The two chambers at the bottom of the heart are called ventricles. Their job is to squirt out the blood. In the middle of the heart there is a thick muscle called the septum that separates the left and the right side. The atria and ventricles work together.The atria fill with blood and when they are full they let the blood into the ventricles.When these ventricles pump the blood out of the heart, the atria fill up again and the whole process starts again. There are valves in the heart that make sure that the blood is pumped in the right direction.When the heart contracts, it pumps blood from its chambers into the body.he atria fill with blood and when they are full they let the blood into the ventricles. When these ventricles pump the blood out of the heart, the atria fill up again and the whole process starts again. There are valves in the heart that make sure that the blood is pumped in the right direction.