After birth, blood flow to the lungs increases; subsequently, the PaO2 increases, which narrows the ductus arteriosus and decreases the shunting through the duct. This causes more blood to flow into the left atrium, thereby increasing the pressure in the left atrium over the right atrium and causing the functional closure of the foramen ovale. Right atrial pressure that is higher than the left atrial pressure leads to a patent foramen ovale. Ventricular wall thickness of either the right or the left ventricle does not directly affect the functional closure of the foramen ovale.