Five measurements of blood oxygen saturation taken in a fetus are: 67, 61, 56, 52 and 31%. The highest value is most likely recorded in which of the following vessels?

Question

Five measurements of blood oxygen saturation taken in a fetus are: 67, 61, 56, 52 and 31%.  The highest value is most likely recorded in which of the following vessels?

Quizplus · Accepted Answer

The fetal circulation differs considerably from the adult circulation. Oxygenated blood from the placenta is delivered to the fetus via the umbilical vein. Thus the umbilical vein has the highest oxygen content in the fetal circulation. (The ligamentum teres is the remnant of the umbilical vein in the adult.) Blood from the umbilical vein is first delivered to the liver, where it bypasses the hepatic circulation via the ductus venosus and enters the inferior vena cava (IVC). Thus, of the choices listed, the IVC has the highest oxygen content. From the IVC, blood is delivered to the heart where it is either pumped into the pulmonary circulation or crosses directly from the right heart to the left heart via the foramen ovale. A fraction of the blood pumped into the pulmonary circulation may bypass the lungs via the ductus arteriosus and pass directly into the descending aorta. From the aorta, blood circulates to all of the fetal tissues. Deoxygenated blood is delivered from the fetus back to the placenta by the paired umbilical arteries. (Choice A) The ductus arteriosus shunts blood from the pulmonary artery into the descending aorta. This blood is mixed with completely deoxygenated blood from the systemic circulation. In the adult, the ductus arteriosus becomes a fibrotic band, the ligamentum arteriosum. (Choice B) The superior vena cava does not participate in the circulation of oxygenated blood in the fetus; it contains only deoxygenated blood returning from the fetal head and upper extremities. (Choice D) The pulmonary trunk contains a mixture of oxygenated and deoxygenated blood that it delivers both to the fetal lungs and to the descending aorta via the ductus arteriosus. (Choice E) The descending aorta contains the same partially deoxygenated blood that is present in the fetal heart and pulmonary trunk. It delivers this blood to the fetal tissues and then to the umbilical arteries for reoxygenation. (Choice F) The umbilical arteries originate from the fetal internal iliac arteries and carry deoxygenated blood back to the placenta via the umbilical cord. Educational Objective: The most highly oxygenated blood in the fetus is carried by the umbilical vein, which empties directly into the inferior vena cava via the ductus venosus.

Five Measurements of Blood Oxygen Saturation Taken in a Fetus