The two upper chambers are called auricles or atria(singular; atrium). The two large lower chambers are known as the ventricles. The right side of the heart completely separated from the left side by means of a muscular wall called the median septum which runs down the middle of the heart. In this way, deoxygenated blood in the right side is unable to mix with the oxygenated blood in the left side. Mixing of deoxygenated blood with the oxygenated blood will reduce the amount of oxygen carried to the tissue cells.