The Fontan physiology is the end result of staged reconstruction of the heart and the major blood vessels in patients who have a single ventricle. After completion of the reconstruction, the great veins which usually bring blood back to the heart are connected directly to the pulmonary arteries, allowing blood from the body to bypass the heart and flow directly into the lungs. In this system, blood flow through the lungs is passive (not pumped) and the efficiency of flow through the cardiovascular system is related to the resistance to blood flow in the vessels of the lungs.