development: when the heart is 1at developing a tissue called the septum primum between left and right atria grows downward, slowly creating 2 separate chambers by closing this gap/ ostium primum, which means "1st opening." This septum primun the fuses with the endocardial cushion and closes the gap completely. Meanwhile, a hole appears in the upper area called the ostium secundum/ "2nd opening." We also have the septum secondum which grows downward just to the right of the septum primum and covers the ostium secondum like a cutain, leaving a small opening called foramen ovale & essentially creating a makeshift valve that allows blood to go from the right atrium to the left atrium, but no the other way. the developing newborn gets oxygenated blood from the placentae, which goes from the umbilical vein over to the right atrium, which is diffrent because after development, only oxygenated blood goes to the right atrium, which is then sent to the lungs to pick up some oxygen. In the developing fetus though, its already oxygenated, so instead of going to the right ventricle and to the lungs and back to the left atrium, it just bypasses the right ventricle and lungs through the foramen ovale and into the left atrium. The oxygenated blood then goes to the left ventricle to be pumped to the body. At birth, the septum secundum and septum primum slap shut and then fuse and close this foramen ovale, so we can rely on our own lungs for oxygen.