What prevents the backflow of blood in the separate chambers of the heart?

Veins are provided with "one-way" valves along their route back to the heart. I used to notice that the veins on my forearm inflated and bulged when if I swept my fingers down from my inner-arm elbow to my wrist when I was young and skinny. This clever arrangement results in the reduction of effort on the heart to pump blood during muscular exercise.