Fetal Circulation In Utero

Overview:

Fetal circulation in utero involves several key shunts that bypass the non-functional fetal lungs. The placenta provides oxygenated blood to the fetus via the umbilical vein, which then passes through the ductus venosus to the inferior vena cava, mixing with deoxygenated blood from the fetal body. The foramen ovale allows blood to flow directly from the right atrium to the left atrium, bypassing the lungs, while the ductus arteriosus connects the pulmonary artery to the aorta, further diverting blood away from the lungs. These shunts close after birth as the newborn begins to breathe air, initiating normal postnatal circulation.

1. Fetal Circulation In Utero - Overview

In utero, the developing baby receives oxygen from the mother through a specialized process facilitated by the placenta. The placenta plays a crucial role in exchanging oxygen and other gases between the maternal and fetal circulatory systems. Here's an summary of how the baby gets oxygen in utero: 

• Oxygen Exchange at the Placenta: 

• The placenta is an organ that develops during pregnancy, attached to the uterine wall and connected to the baby via the umbilical cord. 

• Oxygen and other nutrients from the mother's blood diffuse across the placental membrane into the fetal blood, while carbon dioxide and waste products move from the fetal blood to the maternal blood. 

• Umbilical Cord: 

• The umbilical cord is a flexible tube-like structure containing blood vessels (two arteries and one vein) that connect the baby to the placenta. 

• The umbilical vein carries oxygenated blood from the placenta to the baby, supplying the developing fetus with the oxygen necessary for growth and development. 

• Oxygen-Rich Blood Flow: 

• Oxygen-rich blood from the placenta flows through the umbilical vein and into the baby's liver. 

• A specialized vessel called the ductus venosus allows some of this blood to bypass the liver and enter the inferior vena cava, which carries the oxygenated blood toward the baby's heart. 

• Fetal Circulatory Pathways: 

• Blood entering the right atrium of the baby's heart is directed to the left atrium through a temporary opening known as the foramen ovale. 

• From the left atrium, the oxygenated blood is pumped into the left ventricle and then into the aorta, supplying the rest of the fetal body with oxygen. 

• Gas Exchange in Fetal Lungs: 

• While the baby's lungs are present in utero, they are not fully functional for breathing. Therefore, the majority of oxygenation occurs through the placenta, and only a small portion of blood flows through the fetal lungs. 

• The majority of blood in the pulmonary arteries is shunted away from the lungs through the ductus arteriosus, connecting the pulmonary artery to the aorta. 

• Return of Deoxygenated Blood: 

• Deoxygenated blood returns to the placenta through the two umbilical arteries, where it releases waste products and picks up more oxygen for the cycle to continue. 

This process ensures that the developing fetus receives a continuous supply of oxygen and nutrients required for growth and development. The efficient exchange of gases at the placenta allows the baby to obtain oxygen from the mother's bloodstream while maintaining a separate and distinct circulatory system. This specialized fetal circulation supports the oxygen needs of the developing fetus until the transition to independent postnatal breathing occurs at birth. 

2. The Two Cardiac Shunts: 

• Connection between the right and left atria via foramen ovale. 

• Connection between the pulmonary artery and the aorta via the ductus arteriosus. 

3. Fetal Circulation: 

Umbilical Vein 

• In the fetal circulatory system, the umbilical vein transports blood rich in 02 and nutrients from the placenta to the fetal body. 

• The umbilical vein enters the body through the umbilical ring and travels along the anterior abdominal wall to the liver. 

• About ½ the blood it carries passes into the liver. 

• The other ½ of the blood enters a vessel called the ductus venosus which bypasses the liver. 

Ductus Venosus 

• The ductus venosus travels a short distance and joins the inferior vena cava. 

• There, the oxygenated blood from the placenta is mixed with the deoxygenated blood from the lower parts of the body. 

• This mixture continues through the vena cava to the right atrium. 

Pulmonary Flow Differences 

• In the heart, blood flows from the right atrium to the right ventricle then through the pulmonary arteries to the lungs. 

• In the fetus however, the lungs are nonfunctional, and the blood largely bypasses them. 

Foramen Ovale 

• As the blood from the inferior vena cava enters the right atrium, a large proportion of it is shunted directly into the left atrium through the opening called the foramen ovale. 

• A small valve, septum primum, is located on the left side of the atrial septum overlies the foramen ovale and helps prevent blood from moving in the reverse direction.  

Fetal Pulmonary Flow 

• The rest of the fetal blood entering the right atrium, including a large portion of the deoxygenated blood entering from the superior vena cava passes into the right ventricle and out through the pulmonary trunk. 

• Only a small volume of blood enters the pulmonary circuit, because the lungs are collapsed, and their blood vessels have a high resistance to flow. 

• Enough blood reaches the lung tissue to sustain them, approximately 10%. 

Ductus Arteriosus 

• Most of the blood in the pulmonary trunk bypasses the lungs by entering a fetal vessel called the ductus arteriosus which connects the pulmonary trunk to the descending portion of the aortic arch. 

• As a result of this connection, the blood with a relatively low 02 concentrations which is returning to the heart through the superior vena cava, bypasses the lungs. 

Aorta: 

• The more highly oxygenated blood that enters the left atrium through the foramen ovale is mixed with a small amount of deoxygenated blood returning from the pulmonary veins. 

• This mixture moves into the left ventricle and is pumped into the aorta. 

• Some of it reaches the myocardium through the coronary arteries and some reaches the brain through the carotid arteries. 

Umbilical Arteries 

• The blood carried by the descending aorta is partially oxygenated and partially deoxygenated. 

• Some of it is carried into the branches of the aorta that lead to various parts of the lower regions of the body. 

• The rest passes into the umbilical arteries, which branches from the internal iliac arteries and lead to the placenta. 

• There the blood is reoxygenated.