The pulmonary veins do not connect to the left atrium; instead, the entire pulmonary venous return enters systemic venous circulation through various connections (in this case the connecting vein is supracardiac or above the heart). Connecting veins can also be below the heart (infracardiac) or behind the heart. Systemic blood flow depends on right-to-left atrial shunting.

AO = aorta; IVC = inferior vena cava; LA =left atrium; LV = left ventricle; PA = pulmonary artery; PV =pulmonary veins; RA = right atrium; RV = right ventricle; SVC = superior vena cava.