Biology Review Notes - Ch. 5, Question 12

Hey Alex,

I was reading through the assigned chapters for biology and came upon a question in chapter 5 at the end of the chapter about fetal circulation. It's question # 12. If you could please look at it, and explain it a little better than they do in the book, I'd really appreciate it. Thanks!
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This question asks us to determine which is FALSE:
A.  In fetal circulation, blood is oxygenated in the placenta.
B.  A small amount of blood reaches the fetal lungs.
C.  After birth, the blood pressure in the right atrium decreases.
D.  In fetal circulation, the blood delivered via the aorta has a higher partial pressure of oxygen than the blood that was delivered to the lungs.


Let's go through these one by one.  We'll be talking about fetal circulation at Biology II, so I'll skip a general overview of the entire topic and just focus on these answers.


A.  Remember that blood leaves the fetus via the paired umbilical arteries (which come off the internal iliac arteries in the lower pelvis - not needed knowledge for the MCAT) and goes to the placenta where it interfaces with the maternal blood that bathes the chorionic villi.  Gases and solutes are exchanged, and the blood gains oxygen.  It returns to the body via the single umbilical vein, which enters through the umbilicus and goes to the inferior vena cava directly -- bypassing the liver via the ductus venosus (also not needed knowledge:  this becomes the ligamentum venosum after birth when it closes, and it can be seen coursing over the bottom of the liver, dividing the left lobe from the caudate lobe; the umbilical vein becomes the round ligament of the liver, which divides the quadrate lobe from the left lobe.  You'll note that I really love anatomy in these questions).  So remember that gas exchange does not occur in the lungs in the fetus, but at the placenta.  Also, remember that umbilical arteries and veins are "backwards" in oxygenation (that is, the arteries are deoxygenated and the veins are oxygenated -- remember that artery and vein always refer to direction, not oxygenation) and therefore are very often tested on the MCAT.  The other example is pulmonary circulation after birth.


B.  The lungs are some of the latest structures to develop.  They require relatively low blood pressures prior to birth so that they can develop properly.  This is okay, because remember that the placenta is where oxygenation occurs.  So the pulmonary circulation is quite low.  The foramen ovale and ductus arteriosus both help shunt blood from the right side of the heart to the left side, increasing the amount of blood flowing to the body, rather than the lungs.  So, in prenetal life, pulmonary circulation is actually quite low.


C.  The pressure in the right atrium can be thought of best by conceptualizing what the right atrium is pushing against.  In prenatal life, the right atrium takes in all of the blood from the body (directly from the placenta, through the vena cavae, at a very high pressure).  It pushes blood into the right ventricle, as well as over to the left atrium and thus to the whole systemic circulation.  After birth, the right heart is at much lower pressure -- it's only pushing blood to the right ventricle and then off to the lungs (at low pressures anyways).  So, one of the transitions after birth, when the ductus arteriosus closes and foramen ovale slams shut is that the right atrium will have lower pressures (another side note:  it actually is this decrease in pressure of the right side of the heart that allows the foramen ovale, which essentially is like a one-way flap, to close and form the fossa ovalis -- a thin epithelial remnant where the foramen ovale seals shut).


D.  This asks us to think about the oxygenation in the pulmonary artery, essentially (which supplies blood to the lungs) to oxygenation in the aorta.  Again, think about the shunts (you'll note that on the MCAT, all of the fetal circulation questions have to do with the three shunts).  The blood that goes to the pulmonary artery has what path?  It is oxygenated at the placenta, enters via the umbilical vein, goes to the IVC, to the right atrium, to the right ventricle, and to the pulmonary artery.  This is part of the (small) proportion of blood that does NOT pass through the foramen ovale.  It is almost maximally oxygenated -- it was fully oxygenated in the IVC, and then got some partially-deoxygenated blood from the SVC when it entered the right atrium, which drains blood from the head.


Now think about the aorta.  The blood here can come from a few sources.  One is blood that goes across the foramen ovale, directly to the left atrium, to the left ventricle and out to the aorta.  Another portion comes back from the lungs, and is less oxygenated because it loses some oxygen in the lungs (remember that gas exchange no longer occurs here, so it actually loses some oxygen by feeding the tissues).  This portion then goes through the four pulmonary veins, into the left atrium and eventually to the aorta.  Another portion is from the pulmonary artery, and then goes across the ductus arteriosus and into the aorta.  This portion is also slightly less oxygenated than the blood coming back from the SVC since it's mixed a bit with the IVC.


How do they compare?  Well, the pulmonary artery is almost completely oxygenated (with a little mixing from the SVC).  The aorta has a bunch more deoxygenated blood -- which passed through the lungs, the ductus arteriosus, and some through the foramen ovale.  Thus D is the FALSE statement here.