(A) The normal cardiovascular circulation. The pulmonary circulation (P) is connected in series with the systemic circulation (S). The right ventricle maintains the right atrial pressure lower than the left atrial pressure, and provides enough energy to the blood to pass the pulmonary resistance. (B) The patient with a univentricular heart. The systemic and pulmonary circuits are connected in parallel, with a considerable volume overload to the single ventricle (V). The width of the line reflects the degree of volume load. There is complete admixture of systemic and pulmonary venous blood, causing arterial oxygen desaturation. (C) The Fontan circulation. The systemic and pulmonary circulations are connected in series. The right atrium (RA) or systemic veins are connected to the pulmonary artery (PA). The volume overload to the single ventricle is now less than expected for body surface area. In the absence of fenestration, there is no more admixture of systemic and pulmonary venous blood, but the systemic venous pressure is notably elevated. Ao; aorta; LA, left atrium; LV, left ventricle, RV, right ventricle.