Props to the placenta. If you remember your Health 101 course from high school, the placenta allows woman and fetus to share nutrients, immune cells, and even oxygen without sharing the same blood. (It also can make almost every hormone the rest of the body’s organs can make, but that’s for another day.) Oxygen floats around your blood inside a molecule called hemoglobin (one part of the hemoglobin molecule complex contains iron, which is what makes your blood red). But how does the oxygen get transfered from one blood system to the other? Simple. As the woman’s hemoglobin starts to dump off her oxygen, the fetus’s hemoglobin scoops it up.
Take a look at the graph to the right. (Don’t run away! Please!) On the Y-axis, you’ve got hemoglobin O2 saturation (ie: the percentage of hemoglobins that are still carrying oxygen, and haven’t released it yet). On the X-axis, you’ve got the pressure of oxygen in your blood (ie: how much oxygen’s in your blood at a certain spot in your body – it decreases the further you get from the heart). Now take a look at the blue line–that’s the adult line. At 40% pressure, about half of the hemoglobins still have their oxygen. But as you get further from the heart, hemoglobins start dumping their oxygens more rapidly.
Now look at the fetal, red line. It’s above the blue one. So, at the same pressure, more of the fetal hemoglobin still has its oxygen. Sum it all up: when the adult is starting to dump off its oxygen, the fetus is still holding tighter to its oxygen, so it collects any oxygen that the woman’s hemoglobin is releasing.
Here’s the trick–fetal hemoglobin is just slightly different from adult hemoglobin. At birth, the baby stops producing its fetal hemoglobin, and starts producing adult hemoglobin, since it’s ready to start breathing on its own.