TPR Test 3: Biol Sci #15

[sshah92]

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Hey guys, need help with this question. Instead of a tl;dr I highlighted the important sentences relevant to the question.

"Needed help with this question. In contrast to those of an adult, the lungs of a human fetus are respiratorily inactive. The fetus depends on circulation to the placenta for oxygen and carbon dioxide exchange as well as nutrient supply.

Fetal blood flows from the umbilical arteries into the placenta, where the exchange of CO2, nutrients, and metabolic waste products occurs. Because there is little difference in oxygen partial pressure between fetal and maternal blood, fetal hemoglobin has a higher binding affinity for oxygen.

Oxygenated fetal blood returning from the placenta passes through the umbilical vein before being diverted past the liver through the ductus venosus and then empties into the inferior vena cava, where it is joined by blood returning from the lower trunk and extremities. Although there is some blood flow through the high-resistance pulmonary circuit, most of the fetal blood is shunted from the right atrium through the foramen ovale to the left atrium and from the pulmonary artery through the ductus arteriosus to the aorta.

At birth, the umbilical vessels, ductus venosus, and ductus arteriosus close due to the contraction of their muscle layers. The decrease in pulmonary vascular resistance caused by inflation of the lungs is the main factor that reverses the pressure gradient between the atria, thereby physically closing the foramen ovale.

Underdeveloped newborns suffering from respiratory distress syndrome are deficient in pulmonary surfactant. As a result, their alveoli are collapsed and highly resistant to expansion. Fortunately, these newborns may be given surfactant until their alveolar type II cells are mature enough to produce pulmonary surfactant in sufficient quantities."

Soon after birth, pressure will increase significantly in the:
A. pulmonary artery.
B. right atria.
C. left ventricle.
D. right ventricle.

I'm a little confused as to why pressure only increases in left ventricle but not the other choices -- I would think pressure increases in pulmonary artery as more blood is funneled through pulmonary circuit and that pressure increases in right ventricle as well because more blood is funneled to this compartment instead of directly from right atrium to left atrium via the foramen ovale.

Thank you!

 

[mehc012]

Hey guys, need help with this question. Instead of a tl;dr I highlighted the important sentences relevant to the question.

"Needed help with this question. In contrast to those of an adult, the lungs of a human fetus are respiratorily inactive. The fetus depends on circulation to the placenta for oxygen and carbon dioxide exchange as well as nutrient supply.

Fetal blood flows from the umbilical arteries into the placenta, where the exchange of CO2, nutrients, and metabolic waste products occurs. Because there is little difference in oxygen partial pressure between fetal and maternal blood, fetal hemoglobin has a higher binding affinity for oxygen.

Oxygenated fetal blood returning from the placenta passes through the umbilical vein before being diverted past the liver through the ductus venosus and then empties into the inferior vena cava, where it is joined by blood returning from the lower trunk and extremities. Although there is some blood flow through the high-resistance pulmonary circuit, most of the fetal blood is shunted from the right atrium through the foramen ovale to the left atrium and from the pulmonary artery through the ductus arteriosus to the aorta.

At birth, the umbilical vessels, ductus venosus, and ductus arteriosus close due to the contraction of their muscle layers. The decrease in pulmonary vascular resistance caused by inflation of the lungs is the main factor that reverses the pressure gradient between the atria, thereby physically closing the foramen ovale.

Underdeveloped newborns suffering from respiratory distress syndrome are deficient in pulmonary surfactant. As a result, their alveoli are collapsed and highly resistant to expansion. Fortunately, these newborns may be given surfactant until their alveolar type II cells are mature enough to produce pulmonary surfactant in sufficient quantities."

Soon after birth, pressure will increase significantly in the:
A. pulmonary artery.
B. right atria.
C. left ventricle.
D. right ventricle.

I'm a little confused as to why pressure only increases in left ventricle but not the other choices -- I would think pressure increases in pulmonary artery as more blood is funneled through pulmonary circuit and that pressure increases in right ventricle as well because more blood is funneled to this compartment instead of directly from right atrium to left atrium via the foramen ovale.

Thank you!

Keep in mind that flow always occurs from an area of high pressure to an area of lower pressure. We know that the fetus's lungs are an area of high pressure, right? That means that, in order for even a small amount of flow to occur, the pulmonary artery must be high pressure...and therefore the right ventricle must be generating high pressure. After birth, the pressure of the lungs decreases, requiring less pressure from everything upstream in order to generate flow.

The passage directly states that the pressure in the right atrium is high before birth (as flow goes from RA to LA) and lowers after birth (flow now goes from LA to RA).

That eliminates everything but the left ventricle!

 

[jungb024]

Void

 

[sshah92]

All you really need to consider is the closure of the foramen ovale here. The passage states that the closure is caused by a reversal in the pressure gradient. So now we know that the pressure gradient swapped such that the right chambers of the heart increased in pressure (we know this because the purpose of the foramen ovale in the fetus is to allow flow from LA to RA). However the closure of the foramen ovale causes all the blood in this now high pressure area to flow into the right ventricle. In other words the pressure increases in both the right atrium and right ventricle, but the much more significant increase will be in the ventricle.
hope this helps!

