Fetal Circulation

Introduction

Fetal circulation is a specialized vascular arrangement that ensures continuous delivery of oxygen and nutrients from the mother to the developing fetus. At the same time, placental mechanisms help limit fetal exposure to many potentially harmful substances, thereby supporting normal growth and development.

Functional Aspects

• Fetal circulation differs markedly from postnatal circulation because the lungs are functionally inactive before birth.

Design of Fetal Circulation

• The fetus depends entirely on the placenta for oxygen, nutrients, and removal of metabolic waste products.
• Oxygenated blood returns from the placenta through a single umbilical vein.
• Umbilical venous blood is approximately 80% saturated with oxygen.
• Part of this blood perfuses the liver, whereas a substantial portion bypasses the hepatic circulation through the ductus venosus and enters the inferior vena cava.
• Blood reaching the right atrium is preferentially directed toward the left atrium through the foramen ovale.
• This right-to-left shunt allows relatively well-oxygenated blood to enter the left ventricle and subsequently the ascending aorta.
• The remaining blood passes into the right ventricle and is ejected into the pulmonary artery.
• Because fetal lungs are collapsed, pulmonary vascular resistance remains high and pulmonary blood flow is minimal.
• Consequently, most blood from the pulmonary artery bypasses the lungs through the ductus arteriosus and enters the descending aorta.
• The aorta distributes blood to fetal tissues.
• Deoxygenated blood returns to the placenta through two umbilical arteries, which arise from the internal iliac arteries.
• Thus, the placenta functions as the fetal organ of gas exchange during intrauterine life.

Special Features

• Pulmonary vascular resistance is high in the fetus because the lungs are collapsed and contain minimal blood flow.
• Consequently, the right ventricle ejects blood against a relatively high pulmonary resistance.
• Most right ventricular output bypasses the lungs and enters the descending aorta through the ductus arteriosus.
• A substantial proportion of blood entering the right atrium is shunted directly into the left atrium through the foramen ovale.
• The placenta offers low vascular resistance, whereas systemic fetal vessels exhibit comparatively higher resistance.
• Therefore, a significant fraction of fetal cardiac output returns to the placenta for gas exchange and nutrient transfer.
• Fetal arterial oxygen saturation is lower than that of postnatal life.
• Nevertheless, fetal tissues tolerate lower oxygen levels because of reduced metabolic demands and the presence of fetal hemoglobin, which has a higher affinity for oxygen.
• Cardiovascular regulation in early fetal life depends predominantly on local mechanisms, while autonomic reflexes such as baroreceptor and chemoreceptor responses mature progressively during late gestation.

Changes Occurring at Birth

• At birth, clamping of the umbilical cord interrupts placental circulation and terminates blood flow through the umbilical vessels.
• Cessation of umbilical venous flow leads to functional closure of the ductus venosus.
• The newborn’s first breaths expand the lungs, causing a marked fall in pulmonary vascular resistance.
• Expansion of the lungs greatly increases pulmonary blood flow and enhances venous return to the left atrium.
• Simultaneously, closure of the umbilical vein reduces venous return to the right atrium.
• Clamping of the umbilical arteries removes the low-resistance placental circulation, thereby increasing systemic vascular resistance and arterial pressure.
• Consequently, left atrial pressure exceeds right atrial pressure, producing functional closure of the foramen ovale.
• Anatomical fusion of the interatrial septum occurs gradually during infancy.
• The fall in pulmonary arterial pressure together with the rise in aortic pressure alters blood flow across the ductus arteriosus.
• Increased arterial oxygen tension and reduced circulating prostaglandins promote constriction of the ductus arteriosus, which usually closes functionally within the first 24–48 hours after birth.
• Before birth, both ventricles have nearly equal wall thickness.
• After birth, right ventricular workload decreases, whereas the left ventricle undergoes hypertrophy to support systemic circulation.

Clinical Importance

• Failure of postnatal closure of the foramen ovale results in an atrial septal defect or patent foramen ovale.
• In most cases, blood flows from the left atrium to the right atrium because left atrial pressure is higher.
• This left-to-right shunt increases pulmonary blood flow and may eventually cause right heart enlargement and pulmonary hypertension.
• Persistence of the ductus arteriosus after birth is termed patent ductus arteriosus.
• In this condition, blood flows from the aorta into the pulmonary artery owing to the higher systemic arterial pressure.
• The resulting left-to-right shunt increases pulmonary circulation and places an additional workload on the left ventricle.
• Tetralogy of Fallot is a cyanotic congenital heart disease characterized by ventricular septal defect, pulmonary stenosis, overriding of the aorta, and right ventricular hypertrophy.
• Severe obstruction to pulmonary blood flow produces a right-to-left shunt, causing systemic hypoxemia, cyanosis, and tissue hypoxia.