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Chapter 33. Circulation Through Special Regions

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A scientist uses the model of a pregnant sheep to understand how oxygen is delivered from the mother to the developing fetus. She measures Po2 in various vessels of the maternal–fetal circulation. Which of the following blood vessels would be expected to have the lowest Po2 value?

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A. Maternal artery

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B. Maternal uterine vein

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C. Maternal femoral vein

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D. Umbilical artery

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E. Umbilical vein

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The correct answer is D. Oxygenated blood flows from the maternal arteries and its oxygen tension will be progressively reduced as it traverses various vascular beds (rules out option A). Maternal arteries supply oxygenated blood to the placenta, where it equilibrates with the fetal blood in the fetal villi and drains into the umbilical veins (rules out option E). Blood draining from the uterus via the uterine veins will have a lower oxygen tension than that in the femoral veins, due to delivery of oxygen to the fetus (rules out option C). However, the specific properties of fetal hemoglobin compared to that in the maternal circulation allow for oxygen delivery to the fetal tissues and will significantly deplete oxygen in the blood returning to the placenta via the umbilical artery (rules out option B). It is important to recall that the designation of vein and artery in the fetal circulation refers to their anatomical relationship to the fetal heart, and unlike adult arteries, the umbilical artery in the fetus carries deoxygenated blood.

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The pressure differential between the heart and the aorta is least in the

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A. left ventricle during systole.

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B. left ventricle during diastole.

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C. right ventricle during systole.

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D. right ventricle during diastole.

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E. left atrium during systole.

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The correct answer is A. Blood flows to the coronary circulation from the aorta, and aortic pressure declines from a high of around 120 mm Hg during systole to a low of around 80 mm Hg during diastole. Left ventricular pressure is minimal during diastole, when the ventricle relaxes to allow for filling (rules out option B). It then rises during systole until it exceeds the pressure in the aorta, when the aortic valve opens. The pressure continues to increase to allow for ventricular ejection. On the other hand, the right ventricle needs to pump only against the low pressure pulmonary circulation during systole, and is relaxed during diastole (rules out options C...

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