In a healthy individual, the veins allow the blood to go back from the periphery to the heart, and to do such a function there are valves inside those veins that only allow the blood one way. For several reasons those valves can get damaged, and once they get damaged the blood will be going with gravity to the periphery, and this is the beginning of the venous disease.
Symptoms of venous disease can vary a lot, can be as simple as some swelling, discomfort, and aching in the legs, or it even can be big varicose veins that can be very unsightly. In the more extreme end, there will be damage to the skin itself. It can be sores that don't heal, and even bleeding from those veins.
