The normal heart has four chambers - the right and left Atriums (RA & LA) and the right and left Ventricles (RV & LV). The atriums collect blood after it returns to the heart via veins. Blood from the circulation round the body ("Main" circulation) returns into the right atrium and from the "Lung" circulation to the left atrium. From the atriums, blood passes to the ventricles, which pump it out, under pressure. The right ventricle pumps into the 'Pulmonary Artery', which carries it to the lung circulation (low pressure) and the left ventricle into the 'Aorta', which is the main artery for the main circulation (high pressure).

In the newborn infant two communications allow blood to pass between the two circuits. The "Ductus Arteriosus" is still open providing a connection between the aorta and the pulmonary artery. In adition the "Foramen Ovale" allows blood to pass between the left and right atriums (LA & RA). Within a few days the ductus closes off completely. The Foramen Ovale closes gradually over several weeks or months, sometimes remaining open as a tiny slit into adolescence or beyond.Flow of blood through the heart is controlled by the four heart valves, which work like 'swing doors'allowing blood to flow forwards but not to 'leak' (regurgitate)
backwards. The valves are called "Aortic" and Pulmonary" (guarding the outlet from the ventricles to each artery) and "Mitral" and "Tricuspid" (at the junction of the atriums with the ventricles).