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Normal Fetal Circulation
Umbilical Arteries and Vein
The umbilicus has two umbilical arteries and one umbilical vein. Fetal blood goes into the placenta through umbilical arteries (58% O2 saturation). These arteries further divide into chorionic arteries in the chorionic villi where exchange of substances takes place. After exchange, oxygenated blood moves from placenta to the fetal circulation through umbilical vein (80% O2 saturation).
What is Ductus Venosus?
Further this blood moves towards the liver where sinusoids receive a small amount of the blood and most of the blood goes into inferior vena cava through ductus venosus (connection between umbilical vein and IVC). A little mixing of the blood takes place in the liver.
What is Foramen Ovale?
As inferior vena cava receives blood from the lower part of the body so mixing of blood also takes place in the inferior vena cava. Blood goes into the right atrium where blood again mixes with less oxygenated blood because right atrium also receives the less oxygenated blood from superior vena cava.
Most of the blood goes from right atrium into left atrium through an opening called foramen ovale; present in septum secundum. Left atrium also receives very little amount of blood coming from lungs so mixing of blood also takes place in the left atrium and it pushes the blood into left ventricle and eventually blood goes into aorta.
What is Ductus Arteriosus?
Some blood also goes from right atrium to right ventricle and the pulmonary arteries. As fetal lungs are not functional, all the blood cannot go to the pulmonary vasculature. Collapsed fetal lungs and compressed pulmonary vasculature provide more resistance to the incoming blood, so little blood goes to the lungs. Due to this reason, most of the blood is pushed from pulmonary artery into aorta through a special connection called ductus arteriosus, due to which mixing of the blood takes place in the aorta.
Cerebral arteries and coronary arteries originate from the early part of the aorta (early to ductus arteriosus) so these arteries have highly oxygenated blood. Aorta moves further and supplies the blood to the body and blood eventually reaches to umbilical arteries.
Sites Where Mixing of Blood Takes Place
Inferior vena cava.
Changes in Fetal Circulation After Birth
There are following changes which take place after birth:
Closure of umbilical arteries.
Closure of umbilical vein.
Closure of ductus venosus.
Closure of ductus arteriosus.
Closure of Foramen Ovale.
Closure of Umbilical Vessels
There are two changes which take place after birth; baby starts breathing by its own respiratory system and baby is disconnected from the placenta.
After cutting the umbilical cord, first umbilical arteries constrict then immediately umbilical vein constricts so that baby can receive more blood from placenta
Closure of Ductus Arteriosus
After first baby’s cry, lungs expansion leads to opening of pulmonary vasculature resulting in less resistance to the blood flow. So blood starts going to pulmonary system and oxygenation takes place in the lungs. Well oxygenated blood moves from the lungs into left atrium, left ventricle and eventually into aorta. Now aorta becomes a relatively high pressure system as compared to pulmonary system. Due to high pressure, well oxygenated blood moves from aorta to pulmonary system through ductus arteriosus.
Closure of Ductus Arteriosus Takes Place by Following Mechanisms:
When blood with high oxygen content passes through the ductus arteriosus, it causes the constriction of smooth muscles because ductus arteriosus is very sensitive to oxygen.
After birth when lungs undergo their first inflation, they release bradykinin that may lead to constriction of ductus arteriosus
Before birth the cells in ductus arteriosus produce PG-E2 at the low oxygen tension to keep the ductus arteriosus open, but after birth when well oxygenated blood passes through the ductus arteriosus, production of PG-E2 becomes less, relaxation failure of smooth muscles occurs and eventually closure of ductus arteriosus takes place.
Closure of Foramen Ovale
After birth when oxygenated blood moves from the lungs into left atrium, this increases the pressure in left atrium. At the same time, right atrium receives less blood due to the constriction of umbilical vein so right atrial pressure is less than left atrial pressure. Due to this pressure difference blood moves from left atrium to right atrium and pushes the septum primum on the foramen ovale. At the end there is a little depression in the wall that is called fossa ovalis.
Initially, all the formen are closed temporarily and eventually they become fibrotic after many months. For example; foramen ovale is anatomically closed after one year of time. In 20% population, it never closes completely but has little communication.
C- Postnatal Remnants of Fetal Circulatory Components
There are five postnatal remnants of fetal circulatory components:
Ligamentum teres: Umblical vein, after closure, becomes fibrotic and called ligamentum teres.
Ligamentum venosum: Ductus venosus closes and called ligamentum venosum.
Fossa ovalis: after closure of foramen ovale, a depression on the wall is called fossa ovalis.
Ligamentum arteriosum: Ductus arteriosus when becomes fibrotic is called ligamentum arteriosum.
Medial umbilical ligament: After birth, some part of the umbilical artery works as an internal iliac artery and supply the blood to urinary bladder through superior vesical artery and the remaining part of the umbilical arteries become fibrotic; called medial umbilical ligament.
What is Patent Ductus Arteriosus (PDA) and Treatment?
Patent Ductus Arteriosus
In premature babies ductus arteriosus does not close and due to high blood pressure in aorta, blood keeps on moving from aorta into pulmonary artery and lungs become overloaded with blood. This abnormal blood flow causes a murmur that changes its intensity during systole and diastole. As this murmur is heard during whole cardiac cycle with different intensity so this is called machinery like murmur.
Medical: Indomethacine. It inhibits the production of PG-E2 that may cause the closure of ductus arteriosus.
Surgical: Ligation of PDA.