Now, before we move on to the right atrium, let's focus on the lungs. So what happens in the lungs of the fetus? So, just like the liver is not yet fully functional, the lungs inside the fetus are also not functional. 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.