The heart continuously pumps blood around the body to distribute nutrients and remove waste. The blood is taken away from the heart in arteries under high pressure, passes through meshes of tiny blood vessels in the tissues (e.g. leg muscles) and is returned to the heart via veins which are thin walled collapsible tubes normally under relatively low pressure.
As veins are thin walled and collapsible, the flow within them must rely on pumping from behind, rather than sucking from the heart in front as all this would achieve would be to collapse them.
In the legs there are two main sets of large veins running in parallel. The superficial (surface) system and the deep system. Blood in the superficial system flows into the deep system before being carried out of the leg up to the heart.
Humans spend a long time standing up and in this position the heart is about a metre or so higher than the veins in the lower leg. This makes it very difficult to get blood back to the heart from the legs. There are two main mechanisms which normally help the blood back up to the heart. First is the muscle pump in the calf. As the muscles contract they squeeze the blood out of the deep veins lying between them. Some of this blood will tend to be squeezed up and some down. This is where the second mechanism comes in; the valves in the veins which, when functioning, allow blood to flow upwards only.
Consequently in healthy legs when the muscles contract they squeeze blood out of the deep veins and, as the valves stop it going down, it all goes up in the direction of the heart. On relaxation of the calf muscles, the pressure in the deep veins lowers and blood flows from the superficial into the deep system. There are also valves which stop blood flowing the wrong way from deep to superficial. Blood flow into the legs is nicely balanced by blood being pumped out and pressure in the veins is kept low.
Normal veins are largely invisible being either near the surface and very small or, if larger, much deeper. In thin or muscular people, especially men, there may be prominent but normal surface veins but these are easy to recognise, cause no problems and require no treatment.
Varicose veins and spider veins are formed when this normal mechanism of venous blood flow fails.