In the Heart, The upper two chambers are the atria, and the lower two are the ventricles.The chambers are separated by a wall of tissue called the septum. Blood is pumped through the chambers, aided by four heart valves. The valves open and close to let the blood flow in only one direction. The functiom of atria is to receive blood from the veins and pump it to the ventricles whereas the ventricles function in pumping blood out of the heart towards the body.