And that's because the alveoli of the lungs are completely filled with fluid. And what that means is there is no oxygen exchange actually taking place within the lungs. Remember, all the oxygen and carbon dioxide is exchanged inside the placenta of that developing fetus. So because the lungs are filled with fluid, what that means is there is a high resistance inside that fluid, inside the lungs, and that creates a high pressure. Now, we know that blood, as any other fluid, will always flow from a high pressure to a low pressure. So because the lungs have a high pressure, that creates a high pressure in the pulmonary trunk, and it also creates a high pressure in the right ventricle and the right atrium.