Blood moves from a region of high pressure to low pressure. Blood flows through the circulatory system in one direction. Valves prevent pressure differences in the circulatory system from resulting in blood flowing in the wrong direction.Atrio-ventricular ValvesThese are located between the atria and ventricles. When the atria contract they allow blood to flow from the atria to ventricles. When the ventricles contract they shut, preventing the backflow of blood from the ventricles back to the atria.Semi-Lunar ValvesThese are located in the aorta and pulmonary artery. When the ventricle contract blood is forced through the valves into the aorta and pulmonary artery. When the ventricles relax the pressure in the aorta and pulmonary artery exceeds that of the ventricles so the valves shut, preventing the backflow of blood.Pocket ValvesThese occur in veins. The pressure in veins is low and many times blood flow is in the opposite direction to gravity. Major veins run through or close to skeletal muscles. When the muscles contract, the veins are squeezed. Valves ensure that the blood flows towards the heart.