A fetus doesn't breathe air!

Of course you knew that. But if you think about it, that is a pretty good trick considering our circulatory system. We have a systemic and a pulmonary circuit. What would a fetus do with its pulmonary circuit?

Also, if the fetus doesn't breathe air, it still has to be able to exchange gases. You know that the fetus will get oxygen from its mother, and you may even know that has to occur across the placenta, but how? You see, oxygen diffuses into our blood from the air because there is more oxygen in the air than in our blood. But a pregnant woman sends her blood to the placenta for gas exchange there, how does the fetus get enough oxygen from its mother's blood?

When you read the material in chapter 23 for this page, you will see that it goes into more detail than I do. That's primarily because we haven't talked about all the blood vessels yet. So I am not expecting you to give me all the detail that is in your book. I do, however, want you to know enough to answer those two questions above. So let's get to it.

The pulmonary circuit does two things in an adult: 1) it picks up oxygen from the air; and 2) it supplies the lung tissue with nutrients and picks lung tissue waste. In a fetus the pulmonary circuit is not necessary for picking up oxygen from the air, but it is still necessary for servicing developing lung tissue. In the adult, a lot of blood goes off into the pulmonary circuit with every cardiac cycle. But in the fetus, that does not need to be the case.

Therefore, there is no need to pump all the blood from the right atrium into the right ventricle and into the lungs in the fetus. Therefore, it doesn't happen. Instead, the two atria are connected via the foramen ovale, a hole between the atria. This foramen even has a valve that helps to ensure that blood doesn't back up from the left atrium to the right atrium.

That means that the blood that normally comes into the right atrium is the blood that is being pumped out into the body. Where does the oxygen get in? The oxygen enters the blood from the placenta, which the fetus accesses by sending its blood away from the heart in the umbilical arteries. The blood returns from the placenta in the umbilical vein, carrying oxygen. To connect this with what you know about blood vessels, the left ventricle sends its blood into the aorta. The aorta has many smaller arteries coming off it, including those that give rise to the umbilical arteries (one near each hip). The oxygen is picked up through umbilical capillaries in the placenta, and then oxygenated blood returns in the single umbilical vein. On its way back to the heart (so that oxygenated blood can be sent out through the body again), some of the blood detours to the liver, but the rest goes through the inferior vena cava and into the right atrium. From there, the oxygenated blood spills over to the left atrium, and the entire thing starts over. This is simplified, but it will do for now.

How is it possible for enough oxygen to diffuse from the mother's blood into the fetus' blood? It has to be able to pick up lots of oxygen, since the fetal circulatory system is not quite as efficient at sending as much blood for oxygen pick-up as the adult one.

Fetal blood contains much more hemoglobin. In fact, if we have 100% of our hemoglobin in our adult blood, a fetus has 150%! In addition, fetal hemoglobin is different from adult hemoglobin. (That is not uncommon, by the way... there are other proteins that are known to have different forms in the embryo than in the adult, like the acetylcholine receptor). The thing about the fetal hemoglobin is that it is much better at attracting oxygen than the adult hemoglobin.

The end result of having more and better hemoglobin as a fetus is that it can really grab a lot of the oxygen off the maternal hemoglobin. So, it can certainly get enough.