For you created my inmost being; you knit me together in my mother's womb. I praise you because I am fearfully and wonderfully made; your works are wonderful, I know that full well. Psalms 139:13-14 (NIV)
Fetal Circulatory System
Before birth, the placenta supplies oxygen and nutrients to the developing fetus, as well as removing waste products. While in the womb, a baby's lungs are collapsed and filled with fetal-lung fluid (this is not the same as the amniotic fluid that surrounds the baby in the womb). The pressure of this fluid on the blood vessels prevents much blood from being able to enter the lungs, so they only receive the amount necessary to provide nutrients. While all the blood in an adult must pass through the lungs to receive oxygen, this is not necessary for the fetus because before birth blood is oxygenated in the placenta, not the lungs. Remarkably, only 5 percent of the baby's total blood volume reaches the lungs, while 50 percent of their blood is being oxygenated in the placenta at all times.
A baby does not breath inside the womb; the exchange of carbon dioxide and oxygen occurs in the placenta. However, babies do go through the breathing motions. This makes them appear to be "breathing" the amniotic fluid into their lungs, but they are actually shifting the fetal-lung fluid within their lungs.
There are three differences, called shunts, between the circulatory system of a fetus and that of an adult (see image at right). These three shunts cause the blood to flow through a different pathway. The ductus arteriosus connects the pulmonary artery to the aorta, the ductus venosus connects the umbilical vein to the inferior vena cava, and the foramen ovale allows blood to go directly from the right atrium to the left atrium. The pathway of blood through the fetal circulation system is as follows:
- In the placenta, the mother's blood brings nutrients to the fetal blood and carries away waste products. This happens through diffusion across the placental membrane. The umbilical cord, containing the umbilical vein and arteries, connects the fetus to the placenta of the mother. It enters the fetus at the umbilical ring and brings the newly oxygenated blood into the developing baby. The umbilical vein carries oxygen-rich blood into the fetus.
- The vein divides into two branches before reaching the liver. The larger branch continues into the liver, while the smaller branch, called the ductus venosus, shunts oxygenated blood directly into the inferior vena cava. After traveling through the liver, the rest of the blood enters the inferior vena cava through the hepatic veins.
- Well-oxygenated blood from the inferior vena cava enters the right atrium (the upper right chamber of the heart). The fetal circulation system differs from an adult's circulatory system in that most of the blood flows into the left atrium through a hole called the foramen ovale. A valve called the septum primum covers the foramen ovale on the left side of the atrial septum (the wall that separates right atrium from the left atrium) to stop blood from flowing back into the right atrium.
- A small amount of low-oxygen blood returning from the pulmonary veins enters the left ventricle and mixes with the blood flowing through the foramen ovale.
- Blood flows from the left atrium through the left ventricle into the ascending aorta to be carried to the head and upper body. Thus the well-oxygenated blood supplies nutrients to the brain, where they are most necessary.
- A small portion of the well-oxygenated blood from the inferior vena cava follows the same pathway as in an adult, entering the right ventricle along with most of the deoxygenated blood from the superior vena cava.
- The mostly deoxygenated blood in the right ventricle is pumped into the pulmonary artery that leads to the lungs. En route to the lungs, the ductus arteriosus diverts even more blood, sending it directly into the descending aorta and bypassing the lungs. The blood that reaches the lungs in the fetal circulation system is only enough to supply nutrients to the lungs.
- Some of the blood in the descending aorta circulates throughout the lower body, while the rest returns to the placenta via the umbilical arteries to exchange waste products for oxygen and nutrients.
Changes at Birth
Baby's first breath
Before birth, a baby's body begins preparing to make the change from receiving oxygen in the placenta to breathing oxygen into the lungs. At about week 35 of gestation, the alveolar cells in the lungs start producing surfactant (surface active material), which will prevent the alveoli from collapsing when the baby exhales. During labor, hormones and other factors cause fluid to stop being produced in the baby's lungs.
As the baby goes through the birth canal, its chest is compacted (the ribs are cartilaginous at this point), forcing 5 to 10 milliliters of fluid out of the lungs. When the baby emerges, the release of pressure on the chest causes air to be drawn into the lungs. However, the amount of air is not enough to fill the alveoli; the baby must accomplish that on its own.
After being born, a baby's first few breaths are shallow and irregular because the lungs still contain some fluid. However, as the circulatory and lymphatic systems remove the fluid and clear the lungs, breathing becomes easier.
When the baby draws its first breath, several remarkable changes take place. The alveoli in the lungs open and fill with air, reducing the pressure in the pulmonary tissue. Blood from the right ventricle flows more quickly into the dilated capillaries of the lungs, decreasing the pressure in the right atrium. The pulmonary arteries stretch and their walls become thinner to accommodate the greater volume of blood. The pressure in the aorta increases and becomes greater than the pressure in the pulmonary arteries, preventing blood from entering the aorta through the ductus arteriosus. Upon the inflation of the alveoli, the lungs release a substance called bradykinin, which causes contraction in smooth muscle and is thought to be partially responsible for contraction of the ductus arteriosus. The ductus arteriosus begins to contract when the percent of oxygen in the blood reaches about 50 mm Hg. After a few days, the ductus arteriosus, no longer needed to divert blood from the lungs, closes off completely.
