in the 17th century, william harvey discovered the circulation of the blood, describing at this time the pumping function of the heart and the “in series” disposition of the pulmonary and systemic circulations. these concepts provided the foundation for the development of modern cardiac physiology. during the three centuries that followed the discovery of circulation, many scientists studied and expanded our knowledge of cardiac and pulmonary function. with the description of complex congenital cardiac diseases, in particular functionally univentricular hearts, and with the development of their palliative surgical treatment, we have uncovered another type of cardiac physiology. in these cases, the circulation of the blood is characterized by an “in parallel” disposition of the pulmonary and systemic circulations, with direct venous–arterial connections in the absence of one pumping ventricle.