Right atrium receives blood from the SVC, IVC (ostium guarded by the eustacian valve), and coronary sinus (guarded by the Thebesian valve). Fossa ovalis (remnant of foramen ovale) is located in the intraatrial septum. The purpose of the eustachian valve in utero is to direct blood from the IVC across the foramen ovale to the left atrium.
Left atrium is larger than the right and receives 1-2 left pulmonary veins and 2-3 right pulmonary veins (R lung has more parenchymal tissue). Output throw low-resistance mitral valve (normal area 6-8 cm2).