The fossa ovalis is a depression in the interatrial septum, between the right and left atrium. It is a remnant of a thin fibrous sheet, that covered the foramen ovale during fetal development. During fetal development, the blood passes from right auricle to right ventricle through the foramen ovale bypassing the foetal lungs which are no- functional. After birth, air gets introduced into lungs which decrease the pressure in the pulmonary circulatory system. This pressure closes the foramen leaving a depression called fossa ovalis.