the pulmonary circulation loop is virtually bypassed in fetal circulation. the fetal lungs are collapsed, and blood passes from the right atrium directly into the left atrium through the foramen ovale: an open conduit between the paired atria, or through the ductus arteriosus: a shunt between the pulmonary artery and the aorta. when the lungs expand at birth, the pulmonary pressure drops and blood is drawn from the right atrium into the right ventricle and through the pulmonary circuit. over the course of several months, the foramen ovale closes, leaving a shallow depression known as the fossa ovalis.