Which of the following factors contribute to the significant increase in systemic vascular resistance (SVR) at birth? Select all that apply.
Clamping of the umbilical cord.
First breath and lung expansion.
Increase in prostaglandin E2 levels.
Closure of the foramen ovale.
Activation of the renal system.
Correct Answer : A,B,D
A. Clamping of the umbilical cord eliminates the low-resistance placental circulation, which previously accounted for a significant portion of fetal blood flow. This increases systemic vascular resistance as blood is now redirected through the neonatal systemic circulation.
B. The first breath and lung expansion lead to a dramatic decrease in pulmonary vascular resistance, which increases left atrial pressure. This shift in pressure gradients contributes to the closure of fetal shunts and an increase in systemic vascular resistance.
D. Closure of the foramen ovale occurs due to increased left atrial pressure following lung expansion and decreased right atrial pressure after umbilical cord clamping. This closure redirects blood flow through the systemic circulation, contributing to the rise in systemic vascular resistance.
Rationale for incorrect answers
C. Prostaglandin E2 levels decrease at birth, not increase. This decline facilitates the closure of the ductus arteriosus and other fetal shunts. Increased prostaglandin E2 would maintain patency of these shunts, opposing the rise in systemic vascular resistance.
E. Activation of the renal system is not an immediate factor in the increase in systemic vascular resistance at birth. While renal function begins to regulate fluid and electrolyte balance postnatally, it does not directly influence the acute changes in vascular resistance during the transition to neonatal circulation.
Take home points
- Clamping the umbilical cord eliminates the low-resistance placental circuit, increasing SVR.
- Lung expansion reduces pulmonary vascular resistance and redirects blood flow.
- Closure of fetal shunts, including the foramen ovale, contributes to increased SVR.
- Prostaglandin E2 levels decrease at birth, facilitating shunt closure.
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Related Questions
Correct Answer is ["A","B","D"]
Explanation
Placental function in fetal circulation ensures survival and growth in utero through tightly coordinated gas exchange, nutrient delivery, waste removal, and hormonal signaling. The placenta acts as a surrogate lung, kidney, and gastrointestinal interface. Oxygen transfer occurs at low PaO₂ levels (30–35 mmHg), yet fetal hemoglobin ensures adequate tissue oxygenation. It also facilitates diffusion of glucose, amino acids, and removal of metabolic by-products such as urea and CO₂.
Rationale for correct answers
A. The placenta facilitates gas exchange by transferring oxygen from maternal blood to fetal blood and removing carbon dioxide. This exchange occurs at the chorionic villi without mixing blood directly. Umbilical vein oxygen saturation typically reaches 70–80%.
B. Placental villi mediate active and passive transport of nutrients including glucose, amino acids, fatty acids, and micronutrients from maternal circulation to the fetus. Transport systems ensure nutrient delivery despite varying maternal serum concentrations.
D. Waste products like carbon dioxide, urea, and bilirubin are transferred from fetal to maternal blood via the placenta for excretion. The placenta essentially substitutes renal and hepatic clearance during fetal development.
Rationale for incorrect answers
C. Fetal hemoglobin (HbF) is synthesized within the fetal liver and bone marrow, not by the placenta. HbF appears from the 6th week of gestation and becomes predominant by the second trimester. The placenta plays no direct role in hemoglobin synthesis.
E. The placenta does not directly regulate fetal heart rate. Heart rate is modulated by fetal autonomic nervous system development, oxygenation status, and neurological integrity. While placental function influences oxygenation, it does not exert active cardiac regulation.
Take home points
- Placenta replaces fetal lungs and kidneys in oxygen and waste exchange.
- Delivers essential nutrients from maternal circulation to fetal tissues.
- HbF synthesis occurs in fetal liver, not placenta.
- Fetal heart rate is regulated neurologically, not placentally.
Correct Answer is C
Explanation
Umbilical circulation in fetal life is specialized to facilitate oxygen and nutrient exchange between the fetus and the placenta. The umbilical vein is the only fetal vessel that carries oxygenated and nutrient-rich blood from the placenta directly toward the fetus. Blood in this vessel has an oxygen saturation of approximately 80%, the highest in fetal circulation. After entering the fetus at the umbilicus, blood from the umbilical vein bypasses the hepatic microcirculation through the ductus venosus and enters the inferior vena cava (IVC), where it mixes with deoxygenated blood before reaching the heart.
Rationale for correct answers
C. The umbilical vein carries highly oxygenated, nutrient-rich blood from the placenta to the fetus. It travels from the placenta through the umbilical cord and connects to the ductus venosus, delivering blood to the IVC and then to the right atrium.
Rationale for incorrect answers
A. The umbilical arteries carry deoxygenated, nutrient-poor blood from the fetus back to the placenta. They originate from the internal iliac arteries and are responsible for eliminating waste and carbon dioxide via placental exchange.
B. The pulmonary artery carries blood from the right ventricle to the fetal lungs. However, in fetal life, the lungs are nonfunctional, and most of this blood is diverted through the ductus arteriosus to the descending aorta. This blood is deoxygenated.
D. The aorta distributes mixed oxygenated blood to the fetal body. Although it carries blood to systemic tissues, it does not deliver blood from the placenta and is not the most oxygenated vessel.
Take home points
- The umbilical vein carries oxygenated blood from the placenta to the fetus.
- It has the highest oxygen saturation in fetal circulation (~80%).
- The umbilical arteries return deoxygenated blood to the placenta.
- The ductus venosus allows umbilical vein blood to bypass the liver.
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