It says that the purpose of the foramen ovale is to allow flow from the RA to LA actually. And since blood flows from region of high pressure to region of low pressure, the LA must have low pressure. And then the passage says that the pressure gradient is reversed after birth -- which would mean that blood flows from LA to RA right? So that is why blood must be pumped in heart across right ventricle to pulmonary circuit and then again from left ventricle to aorta?

From taking system phys I know that the highest pressure in the body is in the left ventricle, then the aorta, and the lowest pressure in the body is the right aorta. But I'm still not able to follow the flow of logic in the passage to get to that answer.. and it's not one of those random facts I remembered while taking the practice test 🙁

 

[jungb024]

I'm sorry I should have read over my answer better...

What I meant to say is that the flow is originally (before birth) from RA to LA through the foramen ovale. After birth the passage suggests that this is reversed meaning the Left is now higher pressure. However the flow is not allowed from LA to RA after birth because of the closure of the foramen ovale...

I don't think my explanation before was very clear (it made sense in my head at the time), but let me try from a different angle...

Consider that the heart in the fetus is really doing the same thing as in the baby, the difference is the pathway of blood flow. So here lets consider the ductus arteriosus. Before birth the blood moving through the body is pushed by the left and right ventricle contractions (since blood is allowed to flow from the pulmonary vein through the ductus arteriosus). Lets assume that after birth the same volume of blood per unit time is flowing through the body. With this being the case the left ventricle is now doing the work that the right and left combined were doing before (because of the closure of the ductus arteriosus). Thereby using logic the pressure must have increased significantly in the left ventricle..

I will admit this is a challenging question, so if I'm way off I apologize!

 

[Mommymed]

Hey guys, need help with this question. Instead of a tl;dr I highlighted the important sentences relevant to the question.

"Needed help with this question. In contrast to those of an adult, the lungs of a human fetus are respiratorily inactive. The fetus depends on circulation to the placenta for oxygen and carbon dioxide exchange as well as nutrient supply.

Fetal blood flows from the umbilical arteries into the placenta, where the exchange of CO2, nutrients, and metabolic waste products occurs. Because there is little difference in oxygen partial pressure between fetal and maternal blood, fetal hemoglobin has a higher binding affinity for oxygen.

Oxygenated fetal blood returning from the placenta passes through the umbilical vein before being diverted past the liver through the ductus venosus and then empties into the inferior vena cava, where it is joined by blood returning from the lower trunk and extremities. Although there is some blood flow through the high-resistance pulmonary circuit, most of the fetal blood is shunted from the right atrium through the foramen ovale to the left atrium and from the pulmonary artery through the ductus arteriosus to the aorta.

At birth, the umbilical vessels, ductus venosus, and ductus arteriosus close due to the contraction of their muscle layers. The decrease in pulmonary vascular resistance caused by inflation of the lungs is the main factor that reverses the pressure gradient between the atria, thereby physically closing the foramen ovale.

Underdeveloped newborns suffering from respiratory distress syndrome are deficient in pulmonary surfactant. As a result, their alveoli are collapsed and highly resistant to expansion. Fortunately, these newborns may be given surfactant until their alveolar type II cells are mature enough to produce pulmonary surfactant in sufficient quantities."

Soon after birth, pressure will increase significantly in the:
A. pulmonary artery.
B. right atria.
C. left ventricle.
D. right ventricle.

I'm a little confused as to why pressure only increases in left ventricle but not the other choices -- I would think pressure increases in pulmonary artery as more blood is funneled through pulmonary circuit and that pressure increases in right ventricle as well because more blood is funneled to this compartment instead of directly from right atrium to left atrium via the foramen ovale.

Thank you!

Sorry I'm a few days late in responding to the question. The major reason all the other answer choices can be eliminated, is because pressure is extrememly high BEFORE birth in 3/4 choices, and the question wants to know where pressure increases AFTER birth.

Fetal lungs are filled with fluid, which essentailly collapses the alveolli. In doing so, pressure increases greatly in the lungs (the phenomina is called hypoxic pulmonary vasoconstriction). Think about it like this. Make a large circle using your thumb and index finger and put your pointer finger from your other hand through the hole. The finger should go right into the hole with no resistance. This represents an open alveolar sac and the air going in. NOW "collapse the allveoli" by making the hole in your fingers as small/tight as you can, and try to stick your finger in now. See how much force you have to apply to get your finger in the hole. Long story short, the pressure in the lungs is really high in fetal lungs because of the collapsed alveolli

To get (essentially force) blood into the lungs, alot of force is going to be required. This pressure builds up in the fetal pulmonary artery (remember, Arteries carry Away from the heart) and the same force is going to be required from the right ventrical to drive blood into the artery. Of course, it gets backed up even more and the right atrium needs to force blood into the right ventrical, so pressure is high throughout the entire Right side of the heart.

Also, there is a spot in the wall of the Atrium that is extremely weak, and the pressure that has built up in the R Atrium rips a flap in the wall, called the foramen ovale, which allows blood to flow from the RA into the LA. Not ALL blood flows through this hole, however a good amount does so. After birth, this hole CLOSES. BLOOD IN ADULTS SHOULD NEVER FLOW FROM THE LEFT ATRIUM INTO THE RIGHT ATRIUM. Blood in the LA is OXYGENATED. You want it to go out into the body, NOT back into the RA with deoxygenated blood!

Hope this helps!