In dilated segments of the veins, the speed of blood flow often decreases significantly. Consequently, white blood cells and platelets group together and emit substances that cause inflammation that spreads rapidly on the vein wall.
Sometimes thrombosis can also occur in these conditions. When a thrombus detaches from the wall of the vein (including a deep vein in the legs), it can quickly reach the heart and from there to the lungs. In the lungs, the thrombus becomes entangled in the fine capillaries. The obstruction of a pulmonary artery is called "pulmonary embolism", and in many cases it leads to death.
The extreme danger of a thrombosis in the deep vein of the leg is confirmed by the high mortality rate from pulmonary embolism: in Germany, approximately 30,000 deaths occur every year for this reason.
In the case of long journeys by plane, car or bus, the lack of movement and the sitting position with crossed legs cause increased blood congestion (the so-called “economy class syndrome”) with the risk of the formation of an embolus.