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. Now, the type of hemoglobin molecule found inside adults is different than the type of hemoglobin that is found inside the fetus. Before we actually see why this is the case, let's discuss what the difference is between the adult and the fetal hemoglobin. So, in adults, we know that the hemoglobin consists of four subunits.