System of blood vessels and special structures through which blood moves in the fetus. The fetus is attached to the mother’s placenta through the umbilical cord. Maternal oxygenated blood from the placenta travels through the umbilical vein to the liver of the fetus and from there through the inferior vena cava to the right atrium of the heart. It then passes through the foramen ovale into the left atrium and from there to the left ventricle and circulation through the head and upper body parts. The returning blood flows through the superior vena cava into the right atrium, from which at low pressure it flows into the right ventricle and then through the pulmonary artery to the descending aorta and circulation through the lower body parts. The wastes of the fetus are carried by the blood through the umbilical arteries back to the placenta, where they diffuse into the mother’s bloodstream for eventual excretion. This system, which differs in many ways from the circulatory path in an adult, allows oxygen and nutrients to be supplied to the fetus and wastes to be carried away. At birth several changes occur, including closure (at least partial) of the foramen ovale so that blood no longer flows from the right atrium to the left atrium. Circulation through the baby’s lungs begins with the first breath.
The course of the flow of blood in a fetus. Oxygenated in the placenta, blood passes through the umbilical vein and ductus venosus to the inferior vena cava and thence to the right atrium. It then follows one of two courses: through the foramen ovale to the left atrium and thence through the aorta to the tissues, or through the right ventricle, pulmonary artery, and ductus arteriosus to the aorta and thence to the tissues. In either case the blood bypasses the lungs, which do not function before birth. Blood returns to the placenta through the umbilical arteries, which are continuations of the hypogastric arteries. At birth or shortly after, the ductus arteriosus and the foramen ovale close, establishing the postpartum circulation. If either fails to close, the baby may be hypoxemic.
The blood circulation in a fetus differs from the circulation after birth (refer to the circulatory system). While in the womb, the fetus does not breathe or consume food directly. Instead, it receives oxygen and nutrients from the mother’s blood through the placenta and umbilical cord. Another crucial distinction is that fetal blood bypasses the lungs.
The placenta, an organ attached to the inner lining and wall of the uterus, serves as the gateway for oxygen and nutrients to enter the fetal blood. It is connected to the fetus through the umbilical cord. Within the placenta, a thin membrane separates the maternal and fetal circulations, enabling the exchange of nutrients and waste products between them.
Oxygenated and nutrient-rich blood travels from the mother to the fetus through a vein in the umbilical cord, entering the right atrium (upper chamber) of the heart. Instead of flowing to the lungs, it bypasses them through an opening called the foramen ovale, entering the left atrium. Subsequently, the blood moves to the left ventricle (lower chamber) and is pumped to the upper parts of the body, providing oxygen to the tissues.
Upon returning to the heart, the blood enters the right atrium and then moves into the right ventricle. During this stage in the fetus, the blood is only partially deoxygenated and must supply more tissues with oxygen. By avoiding the lungs once more, it travels from the pulmonary artery to the aorta through a passage called the ductus arteriosus. The aorta carries this blood to the lower parts of the body. Subsequently, the fully deoxygenated blood is transported via the umbilical cord to the placenta, where carbon dioxide and other waste products diffuse into the mother’s blood.
Following birth, both the foramen ovale and the ductus arteriosus typically close as part of the normal process. Blood pumped from the right ventricle flows through the pulmonary artery to the lungs, where it undergoes reoxygenation and gets rid of carbon dioxide and other waste products. However, in rare instances, the foramen ovale or ductus arteriosus may fail to close properly after birth, resulting in a congenital heart disorder.
The flow of blood in the developing fetus through the umbilical cord and the placenta.