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In this lesson I will explain how fetal circulation works in utero and how this transition occurs once the baby is born.
So let’s understand some of the big picture before we break it down. It is a closed circulatory system. So this means the maternal blood doesn’t mix with the fetal blood. So how does this happen? The nutrients from the maternal blood supply at the uterus is absorbed. So the oxygen and nutrients will now diffuse through fetal circulation by the umbilical cord to the fetus. Ok so now this umbilical cord. Remember AVA. There are two arteries and one vein. Now if you ask me it seems a little backwards on their role so let’s look at that. The two small arteries are taking waste and deoxygenated blood away from the fetus and the veins are taking oxygen rich blood to the fetus. With fetal circulation there are little bypasses throughout because the fetus doesn’t need blood to go places like the lungs and liver. So the bypasses move the blood around and get oxygenated blood where it needs to go quickly. And the last big picture point is that gas exchange does not occur by the alveoli because they are filled with fluid. They don’t need to breathe to exchange oxygen and the fluid that is there creates a high pressure and therefore high resistance in lungs, which plays a role in these bypasses that we will discuss.
Ok so now onto these bypasses. The first is the ductus arteriosus. This one connects the pulmonary artery and aorta. Blood will move from the right ventricle to the pulmonary artery then through ductus to aorta and this will bypass the fetal lungs. In this image you can see here is the ductus and blood is moved from the pulmonary artery to the aorta to bypass the lungs.
The foramen ovale will connect the left and right atrium. So blood is shunted from the right to the left and this will also bypass the lungs. In this image you can see the foramen ovale is here so blood is shunted from the right atrium to left and bypass the pulmonary artery and lungs.
Our final bypass is the ductus venosus. Here blood is shunted from the umbilical vein up into the inferior vena cava so this will bypass the liver. Bypassing the liver is critical to get oxygenated blood to the fetal brain quickly. So in this image you see blood is coming through the umbilical vein to the inferior vena cava, which will bypass the liver taking oxygenated blood to the heart and quickly through the body.
For management of this patient we want to assess on ultrasound that proper circulation is occurring and that there is good blood flow through the umbilical veins. After birth we want to ensure that these bypasses and adaptations have closed on their own after birth. Usually by 24 hours they have. If they do not then the newborn could have difficulties with perfusion and oxygenation. Murmurs are normal so reassuring families that if a murmur is heard in the first 24 hours it is usually because these pathways are closing. For more information on when these pathways do not close refer to the Congenital Heart Defects lesson. Murmurs will really be the only education is on murmurs after delivery if they are present. Fetal circulation is a difficult topic for a nonmedical parent to understand so they do not need to be educated on so many details.
Our nursing concepts are reproduction, perfusion because it is cardiac and human development because all of this is part of development.
Ok so I don’t know about you but I think the heart is fascinating but also confusing and especially in fetal circulation so let’s review. The umbilical cord has 2 arteries and 1 vein. Remember AVA. The arteries carry the deoxygenated, nutrient depleted blood from the fetus to the mother for waste removal. The 1 vein brings the oxygenated nutrient rich blood to the fetus. For the ductus arteriosus blood moves from right ventricle to pulmonary artery then through ductus to aorta to bypass the lungs. With the foramen ovale the blood is shunted right to left between the atriums and bypasses the lungs. With the ductus venosus there is shunting from the umbilical vein into the inferior vena cava and this bypasses liver. And last the lungs. It is important to remember that they do not do the gas exchange. This is done at the placenta.
Make sure you check out the resources attached to this lesson and review all these pathways. Sometimes drawing it out with arrows pointing to the direction of the blood flow can help commit it to memory. Now, go out and be your best selves today. And, as always, happy nursing.