Fetal circulation and respiration

1 Fetal Heart and Placenta

Development of heart is completed at fourth week of intrauterine life, and it starts beating at the rate of 65 per minute. Along with heart, the blood vessels also develop.

Heart rate gradually increases and reaches the maximum rate of about 140 beats per minute just before birth.

Fetal circulation is different from adult circulation because of the presence of placenta.

Since fetal lungs are nonfunctioning, placenta is responsible for exchange of gases between fetal blood and mother’s blood. So, the blood from right ventricle is diverted to placenta.

Fetus is connected with the mother through placenta. Fetal blood passes to placenta through umbilical vessels and the maternal blood runs through uterine vessels.

Both the sets of blood vessels lie in close proximity in placenta, through which exchange of substances takes place between mother’s blood and fetal blood.

However, there is no direct admixture of maternal and fetal blood.

2 Blood Vessels in Fetus

Due to nonfunctioning of fetal lungs, fetal heart pumps a large quantity of blood into the placenta for exchange of substances.

Blood from the Placenta

From placenta, umbilical veins collect the blood, which has more oxygen and nutrients.

Umbilical vein passes through liver. Some amount of blood is supplied to liver from umbilical vein.

However, a large quantity of blood is diverted from umbilical vein into the inferior vena cava through ductus venosus.

Liver receives blood from portal vein also.

In liver, oxygenated blood mixes slightly with deoxygenated blood and enters the right atrium via inferior vena cava.

From right atrium, major portion of blood is diverted into left atrium via foramen ovale.

Foramen ovale is an opening in intra-atrial septum.

Blood from the Upper Part of Body

Blood from upper part of body enters the right atrium through superior vena cava.

From right atrium, blood enters right ventricle. From here, blood is pumped into pulmonary artery.

From pulmonary artery, blood enters the systemic aorta through ductus arteriosus.

Only a small quantity of blood is supplied to fetal lungs.

Blood from left ventricle is pumped into aorta. 50% of blood from aorta reaches the placenta through umbilical arteries.

Umbilical vein → Liver → Ductus venosus → Inferior vena cava → Right atrium → Foramen ovale → Left atrium

Superior vena cava → Right atrium → Right ventricle → Pulmonary artery → Ductus arteriosus → Systemic aorta

3 Fetal Lungs

Pulmonary vascular resistance is the resistance offered to blood flow through pulmonary vascular bed.

This resistance is very high in fetus because of the nonfunctioning of fetal lungs.

High resistance in fetal lungs increases the pressure in blood vessels of lungs.

Because of high pressure, the blood is diverted from pulmonary artery into aorta via ductus arteriosus.

4 First Breath of the Child

When fetus is delivered and umbilical cord is cut and tied, the lungs start functioning.

When placental blood flow is cut off, there is sudden hypoxia and hypercapnia.

Now, the respiratory center is strongly stimulated by these two factors and respiration starts.

Initially, there is gasping, which is followed by normal respiration.

Cutting off placental blood flow → Hypoxia + Hypercapnia → Stimulation of respiratory center → Gasping → Normal respiration

5 Flow of Blood to Lungs

Lungs expand during the first breath of the infant.

Expansion of lungs causes immediate reduction in the pulmonary vascular resistance and a sudden fall in pressure in the blood vessels of lungs.

Therefore, the blood flow from pulmonary artery to lungs increases.

Change after the first breath:
Expansion of lungs → Reduced pulmonary vascular resistance → Fall in pressure in pulmonary blood vessels → Increased blood flow from pulmonary artery to lungs.

6 Closure of Foramen Ovale

When blood starts flowing through pulmonary circulation, the oxygenated blood from lungs returns to left atrium.

It causes increase in left atrial pressure.

Simultaneously, due to stoppage of blood from placenta, pressure in inferior vena cava is decreased.

It leads to fall in right atrial pressure.

Left atrial pressure ↑ | Right atrial pressure ↓

Thus, the pressure in right atrium is less and pressure in left atrium is already high.

This causes the closure of foramen ovale.

Within few days after birth, the foramen ovale closes completely and fuses with atrial wall.

7 Reversal of Blood Flow in Ductus Arteriosus

In fetus, since pulmonary arterial pressure is very high, the blood passes from pulmonary artery into aorta via ductus arteriosus.

However, in neonatal life, since systemic arterial pressure is more than pulmonary arterial pressure, the blood passes in opposite direction in ductus arteriosus, i.e., from systemic aorta into pulmonary aorta.

Fetal life: Pulmonary artery → Aorta through ductus arteriosus

Neonatal life: Systemic aorta → Pulmonary aorta through ductus arteriosus

The reversed flow in ductus arteriosus is heard as continuous murmur in infants.

8 Closure of Ductus Venosus

Due to contraction of smooth muscle near junction between umbilical vein and ductus venosus, the constriction and closure of ductus venosus occurs.

Later, the ductus venosus becomes a fibrous band.

9 Closure of Ductus Arteriosus

Ductus arteriosus starts closing due to narrowing.

It closes completely after 2 days and the adult type of circulation starts.

In some rare cases, the ductus arteriosus does not close.

It remains intact producing a continuous murmur.

The condition with intact ductus arteriosus is known as patent ductus arteriosus.