The dilated veins are filled with excess blood. Every part of the body needs oxygen.
Blood carries oxygen to the body through the arteries.
Arteries have muscular walls that pump the oxygenated blood away from the heart. Unlike arteries, their walls are fairly thin and the blood in them moves more passively. Blood is pushed back towards the heart through the pressure of the blood being pumped away from the heart, helped along by the movement of the muscles and other structures surrounding the veins pushing against the vein walls. Interspersed along the veins are a series of one-way valves. In people who have large varicose veins in their legs, the valves in their veins are not closing properly and some of their blood is pooling in their legs rather than traveling back to the heart.