide to use a blood thinner chronically.
People who are worried about their risk of blood clots would do well to discuss this with their health care professional. For those with moderately increased risk of such events it is best to consider anti embolism stockings, abstinence from smoking, avoidance of alcohol (it "dries you out"), getting up and around, and leg exercises when on long trips. I had a European car once that used to flash a timer every two hours to remind the driver to get out and walk around.