Place your mouse over the picture to start the video.

The picture produced by the Duplex ultrasound is basically black and white. However, any movement of blood is represented by colour - Red or Blue, depending on the direction that the blood is flowing.

The probe is held so that the picture is always horizontal. The head ("up") is on the left and the foot ("down") is on the right. The examiner squeezes the calf muscle to make the blood flow in the veins.

This video shows good flow (blue colour) in the veins on squeezing the calf. The calf is immediately released and there is only a slight glimmer of red, showing the valves are stopping the blood from flowing back down the leg. 

Normal Veins - and normal valves:  (see How Veins Work section)

When Normal Veins - that is veins with normal valves - undergo "pumping" by movement of the muscles in the legs, the veins get squashed. This results in the blood getting forced upwards into the pelvis.

However, when the muscles relax and the veins open again, the blood would rush back into the leg (called reflux) if it weren't for the valves stopping this from happening.

The prevention of this reflux by the valves is the key to how veins protect the legs in normal people. When the valves are working the vein is said to be "competent".