The circulation of blood throughout the human body has two major components: the arterial system which leads the oxygenated blood from the heart to the tissues and the venous system which leads the blood from the tissues to the heart.
The venous system of the inferior limbs has a certain characteristic: it's formed of both the profound veins (situated between the muscles and the bones) and the superficial veins (situated under the skin).
The vertical position of the human body determines the blood to circulate in the veins of the inferior limbs in a direction opposed to that of the force of gravity. In the structure of these veins there is a system of valves which facilitate the "one-way" blood circulation. The destruction of these valves due to a number of causes allows the blood to fall under the influence of the force of gravity.
This disorder of the blood circulation in the superficial veins determines the dilatation of the veins and their sinuous aspect, characteristic for the varicose veins.