Mikhail

Senior Member
7+ Year Member
15+ Year Member
Nov 27, 2003
144
0
New York
www.mdapplicants.com
Status
Hello,

What do you think is the correct answer for this question?

Which is the dominant feature found in a newborn whose ductus arteriosus failed to obliterate?

A) Increased O2 partial pressure in pulmonary arteries
B) Decreased CO2 partial pressure in pulmonary arteries
C) Increased O2 partial pressure in systemic arteries
D) Decreased O2 partial pressure in systemic arteries

This explanation is given

The ductus arteriosus connects the pulmonary artery to the aorta. If the ductus arteriosus remains open or patent after birth, some of the deoxygenated blood from the pulmonary artery will flow through the ductus into the aorta which contains fresh oxygenated blood from the newborn's lungs. Thus the mixing causes a decrease in oxygen and an increase in carbon dioxide partial pressures in the aorta. The aorta leads the blood into systemic arteries and circulation.


Here is my logic. The pressure in the aorta is higher in a newborn than in the pulmonary artery (correct me if I'm wrong). So, the blood should go from the aorta to the pulmonary artery, not vice versa. And the answer for this question would be A.

Thanks
Mikhail
 

QofQuimica

Seriously, dude, I think you're overreacting....
Staff member
Administrator
Lifetime Donor
10+ Year Member
Oct 12, 2004
18,915
4,165
Florida/Fellowship
Status
Fellow [Any Field], Attending Physician
I wouldn't pick answer choice A for the simple reason that it basically says the same thing as answer choice B. Since you can't have two correct answers, both of those answer choices must be wrong. I'd have gone with D.

I'm a chemist, not a biologist, and I have no earthly idea which of those two blood vessels has a higher pressure in a fetus. Both are directly connected to the heart, so I can't even make a decent educated guess. But even if you're correct, since there aren't any valves in arteries, maybe it's because you'd get a backflow through the ductus arteriosus in between heartbeats?
 

Daichi Katase

Senior Member
10+ Year Member
5+ Year Member
Oct 24, 2004
498
0
Status
Blood would need to go through the system before it goes back to the pulomonary artery. The body will consume most of the oxygen in the circulated blood therefore the O2 partial pressure in the pulomunary artery would be even less. Therefore A is wrong. Only D makes sense.

I think you might be confused about what the pulumonary artery is. The pulomonary artery takes the deoxygenated blood collected on the right side of the heart to the lungs for oxygenation. Hope that helps!
 

HistoRocks

Membership Revoked
Removed
10+ Year Member
Nov 2, 2004
175
0
Status
Yeah I agree the answer is D.The pulmonary arteries only deliver deoxygenated blood to the lungs. So no way the oxygen partial pressure can exceed the CO2 partial pressure. That eliminates A and B. The key to the question is here knowing that the blood vessel pressure decreases as you move away from the heart... in other words, the diameter of the arteries decreases. So if the diameter decreases, then the pressure must also decrease. In the case of the ductus arteriosus remaining open, if the oxygen content becomes reduced, then the pressure will also be lower in the systemic arteries (which carry blood away from the heart). Leading to answer D.
 

Wolfkishner

Member
10+ Year Member
5+ Year Member
Nov 27, 2004
28
0
Michigan
Status
HistoRocks said:
Yeah I agree the answer is D.The pulmonary arteries only deliver deoxygenated blood to the lungs. So no way the oxygen partial pressure can exceed the CO2 partial pressure. That eliminates A and B. The key to the question is here knowing that the blood vessel pressure decreases as you move away from the heart... in other words, the diameter of the arteries decreases. So if the diameter decreases, then the pressure must also decrease. In the case of the ductus arteriosus remaining open, if the oxygen content becomes reduced, then the pressure will also be lower in the systemic arteries (which carry blood away from the heart). Leading to answer D.
I agree that the answer would be D, though I think a lot of people are overthinking the question by going into pressures in the various vessels. There is a difference between systemic pressure and the partial pressure of a gas, and this question is asking about the partial pressure of a gas not the total pressure within the vessels. Remember, the partial pressure of a gas is a product of its molar fraction within a mixture of gases and the total pressure of all gases in the mixture, and this partial pressure is a measure of the pressure or force of a gas on the walls of its container. Systemmic pressure is a function of Force and diameter (Force = Pressure * Cross-sectional Area) of vessels and is not the same thing as the partial pressure (or even the total pressure) of gases in a mixture. The logic of vessel pressure is not needed here, and is faulty in relation to examining gas partial pressures.

The key to the answer lies in understanding what the ductus arteriosis does--to shunt oxygenated blood away from the developing fetal lungs to the systemic circulation. In the fetus, this is because oxgenated blood is coming from the umbilical vein, not the lungs. If after birth, the ductus arteriosis fails to atrophy, blood will continue to be shunted away from the lungs. But in the newborn, it is the lungs which are supplying the oxygenated blood. Thus, deoxygenated blood is now being shunted into the systemmic circulation, thereby decreasing the partial pressure of oxygen (as it is decreasing the molar fraction of O2) or increasing the partial pressure of carbon dioxide (as it is increasing the molar fraction of CO2) within the systemmic circulation.

