It is helpful to think about the venous system in our lower extremities as an upside down tree, comprised of branches that feed in to a trunk that then moves upwards to drain vein blood from the leg. For two-thirds of our life, this inverted vein tree system of the lower extremities must work against gravity while we are upright living our lives—either sitting, standing, or walking around. The venous blood is picking up the impure blood that is nourishing all the tissue types of the lower extremities via the arterial system and now must be returned to be passed through the liver, intestines, and lungs to be re-vitalized and returned over and over again to the heart and pumped through the arterial system to all the tissues/organ systems of our body, including those of the lower extremities—skin, muscle, bone, soft tissue, nerves. The lower extremity venous return system relies on hundreds of properly functioning one-way valves and muscle contractions squeezing on the veins, allowing blood to be propelled back to the heart, even while gravity is at play.
Medically relevant vein sufferers most commonly inherit bad valves through family genetics. Once in a while, severe trauma (most commonly blunt trauma) to a lower extremity can cause valve failure in a vein system. Once valve failure manifests, the problem progresses while the affected person is in a gravity-dependent position, causing vein blood back-flow overload problem of the trunk first, and then back-flow randomly affecting one or more of the branches. It is the back-flow in the branches from the back-flowing trunk vein(s) causing too much stretching and eventual vein wall over-distention of the branch veins that causes the visible varicose veins. (“Varicose” actually meaning “tortuous” or “ropey”).
From the above, you can see that it is possible to have trunk vein overload and never any branch vein overload (varicose vein) visible evidence of venous insufficiency. In this latter scenario, a high level of suspicion by a medical provider or concerned individual will lead to proper diagnosis and treatment by a vein specialist.