Thrombosis
A lack of movement, e.g. through bed rest at home or in hospital can cause blood to stagnate in the veins and a blood clot (thrombosis) may form.
This blood clot prevents the return transport of blood to the heart so that blood is stuck in the veins. Swelling, a tense feeling and pain in the legs result.
Deep vein thrombosis, in which the blood clot has formed in a vein deep in the leg muscle or in the pelvic veins, is of particular significance here.
If this clot becomes detached from the venous wall, it can travel to the lungs in the bloodstream where it can cause serious complications (pulmonary embolism).