So we must make sure that oxygen always travels into that fetus and carbon dioxide is always removed from that fetus and moves into the blood capillaries and the blood system of the mother. So there are three factors that affect the efficiency of that placenta, and one of them is the type of hemoglobin molecule that is found inside the blood of that fetus. So recall that oxygen is actually a nonpolar molecule. And what that means is it cannot readily dissolve in the blood plasma. And so it requires a special type of protein carrier to carry it from point A to point B. And this type of protein is known as hemoglobin.