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I am currently practising for an exam and am having trouble understanding one of the exam questions:

The table gives the blood flow rate in various parts of the fetal circulation.

Flow rate in mL min-1 kg-1 bodyweight

Superior vena cava 78

Inferior vena cava 182

Right ventricle 169

Pulmonary veins (together) 13

How many millilitres of blood flow each minute from the left ventricle directly into the aorta of an unborn baby weighing 3 kilograms?

A 91

B 104

C 156

D 273

E 312

Apparently, the correct answer is E and I can't explain why. My first guess was 3kg*13 for the pulmonary veins, since as I understand, all the blood in the left ventricle comes from them. However, this is apparently wrong. Looking at the answer, I guessed around a little and the one way I found to get the number 312 would be to subtract the blood level in the superior vena cava from that in the inferior vena cava. Why, I have no idea.

If someone could explain to me why 312 is the correct answer and how this works, I would be very grateful. Thank you!

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    $\begingroup$ Hint: draw the circulation with all the fetal shunts in place, and use the numbers given to map out how much must travel through each valve or shunt keeping in mind that it's a closed system so the amount of blood entering a compartment must equal the amount leaving. $\endgroup$
    – Bryan Krause ♦
    Commented Apr 6 at 17:40
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    $\begingroup$ Good question. One factor you need to consider is that pulmonary blood flow is low in the fetus, considering the lungs aren't oxygenating anything, but increases as it approaches 40 weeks. Still, at 40 weeks, pulmonary veins only account for up to ~ 25% of left ventricular output. There is also the issue of the two patents/cies: the patent ductus arteriosus and the patent foramen ovale. The majority of blood flow into the fetal LV is from the umbilical vein. What does the umbilical vein empty into? How do the patencies affect ventricular flow? $\endgroup$
    – anongoodnurse
    Commented Apr 6 at 17:46
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    $\begingroup$ @BryanKrause - I was typing when you posted your comment, which is a more elegant exercise (and less loquatious) than mine. $\endgroup$
    – anongoodnurse
    Commented Apr 6 at 17:49
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    $\begingroup$ Less loquaciously: You're calculation is based on neonatal blood flow (as opposed to fetal) with no loss from a PFO or PDA. $\endgroup$
    – anongoodnurse
    Commented Apr 6 at 21:34

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After the helpful comments, I have now been able to solve the question. Since some of the blood directly flows from the right to the left side of the heart in the foetus, the correct calculation would be:

  1. Inferior vena cava + superior vena cava ; This calculates the amount of blood arriving in the right side

  2. Subtract the amount in the right ventricle; The result is the amount that passes to the left side

  3. Add the blood coming from the pulmonary veins; since they are also flowing into the left side, this gives the total amount of blood in the left ventricle

  4. Multiply by 3 for the 3kg

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