Fetal circulation question in kaplan embryology

[orthoboy]

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[blackandgold1]

This question is driving me crazy.

12) Which of the following statements is false?
A) In fetal circulation, blood is oxygenated in the placenta.
B) A small amount of blood reaches the fetal lungs.
C) Are 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.

Why is it D? I know the 3 others are true, but why is the blood delivered via the aorta contains a lower pressure of oxygen? Both oxygenated and deoxygenated blood enter the right atrium, most goes to the left atrium via the foramen ovale and some goes to the right ventricle, and gets pumped to the pulmonary artery. Most goes to the aorta from there by the ductus arteriosus, but some goes to the lungs. WHY would the oxygen partial pressure be higher in the pulmonary than the aorta?? Doesn't both oxygenated and deoxygenated go through this same mechanism??

Ahh this is driving me crazy! someone please help!

It wouldn't.............. statement D is false, meaning that the blood delivered thru the lungs and aorta has the same partial pressure. The question is asking for the false statement.

 

[orthoboy]

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[Zeinne]

In the explanations it said that the oxygen partial pressure in the Aorta is less than the pulmonary artery...That's why it was false. I thought it would be the same partial pressure too!

wow that's confusing. if anything, i'd have thought all statements were true. hehehe.

 

[Code Blah]

The two should have almost the same partial pressure of oxygen, but you have to take into account the oxygen the developing lungs is using. While most blood is shunted away from the pulmonary artery by the ductus arteriosus, some remain and carries oxygen to the developing lungs. When this blood returns to the left atrium, they're deoxygenated, making the blood in the left atrium slightly more deoxygenated than that in right atrium (they aren't going to even out because blood is only flowing from the right atrium to the left atrium from the foramen ovale). This blood will be the same blood that enters the aorta, which will have slightly less partial pressure of oxygen than blood in pulmonary artery. This question is a pain, I would just eliminate the first 3, which are obviously true, and then picked D as answer.

 

[Morsetlis]

The pressure between the Aorta and Pulmonary Arteries are communicated by the Ductus Arteriorsus, which is not a capillary so therefore the pressure differences are obliterated by open communication between close vessels by a big vessel (the ductus).

Also remember that the ductus is for shunting blood from the pulmonary circulation TOWARD the aorta, not the other way around.

In the adult human, there is no ductus, and you don't want a huge pressure that will burst your alveolae in the lungs, so pulmonary arterial pressure would be about 10-25 mmHg, depending systole/diastole (compared to left ventricular pressure at the aorta of 120mmHg).