Umbilical arteries
⏭️ The two umbilical arteries carry blood from the fetus toward the placenta.
⏭️ This blood is relatively deoxygenated because oxygen has already been delivered to fetal tissues.
⏭️ The umbilical arteries arise from vessels associated with the fetal internal iliac arteries.
⏭️ At the placenta, fetal blood comes into close proximity with maternal blood, allowing exchange of oxygen, carbon dioxide, nutrients, and wastes.
⏭️ Maternal and fetal blood normally do not directly mix because exchange occurs across the placental membrane.
⏭️ Carbon dioxide and metabolic wastes move from fetal circulation toward the maternal circulation for elimination.
⏭️ The umbilical arteries therefore form an essential pathway connecting the fetus with the placental circulation.
⏭️ Remember that fetal circulation contains two umbilical arteries carrying blood away from the fetus.
Foramen ovale
⏭️ The foramen ovale is an opening between the fetal right and left atria.
⏭️ It allows oxygen-rich blood entering the right atrium to pass directly into the left atrium.
⏭️ This pathway helps blood largely bypass the fetal lungs, which are not responsible for oxygen exchange before birth.
⏭️ Blood entering the left atrium subsequently moves into the left ventricle and then into the aorta.
⏭️ This arrangement helps deliver relatively well-oxygenated blood to vital structures, particularly the brain and heart.
⏭️ The foramen ovale is necessary because fetal oxygenation occurs through the placenta rather than the lungs.
⏭️ After birth, changes in circulatory pressures normally promote functional closure of the foramen ovale.
⏭️ The foramen ovale is therefore one of the major fetal circulatory shunts.
Ductus venosus
⏭️ The ductus venosus is a fetal vessel that allows much of the oxygenated blood from the umbilical vein to bypass the liver.
⏭️ Oxygenated blood arriving from the placenta enters the fetus through the umbilical vein.
⏭️ Much of this blood passes through the ductus venosus toward the inferior vena cava.
⏭️ This pathway allows relatively oxygen-rich blood to reach the heart rapidly.
⏭️ Some umbilical venous blood still perfuses the fetal liver, but the ductus venosus provides an important hepatic bypass.
⏭️ The ductus venosus is one of the three major fetal shunts, together with the foramen ovale and ductus arteriosus.
⏭️ Following birth and interruption of placental circulation, the ductus venosus normally closes.
⏭️ Its fetal function is therefore to help direct oxygenated placental blood toward the central circulation.
Ductus arteriosus
⏭️ The ductus arteriosus connects the fetal pulmonary artery with the aorta.
⏭️ Because the fetal lungs are fluid-filled and have high vascular resistance, only a limited amount of blood needs to enter the pulmonary circulation.
⏭️ The ductus arteriosus allows most blood entering the pulmonary artery to be diverted into the aorta.
⏭️ This shunt therefore helps bypass the fetal lungs before birth.
⏭️ Blood entering the aorta is distributed to fetal tissues and eventually returns toward the placenta through the umbilical arteries.
⏭️ After birth, expansion of the lungs decreases pulmonary vascular resistance and contributes to major changes in circulatory pressures.
⏭️ Increased oxygen levels and other physiologic changes promote functional closure of the ductus arteriosus after delivery.
⏭️ The ductus arteriosus is therefore another essential fetal circulatory shunt.
Umbilical vein
⏭️ The single umbilical vein carries oxygen-rich blood from the placenta to the fetus.
⏭️ This vessel provides the fetus with its major supply of oxygen and nutrients obtained through placental exchange.
⏭️ The blood carried by the umbilical vein has a higher oxygen concentration than blood in the umbilical arteries.
⏭️ After entering the fetus, much of this blood is directed through the ductus venosus toward the inferior vena cava.
⏭️ The blood then reaches the right atrium, where fetal shunts help distribute relatively oxygenated blood to essential organs.
⏭️ The umbilical vein is unusual because it is a vein carrying oxygenated blood.
⏭️ Placental circulation replaces the fetal lungs as the site where blood receives oxygen before birth.
⏭️ The umbilical vein is therefore essential for transporting oxygenated placental blood to the developing fetus.