Veins ensure that blood returns to the heart, where it can be transported to the lungs and oxygenated so it can travel through the body again. To make sure that the blood in the veins of the lower body reaches the heart, which defies gravity, there are two main mechanisms: venous valves and muscle pumps. The venous valves open and close, and by closing, they divide the veins into small sections. This happens to make sure the blood always goes up to the heart without pooling in the legs. When venous blood travels up the veins, the valves are automatically opened. The veins automatically close again when blood flow stops, to prevent the blood from flowing back down due to gravity. When the venous valves are damaged, the damage can lead to a disrupted blood flow and the pooling of blood under the damaged area. Muscle pumps, on the other hand, help blood flow against gravity. This is necessary because the heart pumps the blood out with significant pressure, but when the blood reaches the venous system, it has lost this pumping power. Contracting muscles can help simulate this pumping motion. The main pump in the body is the calf muscle pump.