In the wall between the right and left atrium we may find a scar (fossa ovalis) in adults. This scar is a remnant of the foramen ovale, which is a duct between the atria present before birth. This duct directed blood directly from the right to left atrium to bypass the pulmonary circulation and the lungs. A developing foetus obtains its oxygen supply from the placenta and therefore does not required a pulmonary circulation for oxygenation.
In addition to the foramen ovale, an additional duct, the ductus arteriosus, is patent in the foetal circulation. This short vein connects the pulmonary vein to the aorta leading blood that is on its way to the lungs directly into the aorta and the systemic circulation. Normally, these ducts close shortly