And more specifically, how exactly does the circulatory system of that fetus transition into the circulatory system of the fully functional adult individual? So, before we examine those questions, however, let's recall some important facts about the way that blood moves inside that developing fetus before the fetus is actually born. So recall that inside the lungs, and more specifically, inside the alveoli of the lungs, we have a fluid. And what that does is it creates a high resistance and a high pressure inside the lungs. And a similar thing exists in the liver. So the lungs and the liver are not functional within that fetus.