Fetal circulation: three shunts, one rule

biology MCAT

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.

Anyone can study hard - but the real key to MCAT success is learning to study smart. So, while all forms of MCAT preparation require you to crunch a lot of material, we focus on helping you to make strategic choices about your areas of focus at every step of the game. Each Cambridge Coaching tutor is a highly-skilled manager of your personal study process. He or she will do more than just target your weaknesses - your tutor’s goal is to identify the sections where you have the greatest potential for improvement, and teach you to wring every last point from them by creating the roadmap for your studying, and helping you stick to it. Right from the start, your tutor will create a customized syllabus for you, and will then modify that syllabus as needed.

Learn more about MCAT tutoring

MCAT Lab Techniques Part 1: Dinosaurs and Gel Electrophoresis

MCAT Lab Techniques Part 2: SDS-PAGE is Still About Dinosaurs


Lydia studied Molecular Biology and Global Health at Princeton, graduating summa cum laude with the Molecular Biology Thesis Prize. She's now pursuing her MD at Washington University in St. Louis.


academics study skills MCAT medical school admissions SAT college admissions expository writing English strategy MD/PhD admissions writing LSAT GMAT physics GRE chemistry biology math graduate admissions academic advice law school admissions ACT interview prep test anxiety language learning career advice premed MBA admissions personal statements homework help AP exams creative writing MD test prep study schedules computer science Common Application mathematics summer activities history philosophy secondary applications organic chemistry economics supplements research grammar 1L PSAT admissions coaching law psychology statistics & probability dental admissions legal studies ESL CARS PhD admissions SSAT covid-19 logic games reading comprehension calculus engineering USMLE mentorship Spanish parents Latin biochemistry case coaching verbal reasoning AMCAS DAT English literature STEM admissions advice excel medical school political science skills French Linguistics MBA coursework Tutoring Approaches academic integrity astrophysics chinese gap year genetics letters of recommendation mechanical engineering Anki DO Social Advocacy algebra art history artificial intelligence business careers cell biology classics data science dental school diversity statement geometry kinematics linear algebra mental health presentations quantitative reasoning study abroad tech industry technical interviews time management work and activities 2L DMD IB exams ISEE MD/PhD programs Sentence Correction adjusting to college algorithms amino acids analysis essay athletics business skills cold emails finance first generation student functions graphing information sessions international students internships logic networking poetry proofs resume revising science social sciences software engineering trigonometry units writer's block 3L AAMC Academic Interest EMT FlexMed Fourier Series Greek Health Professional Shortage Area Italian JD/MBA admissions Lagrange multipliers London MD vs PhD MMI Montessori National Health Service Corps Pythagorean Theorem Python Shakespeare Step 2 TMDSAS Taylor Series Truss Analysis Zoom acids and bases active learning architecture argumentative writing art art and design schools art portfolios bacteriology bibliographies biomedicine brain teaser campus visits cantonese capacitors capital markets central limit theorem centrifugal force chemical engineering chess chromatography class participation climate change clinical experience community service constitutional law consulting cover letters curriculum dementia demonstrated interest dimensional analysis distance learning econometrics electric engineering electricity and magnetism escape velocity evolution executive function fellowships freewriting genomics harmonics health policy history of medicine history of science hybrid vehicles hydrophobic effect ideal gas law immunology induction infinite institutional actions integrated reasoning intermolecular forces intern investing investment banking lab reports letter of continued interest linear maps mandarin chinese matrices mba medical physics meiosis microeconomics mitosis mnemonics music music theory nervous system