Fetal Circulation Physiology and Features by Nurses Note

 Fetal Circulation

Circulation in fetus (before birth) differs significantly from circulation in postnatal life. The main difference is that the fetal lungs are functionally inactive and fetus derives oxygen and nutrients from the placenta.

Design of Fetal Circulation

Fetus receives blood supply from the placenta through umbilical veins. 

1. This blood is only 80% saturated with oxygen as placenta has extracted some of its oxygen. The blood from the umbilical veins enters the liver, and some amount bypasses the liver and enters the inferior vena cava through the ductus venosus. Blood also comes from other sources. 

2. The inferior vena cava drains into right atrium. About 50% of blood from the right atrium enters the left atrium through the foramen ovale, which is located in the interatrial septum. Another 50% of blood from right atrium enters into the right ventricle, from where it is pumped into the pulmonary artery. 

3. As the pulmonary vascular resistance is very high(because fetal lungs are collapsed), only a small quantity of blood reaches left atrium through the pulmonary veins. 

4. The main bulk of the blood from the right ventricle directly enters the aorta through ductus arteriosus. 

5. The blood in the aorta is then distributed to the whole body.

6. However, the umbilical artery, which arises from the aorta, transports blood to the placenta, where it is oxygenated and sent back to the fetus via fetal veins. Thus, placenta functions like lungs during fetal life.

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Special Features of Fetal Circulation

1. Right ventricle pumps blood into pulmonary artery against higher resistance as the pulmonary vascular resistance is very high.

2. Most part of the right ventricular output enters directly into the aorta through the ductus arteriosus.

3. The right ventricle receives blood from the placenta and pumps into the aorta from where about 60% of the blood goes to the placenta for oxygenation and only 40% is distributed to the different parts of the body. This occurs because peripheral resistance of the fetal vessels is high. Whereas resistance of the placenta is relatively low.

4. About 50% of the blood from the right atrium directly enters into the left atrium via foramen ovale.

5. Oxygen saturation of the fetal arterial blood is much lower than that of the adult. However, fetal tissues are highly resistant to the effect of hypoxia. The fetal hemoglobin also has higher affinity to the oxygen.

6. In fetus, the cardiovascular regulatory mechanisms operate mainly by the local factors. The baroreceptor and chemoreceptor reflexes develop at about 30th week of intrauterine life.

 Fetal Circulation Changes Occurring at Birth

1. Closure of the umbilical vessel ceases blood flow through the umbilical veins. This results in closure of ductus venosus.

2. The clamping of umbilical vessels immediately after birth causes asphyxia that activates respiratory center. Respiratory signal is generated. This results in expansion of the lungs. As the lung fills with air, the pulmonary vascular resistance decreases significantly.

3. Closure of the umbilical vessels increases the total peripheral resistance and blood pressure. The left atrial reserve is raised above the volume of blood present in inferior vena cava and right atrium. This occurs because:

 i. The decreased pulmonary resistance increases blood flow through the lungs to the left atrium,

 ii. The decreased flow of blood to the right atrium occurs due to closure of umbilical vein, and,

iii. Occlusion of umbilical artery increases the resistance to the left ventricular output.

The change in pressure gradient across the atria abruptly closes the valve of the foramen ovale, and gradually the interatrial septum closes.

4. The pulmonary arterial pressure decreases significantly due to decreased pulmonary vascular resistance. This along with increased aortic pressure reverses the flow of blood through the ductus arteriosus. However, constriction of ductus arteriosus begins within few minutes and is completely closed within 1–2 days after birth.

5. Before birth, the thickness of both ventricles is equal. After birth, thickness of the right ventricular wall decreases, whereas left ventricular thickness increases.

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Clinical Importance of Fetal Circulation

1. If the opening between the two atria (foramen ovale) does not close after birth, the condition is called atrial septal defect (patent foramen ovale). This results in mixing of oxygenated blood from the left atrium with that of deoxygenated blood in the right atrium. Thus, oxygen supply decreases to the tissues due to left to right shunt.

2. The failure of the closure of ductus arteriosus results in flow of blood from the aorta to the pulmonary artery, as the pressure in the aorta is more. This condition is called patent ductus arteriosus. This is also an example of left to right shunt. Oxygenation of the tissues of the body decreases in this condition.

3. A condition known as tetralogy of Fallot occurs rarely in which there are four cardiovascular defects: ventricular septal defect, pulmonary stenosis, right ventricular hypertrophy, and overriding of the aorta. Due to ventricular septal defect and pulmonary stenosis, the oxygenation of blood is grossly reduced. Therefore, tissues suffer from severe hypoxia, and cyanosis develops.