Hearts are pretty cool, and so are developing fetuses. Unfortunately for the student preparing for the MCAT, they’re also both pretty complicated, and fetal circulation differs from adult circulation in three main structures. In the next couple paragraphs, I’m going to break down these structures with one rule. If you’re not familiar with adult circulation, I highly recommend brushing up on it before diving into this article.
The one rule to rule them all is that a fetus wants to circulate oxygen-rich blood to the body.
Unlike in an adult, fetal lungs do not provide oxygen. Instead, the oxygen source for the fetus comes from the placenta because fetal lungs are filled with fluid. So, let’s trace the path of oxygenated blood from the placenta through the three fetal shunts and see how the rule is always followed.
1. Ductus venosus
Oxygen-rich from the placenta comes through the umbilical cord to the umbilical vein and travels to the liver. Here, we encounter the ductus venosus, which is a fetal shunt that serves to divert blood away from the liver, acting as a shortcut between the umbilical vein and the inferior vena cava. Think of this as “saving” the oxygenated blood for the rest of the body (the one rule!) since the fetal liver isn’t pulling its weight. Bonus: the ductus venosus has a sphincter, which can allow for blood to enter the liver instead of the inferior vena cava. This protects the right atrium from big surges of blood during uterine contractions.
2. Foramen ovale
Once oxygenated blood is in the right atrium, it can pass through the foramen ovale directly into the left atrium. This shunt serves to bypass pulmonary circulation because the lungs are basically useless, effectively prioritizing the rest of the body to receive this oxygen-rich blood (the one rule!).
3. Ductus arteriosus
The third and final shunt connects the pulmonary trunk to the aortic arch. This takes care of the blood that is returning to the heart from the superior vena cava. Oxygen-poor blood returning to the right atrium via the superior vena cava mixes with oxygen-rich blood from the inferior vena cava in the right ventricle and is pumped out of the pulmonary trunk to the lungs. Some of this mixed blood will supply the lungs and nourish them, but since the lungs are useless, the rest of this mixed blood enters the aortic arch via the ductus arteriosus and joins the oxygen-rich blood going to the rest of the body. This is just another way the fetus makes sure as much oxygen as possible is going to the body, where it is needed (the one rule!).
Hopefully you found this one rule a helpful way to conceptualize the function of these three fetal shunts. This concept may not be considered very “high yield” for the MCAT, but with an understanding of adult circulation and this rule, you won’t have to break out into a cold sweat if this shows up on exam day.
The road to medical school is long, and the MCAT is one of its most formidable challenges. You will be relieved to know that what you learned in your premedical courses is actually on the test. But studying for the MCAT is more about taking that knowledge stored way back there in the nooks and crannies of your mind, bringing it to the fore, and then learning to twist and stretch it in the ways the MCAT tests. In reality, studying for the MCAT is no more (or less) difficult than spending late hours on a physics problem set or an entire weekend on an organic chemistry lab report. Just like these other tasks, the MCAT requires endurance and follow-through, but it becomes significantly more manageable when you work with a Cambridge Coaching MCAT tutor to apply a structured, systematic, and strategic approach to your studying.
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