Need help with OB/Fetal Circulation

VickyRN, MSN, DNP, RN · Jan 25, 2006

Fetal circulation differs from neonatal circulation in three areas: the process of gas exchange, the pressures within the systemic and pulmonary circulations, and the existence of anatomic structures (shunts) that assist in the delivery of oxygen-rich blood to vital organ systems.

You may recall that the process of growth and development in the fetus and neonate is cephalocaudal (head first) and proximodistal (from the center out to the periphery). The object of fetal circulation is to supply oxygenated blood as quickly and efficiently as possible first to the fetal brain and heart.

Process of gas exchange:

In fetal circulation, gas exchange occurs in the placenta; the lungs are nonfunctional with very little blood flow. The placenta provides oxygen and nutrients for the fetus, and removes carbon dioxide and other waste products. The mother and fetus’ blood vessels intertwine but do not join (no mixture of blood). The exchange of oxygen, nutrients, and waste materials between the mother and fetus occurs by diffusion.

The umbilical cord connects the fetus to the placenta, and contains two small arteries and one large vein, providing for the transport of blood to and from the fetus and placenta. Oxygenated blood from the placenta flows by way of the umbilical vein to the inferior vena cava. Deoxygenated blood flows back to the placenta by way of the two umbilical arteries (arteries always carry blood "away" from heart).

Pressure Differences:

Oxygenated blood flow (from the placenta) is from right to left.

Oxygenated blood enters right side of fetal heart.

Blood pressure on the right side of the heart > blood pressure on the left side of the heart.

High resistance in fetal lungs (arterioles are constricted, alveoli are filled with fluid).

Resistance to blood flow in the lungs (pulmonary vascular resistance) > Resistance to blood flow in the body (systemic vascular resistance)

Fetal lungs don’t receive much blood.

Shunts: A shunt is a detour that directs blood flow in a different direction.

Ductus venosus: In utero, the fetus receives blood carrying oxygen and nutrients from the placenta through the umbilical vein. The liver is bypassed along the way, by the first fetal shunt—the ductus venosus. Blood routed to the ductus venosus bypasses the liver and is routed to the inferior vena cava. Only a small amount of blood circulates to hepatic tissue. Liver function is minimal in the fetus, so very little blood supply is required.

Foramen ovale: Oxygenated blood from the placenta flows into the right atrium. Two-thirds of the oxygenated blood flows directly to the head, heart, and upper extremities. The brain and coronary arteries receive the blood with the highest oxygen concentration.

In fetal circulation, the pressures on the right side of the heart are greater than pressures on the left side. The foramen ovale is an opening between the atria (top chambers of the heart) that allows blood flow from the right atrium directly to the left atrium. This is the second fetal shunt. Most of the blood received from the inferior vena cava is shunted through the foramen ovale and into the left atrium. From the left atrium, the blood is pumped through the left ventricle into the aorta.

The direction of the blood flow as well as the high blood pressure in the right atrium propels this blood through the foramen ovale into the left atrium.

Most of the lungs are bypassed, supplying the heart and the brain with oxygenated blood. These are the two most oxygen-needy organ systems.

Ductus arteriosus: The remainder of the less oxygenated blood now returns from the upper body, head, neck, and arms, and travels from the superior vena cava into the right atrium, where it mixes with some of the oxygen-rich blood coming up from the inferior vena cava. Because of the direction of the blood flow, the right ventricle pumps most of the blood returning from the superior vena cava. This blood travels through the tricuspid valve and the right ventricle into the pulmonary artery.

A very small portion of this blood flows from the right atrium into the right ventricle and out the pulmonary artery into the nonfunctioning lungs. The pressure in the pulmonary circulation is very high. The lungs are collapsed and filled with fluid. Because of the high pressure, most of the blood bypasses the lungs.

Most of the blood pumped out by the right ventricle bypasses the lungs by flowing through the third fetal shunt, the ductus ateriosus, into the descending aorta. This supplies blood to the lower portions of the body.

The ductus arteriosus is a conduit between the pulmonary artery and the aorta that shunts blood away from pulmonary circulation. Blood then flows from the descending aorta to the other organ systems, then back to the placenta for gas exchange by way of the two umbilical arteries.

Neonatal circulation:

With the neonate’s first breath, gas exchange is transferred from the placenta to the lungs. This means that the baby must use his/her lungs to take in oxygen and get rid of carbon dioxide. With the initiation of respirations, the Pa02 levels are increased and PaC02 levels are decreased. Inspired oxygen dilates the pulmonary vessels, and pulmonary vascular resistance is decreased. Because resistance in the lungs has decreased, blood flow into the lungs increases.

After the first breath, resistance to pulmonary blood flow decreases and a marked increase in pulmonary blood flow follows. Decreased pulmonary resistance (after the first breath) allows for the increased pulmonary blood flow. Resistance in the systemic circulation increases.

Umbilical cord clamped = less blood flow to right side of the heart. Pressures on the left side of the heart become greater than pressures on the right side. Systemic vascular resistance becomes greater than pulmonary vascular resistance. The right heart receives poorly oxygenated blood from the body, while the left heart receives highly oxygenated blood from the lungs. Deoxygenated blood flow starts flowing into the right heart and lungs. Because of these favorable pressure changes, more blood flows to the lungs, where it is oxygenated.

Hope this helps! :)