So once again, if we examine this diagram here, the deoxygenated blood coming from the superior venecave is further mixed with the oxygenated blood that is coming from the inferior venecrava. So we have these two different types of blood coming in and they mix and they enter the right atrium. Now, once they enter the right atrium, there is a high pressure inside the right atrium as a result of the high pressure inside the lungs. So since the fetal lungs are filled with fluid, they contain a high resistance to flow and that creates a high pressure. And as a result, the blood is rerouted and moves to the lower pressure left atrium via the four men ovaly. So there is this tiny one way door that opens as a result of that push, as a result of that difference in pressure.