WebMD Answers
To push blood back to your heart, your veins rely mainly on surrounding muscles and a network of one-way valves. As blood flows through a vein, the cup-like valves alternately open to allow blood through, then close to prevent backflow.
In varicose veins, the valves do not work properly -- allowing blood to pool in the vein and making it difficult for the muscles to push the blood "uphill." Instead of flowing from one valve to the next, the blood continues to pool in the vein, increasing venous pressure and the likelihood of congestion while causing the vein to bulge and twist.
Any condition that puts excessive pressure on the legs or abdomen can lead to varicose veins. The most common pressure inducers are pregnancy, obesity, and standing for long periods. Chronic constipation and -- in rare cases -- tumors also can cause varicose veins. Being sedentary likewise may contribute to varicosity, because muscles that are out of condition offer poor blood-pumping action.
The likelihood of varicosity also increases as veins weaken with age. Genetics also plays a role and if other family members have varicose veins there is a greater chance you will too. Contrary to popular belief, sitting with crossed legs will not cause varicose veins, although it can aggravate an existing condition.
This answer should not be considered medical advice...
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