So the right atrium has its own wall, and the left atrium also has its own wall. And there are two tiny holes, one in the right atrium and one in the left atrium. And that creates this door like structure that essentially opens one way, and it opens from the right atrium to the left atrium. And that's because inside the right atrium, we have a higher pressure than inside the left atrium. And so, because there is a higher pressure inside the right atrium, then inside the left atrium, blood will naturally and spontaneously move from the right atrium into the left atrium. And this is the shunting of blood that takes place between the two atrium of the fetal heart.