Illustration to make clear the functionality of the venous muscle pump. Veins are always surrounded by muscles, because their own muscular layer in the vascular wall is only weakly developed and thus doesn`t have much contractive force. When the skeletal muscles contract (e.g. the leg muscles during walking), the muscle bellies thicken and thereby compress the veins that proceed between them. Because the venous valves only permit the bloodstream to flow in one direction, the blood is pumped back to the heart.