Fetal & Neonatal Circulation Flashcards
Essential functions performed by the placenta are:
A. gas exchange (similar to adult lungs).
B. nutrition (similar to adult GI tract).
C. nutrition and waste removal (similar to adult liver).
D. fluid and electrolyte balance, waste removal (similar to adult kidneys).
When the gas consumption of umbilical arterial blood is compared to umbilical venous blood, the arterial blood has
lower PO2, and higher PCO2
Blood in the umbilical artery is returning from fetal tissues to the placenta for oxygenation and removal of CO2
The answer choices below compare pairs of cardiovascular system variables. Which of these choices best describes the fetal cardiovascular system?
O2 saturation: inferior vena cava > aorta
Fetal blood is oxygenated in the placenta, then flows via the umbilical vein at ~85% saturation into the inferior vena cava (~70%), through the heart, and into the aorta. Saturation has fallen to ~65% by venous admixture during the passage. The fetal cardiovascular system is characterized by its high pulmonary vascular resistance compared with systemic vascular resistance. Shunts (foramen ovale and ductus arteriosus) direct blood past the high-resistance pulmonary circuit, and, therefore, pulmonary blood flow is lower than aortic flow. Fetal blood is enriched with hemoglobin to enhance its O2 carrying capacity.
Blood in which of the following vessels normally has the lowest PO2?
Umbilical artery
The maternal blood coming into the intervillous space has a PO2 of ~100 mm Hg, similar to her systemic arterial blood. However, the diffusion of O2 from the maternal blood into the chorionic villi of the fetus causes the PO2 of blood in the intervillous space to fall to ~30-35 mm Hg. The PO2 of blood in the umbilical vein, which perfuses the fetus, is even less (~30 mm Hg). Blood returning to the placenta via the umbilical arteries has the lowest PO2 (~23 mm Hg).
After birth, the pressure in the pulmonary artery decreases greatly. What is the cause of this?
Pulmonary vascular resistance decrease
The pulmonary vascular resistance greatly decreases as a result of expansion of the lungs. In the unexpanded fetal lungs, the blood vessels are compressed because of the small volume of the lungs. Immediately on expansion, these vessels are no longer compressed and the resistance to blood flow decreases several-fold
In the circulatory system of a fetus, which of the following is greater before birth than after birth?
Right atrial pressure
Right atrial pressure falls dramatically after the onset of breathing due to a reduction in pulmonary vascular resistance, pulmonary arterial pressure, and right ventricular pressure
Which of the following is greater after birth than before birth?
Aortic pressure
Owing to the loss of blood flow through the placenta, systemic vascular resistance doubles at birth. This increases the aortic pressure as well as the pressure in the left ventricle and left atrium.
