This article explores an alternative conception held by high school and first-year university biology students regarding the structure of the left and right ventricles of the heart and the significance of the left ventricular wall being thicker than the right. The left ventricular wall of the heart is thicker than the right ventricular wall due to greater back pressure from the aorta during systole, and not because of the differences in the length of the pulmonary and systemic circuits, which is the current understanding. We discuss reasons this alternative conception has arisen and present an explanation to address the current understanding.