The body has a number of way to help blood return to the heart. Just as the arterial system uses the power of the heart to drive fresh blood into the tissues, the calf muscle acts as a 'second heart' by contracting and relaxing as a person walks, propelling blood upward.
One-way valves in the superficial and deep veins help blood flow back to the lungs and heart. The deep venous structure handles the vast majority of the venous blood volume and is the high pressure system of the venous circulation in the legs. The remaining blood volume is handled by the superficial system. When calf muscles relax, the valves close to prevent blood from flowing backwards into the lower part of the vein. These valves are fragile and can be easily damaged.
Other 'pumps' help push blood toward the heart, such as the ankle, the foot, and the diaphragm.