Feb 18, 2013
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Question from TPR

Q: A child with Down's syndrome has a common arterial trunk that comes off both the left and right ventricles. Which of the following occurs?

A) Aortic arterial blood oxygen saturation is lower than normal.
B) Pulmonary arterial blood oxygen saturation is lower than normal.

Answer: A

I understand why A is true. The blood of the two ventricles mix. However, why is B false? Why would the pulmonary arterial blood oxygen saturation be higher than normal?

Also, just out of curiosity, would the pressures in the ventricles increase or decrease?

Circulatory system is a weakness of mine. Thanks for the help!
 

NextStepTutor_3

MCAT Tutor
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Nov 6, 2014
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Good question! Think about the pulmonary artery in a healthy individual - it carries deoxygenated blood from the right ventricle to the lungs. If, instead, it is connected to the aorta (forming this "common arterial trunk") it will also contain some oxygenated blood. So, B is false for the same reason that you knew A was true - the blood from the two ventricles is able to mix, leaving us with blood that is partially oxygenated.

Now, would the ventricular pressure increase or decrease? That's a good question, and one that I don't think we could know the answer to based on this information alone (though I'd be curious to see what others think). Ventricular pressure is strongly dependent on things like vascular resistance / "how hard the heart has to pump," and since we don't know the diameter of this "common trunk" in comparison to the normal vessels, we wouldn't be sure what would happen to the pressure. But a lot of other serious problems would definitely arise due to hypoxia (decreased oxygen to the tissues).