Schematic representation of the venous muscle pump. Veins are always surrounded by muscles, since their own muscle layer in respective vascular wall is poorly developed and thus they do not have a strong contractive force. By contracting skeletal muscles (e.g. by walking with the legs), the muscle belly thickens and narrows veins that are located between them. Since venous valves just allow unidirectional blood flow, blood is pumped back to the heart.