Within the pulmonary circulation, since blood is already deoxygenated (thus high CO2 partial pressure and low O2 partial pressure) and since the rate of deoxygenation is a measure of your body's metabolism for gas not vessel pressures, shunting blood to the aorta has no significant change in the partial pressures of either gas i.e. they would remain the same. Thus eliminate A and B which predict a change in partial pressure. Between C and D, only D is a true statement based on the physiology, so eliminate C and pick D.

And actually, the O2 partial pressure can exceed the CO2 partial pressure--this is what happens every time oxygenated blood leaves the lungs to the general circulation. Oxygenated blood is not blood in which there is no carbon dioxide, rather it is blood in which the concentration, or molar fraction, of oxygen is higher than CO2 and other gases, thus, its partial pressure is higher than that of CO2. The reverse is true of deoxy blood. Again, deoxy blood is not blood in which there is no oxygen, it is blood in which the concentration, or molar fraction, or CO2 exceeds that of O2, thus in deoxy blood, CO2 has the higher partial pressure.

If you were to try and factor in vessel diameters and relate it to partial pressure of gases, you're in for a rough road, because vessel diameters are not constant, they are constantly changing. Example, during exercise peripheral arteries dialate (increase diameter, thus decrease pressure and force) and central/internal organ arteries constrict (decrease diameter, thus increase pressure and force). Does this affect the partial pressure of gases with the circulation? No, it changes the rate of oxygen delivery to tissues by lowering resistance of flow to tissues with a higher metabolic demand and raising resistance of flow to tissues with a less critical metabolic demand. But gas partial pressure does not change--remember, it's related to molar fraction and total gas pressure and is a measure of the pressure a gas exerts on the walls of its container. Or consider what happens in the case of arthrosclerosis, where again, plaques on the interal wall decrease vessel diameter. This does change systemmic pressure with the vessel (it increases) yet it does not change the partial pressure of gases. What does change the partial pressure of a gas? Things like increasing number of red blood cells (can carry more oxygen thus higher molar fraction) or increasing/decreasing blood volume or increasing/decreasing metabolic demand.

I know it's a long explanation, but I hope that helps get people on the right track. Cheers and Good luck studying!
 

SanDiegoSOD

Milk was a bad choice
10+ Year Member
7+ Year Member
Jul 5, 2004
2,794
10
Sunny California
Status
There is no way that this question is on the MCAT withouth an accompanying passage that gives more information. You don't need to know much about human anatomy for the MCAT.
 

Turkeyman

Trickster Poultry
10+ Year Member
5+ Year Member
Dec 26, 2004
1,164
1
35
Silver Spring, Maryland
Status
SanDiegoSOD said:
There is no way that this question is on the MCAT withouth an accompanying passage that gives more information. You don't need to know much about human anatomy for the MCAT.
Read right under it, he gave the pertinent information.

And yeah, D was my answer choice as well, and it seems everyone else's too =D
 

SanDiegoSOD

Milk was a bad choice
10+ Year Member
7+ Year Member
Jul 5, 2004
2,794
10
Sunny California
Status
Turkeyman said:
Read right under it, he gave the pertinent information.

And yeah, D was my answer choice as well, and it seems everyone else's too =D

Haha... All I read was "this explanation was given", which I took to mean that it was the explanation for the answer, not the info for the passage. My bad - that's what happens when you SDN on the run. ;)


Oh, I also chose D.
 

SanDiegoSOD

Milk was a bad choice
10+ Year Member
7+ Year Member
Jul 5, 2004
2,794
10
Sunny California
Status
Mikhail said:
Here is my logic. The pressure in the aorta is higher in a newborn than in the pulmonary artery (correct me if I'm wrong). So, the blood should go from the aorta to the pulmonary artery, not vice versa. And the answer for this question would be A.

Occams razor, my friend - you are thinking way too hard. The MCAT won't assume that you know the difference between a newborn's physiology and an adults physiology. Either way, the difference is irrelevant. Besides, blood does not flow from one artery to another.
 

09Doc

Junior Member
10+ Year Member
5+ Year Member
Feb 20, 2005
10
0
Status
I didnt have the patience to read the long post above so if this was said, then i apologize... but I dont understand why everyones considering blood pressure and arterioles and all this meaningless stuff for this question! All you need to know is:

Ductus Arteriosus allows circulation to bypass the lungs in the fetus. THEREFORE without the maternal support the child would have less blood to the lungs, therefore less gas exchange, therefore increased CO2 and decreased O2!

Jeez
 

rn2md10

Junior Member
10+ Year Member
Feb 16, 2005
7
0
Status
Yeah too much thinking going on for this question..... Svo2 is measured in the pulmonary arteries so its not a reflection of A or B. With congenital heart disease the child would have less O2 in the system otherwise they wouldnt be in to see a Dr. Answer D is my choice.