Spider veins, like all chronic venous insufficiency problems, are caused by leaky valves in the veins of the leg that allow blood to fall sluggishly downward, instead of briskly flowing upward as it should. During prolonged periods of sitting or standing, the pooling blood accumulates under increasing pressure below the leaky valves, causing the vein to become inflamed and dilated.
But spider veins themselves are just the tip of the iceberg. Two even bigger culprits lurk beneath the skin’s surface. One of these villains goes by the name of “incompetent reticular vein.”
That’s the standard medical term for a larger vein that lies just below the surface of the skin. Many doctors refer to them as feeding green veins. That’s because in fair-skinned patients they’re greenish-blue in color and they “feed” blood at excessive pressures into clusters of spider veins. Other physicians refer to them as blue-green veins or root vein (every villain must have an alias).
The other culprit is the incompetent perforator vein. Perforator veins are short connecting blood vessels that carry blood from your superficial vein system inward to your deep vein system. Your deep veins then transport the blood briskly up your legs and back into your heart and lungs for replenishment with oxygen.
A valve with each perforator vein allows blood to flow in only one direction: from the superficial veins to the deep veins. But if this valve becomes defective and leaks (i.e. the perforator vein becomes incompetent), blood begins to flow backwards from the deep veins to the superficial veins.
Blood flowing in the wrong direction through feeding green veins and incompetent perforator veins creates pressure on the superficial vein system immediately beneath the skin. According to one theory, spider veins form at the surface of the skin when the body attempts to alleviate this pressure.