why is the heart divided into two halves by a thick wall.

Heart is divided into two halves by a thick wall so that the septum separates the left and right ventricle which pumps blood to different parts of the body. The right ventricle pumps the oxygen poor blood to the lungs whereas, the left ventricle pumps oxygenated blood to the rest of the body. If there was no separate septum the oxygen rich and poor blood will mix up and would make the circulatory system less efficient.