Veins start out as a network of capillaries and their role is mainly as a conduit to send blood from the body back to the heart. Small changes in the caliber of the veins can result in large changes in the volume of blood held in the veins and small changes in pressure, so that the veins store roughly 60-70% of the volume of blood in the vascular system, which in physiology is referred to as the volume vessel. Moreover, veins can provide access to drugs in case of illness or need for medication, and the role of veins is mainly composed of these three points.