The greater amount of blood flowing into the lungs returns to the heart via the pulmonary veins and enters the left atrium, causing increased pressure in the left atrium. The pressure is now higher in the left atrium than in the right atrium, forcing the septum primum against the atrial septum. This closes the foramen ovale and separates the heart into left and right sides. With the foramen ovale closed off, all the blood entering the right atrium must enter the right ventricle and be pumped into the pulmonary arteries.
Cutting the umbilical cord
When the baby is born, the muscles surrounding the umbilical vein and arteries constrict. The umbilical arteries (which carry blood to the placenta) close off first, and the umbilical vein (which carries blood from the placenta to the baby) closes off soon afterward, allowing nearly all the blood in the placenta to be transferred to the baby. Doctors wait for a few minutes after birth to cut the umbilical cord to ensure that this process takes place.
After the umbilical cord is cut, a quick transition must be made. The baby can no longer receive oxygen from the mother and must begin oxygenating blood in the lungs. An increased level of carbon dioxide, lower temperatures, and possibly certain other factors all prompt the baby to take its first breath, despite the difficulty of inhaling air into lungs full of fluid. Removal of the placenta from the circulatory pathway increases the baby's blood pressure. The umbilical arteries constrict to prevent blood loss and, after allowing a few minutes for blood to be transferred from the placenta to the baby, the umbilical cord is cut and tied off.
Cutting the umbilical cord also increases vascular resistance in the systemic vascular system, which causes the ductus venosus to close.
Adult Forms of Fetal Vascular Structures
Ductus arteriosus: Within 1-2 days of birth, the ductus arteriosus constricts so that no more blood can flow through it. Within a few weeks, new connective tissue replaces the ductus arteriosus and it becomes known as the ligamentum arteriosum.
Foramen ovale: The foramen ovale closes when pressure in the left atrium increases to exceed the pressure in the right atrium. The pressure change forces the foramen ovale against the atrial septum, preventing blood from passing into the left atrium from the right ventricle and separating the heart into right and left halves. The remnant of the foramen ovale is known as the fossa ovalis.
Ductus venosus: The ductus venosus constricts and closes soon after birth. It becomes the ligamentum venosum, a cord of connective tissue that attaches to the liver.
Umbilical cord: Once the doctor cuts the umbilical cord, the umbilical vein and arteries constrict to prevent blood loss. The cut umbilical cord becomes the belly button. For the first three weeks or so, the newborn baby has an umbilical stump, which should be allowed to dry and fall off on its own. Some doctors recommend cleaning the umbilical stump with rubbing alcohol to prevent infection.
After the umbilical cord is cut, the blood in the umbilical vein and arteries clots, closing off the blood vessels. The remnant of the umbilical vein is the ligamentum teres hepatis, which develops within months of the baby's birth. The umbilical arteries become the medial umbilical ligaments.
Related Heart Defects
Several congenital (present at birth) heart defects can occur if one of the blood shunts in the fetal circulation system does not close properly after birth (a shunt that remains open is said to be patent).
Patent Ductus Venosus (PDV)
A patent ductus venosus is uncommon, being very rare in children and even more so in adults. It can contribute to portosystemic encephalopathy, a central nervous system disorder believed to be the result of liver problems and/or liver failure. Once diagnosed, PDV can be treated using a catheter, surgery, or other more conservative methods.
Patent Ductus Arteriosus (PDA)
If the ductus arteriosus remains open (patent) after birth, it results in abnormal blood flow between the aorta and the pulmonary artery. This allows oxygenated blood from the aorta to mix with oxygen-poor blood in the pulmonary arteries. A small PDA may not have any damaging effects on the baby other than a heart murmur. However, large PDAs are more serious and put extra strain on the heart and lungs. The lungs may become congested and the child may experience difficulty breathing. In addition, the extra blood flowing to the lungs increases pressure in the lung's blood vessels, which may lead to permanent damage if not corrected.
If the PDA is small, it may close on its own and not require treatment. Medication has been effective in treating some premature babies with PDAs. However, for larger PDAs, catheterization or surgery can be used to close off the PDA. Doctors can use a catheter (long thin tube) to insert a device that will block the PDA, or they may surgically close the PDA with a clamp or suture.
Patent Foramen Ovale (PFO)
If the foramen ovale does not properly close after birth, the valve may continue to open when the chest is subjected to pressure, such as while coughing or sneezing. This means that blood can flow between the right and left atrium. People with PFO may have an increased risk of stroke and migraines. In fact, 40 percent of patients who have a stroke with no known cause are diagnosed for PFO. A patent foramen ovale may also be linked to an atrial septal aneurysm, in which the atrial septum is more mobile than normal.
Many people with PFO have no symptoms, but it can be detected by an echocardiogram or a transesophageal echo. If there are no problems resulting from the PFO, treatment may not be needed. However, if the PFO leads to serious problems such as a stroke, a doctor can close it through catheterization or through surgery. Medication may also be prescribed to treat the PFO.
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