If the oxygen level in the fetus is so low then how does it survive?
While it’s developing in the womb the fetal lungs remain closed. It’s the placenta that performs the job of the respiratory system for the fetus. Blood travels from the fetal heart through its umbilical arteries into the placenta. In the placenta fetal blood comes close to the maternal blood allowing some of the oxygen that her blood just picked up from her lungs to move from it to the fetus. This newly oxygenated blood then travels from the placenta through the umbilical vein back to the right side of the fetal heart. Along the way it mixes with deoxygenated blood from the lower part of the fetus. Since the fetal lungs aren’t working this blood is shunted from the right side of the heart straight into the left atrium through an opening called the foramen ovale. From there it goes into the left ventricle and is pumped out to the fetal tissues. Meanwhile, the deoxygenated blood from the top part of the fetus enters the right atrium and goes into the right ventricle where it is pumped into the pulmonary arteries. Since the lungs are closed, another shunt, the ductus arteriosus, directs most of this blood straight into the aorta where it travels to the fetal tissues.
Due to this setup the amount of oxygen in the fetal arterial blood is only about one quarter of what it is in the mother. This is because the placenta uses up oxygen to do its work and is not as efficient as the lungs at moving oxygen into the fetal circulation. Also, the oxygenated blood in the umbilical vein eventually mixes with deoxygenated blood from the lower part of the fetal body on its way back to the right side of the heart and later on deoxygenated blood that comes from the top part of the fetus that is shunted from the pulmonary arteries to the aorta. Together, all of this results in the fetus having a very low level of oxygen in its arteries.
So, the fetus faces a major dilemma because the amount of oxygen in its arterial blood is so low that if that were the case for its mother she couldn’t survive. Here’s why.
Oxygen doesn’t dissolve well in blood. For your tissues to get enough oxygen it has to be carried by a protein called hemoglobin which is housed in the red blood cells that float in your blood. Special cells in the kidneys send out a hormone called erythropoietin which goes to your bone marrow and tells stem cells there to become red blood cells which produce hemoglobin. When the oxygen level in your arterial blood is high (like it would be for the mother) the hemoglobin molecule can carry a maximum amount of oxygen which allows you to be active. But when the oxygen level in your arterial blood is very low, about one quarter of normal (as it is in the fetus), the hemoglobin molecule can only carry about 40% of its full capacity. Given how much oxygen the mother’s body would need to live at rest this would not be enough and she would die. So, what about the fetus?
The fetus solves this problem in three ways. First, as its bone marrow comes into being, since it doesn’t have kidneys yet, its embryonic tissue sends out erythropoietin to tell it to make red blood cells. This job is later taken over by the liver and finally the special kidney cells mentioned above. Second, fetal red blood cells don’t make the same type of hemoglobin as the mother’s. They make something called fetal hemoglobin (as opposed to adult hemoglobin) which has a different molecular structure. This difference allows fetal hemoglobin to hold onto more oxygen.
At one-fourth of the mother’s arterial oxygen level her hemoglobin would be able to only carry about 40% of its full capacity whereas fetal hemoglobin can carry 60%. Third, with the help of erythropoietin telling the bone marrow to make red blood cells, the fetus has more hemoglobin in its blood than the mother. In fact, fetal blood usually has at least one-third more hemoglobin per liter than the mother’s blood.
The setup in the fetus for gas exchange results in its arterial blood having only one quarter of the amount of oxygen than its mother’s. At such a low level the mother’s hemoglobin would only allow her blood to carry 40% of its oxygen capacity which wouldn’t be enough for her to live. By sending out enough erythropoietin to tell its bone marrow to make enough red blood cells with a third more fetal hemoglobin than the mother’s regular hemoglobin this allows the fetus to carry 80% of her oxygen capacity which is enough for it to live, grow and develop. Pretty neat!
Three Questions for Mr. Darwin
- How did life figure out and go about setting things up so that the embryonic support tissue and then the liver would send out enough erythropoietin to make enough fetal red blood cells until the kidneys took over later?
- How did life anticipate and from where did the information come to tell the fetal red blood cells to make fetal hemoglobin rather than adult hemoglobin so it could survive with such low levels of arterial blood oxygen?
- How did life anticipate and from where did the information come for its kidney cells to send out enough erythropoietin to tell the bone marrow to make enough red blood cells that contained more than one-third the amount of fetal hemoglobin compared to the mother’s regular hemoglobin?
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Howard Glicksman M. D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 yrs in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being.
Copyright 2018 Dr. Howard Glicksman. All rights reserved. International copyright secured.
