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Elmoselhi - Fetal Circulation
Fetal and Neonatal Circulation

Additional Physiology Flashcards




What is the role of the placenta?

*The placenta acts as the fetal lung during gestation.


*The placenta is less efficient than the lungs. Villi containing branches of the umbilical arteries and vein project into a lake of maternal blood. This is a less efficient blood flow pattern for gas exchange and the cell layer covering the villi is thicker and less permeable than that of the lungs.


*The less efficient system means that the fetus develops in a "hypoxic" environment.

What are the roles of the umbilical arteries and veins?

*to supply oxygen and remove carbon dioxide


*The umbilical arteries are equivalent to the pulmonary artery (venous blood) and the umbilical vein is equivalent to the pulmonary vein (arterial blood) - the values are very different.


*The PO2 gradient from maternal to fetal blood is very large because of the poor efficiency of the diffusion.


*The PCO2 and pH gradients between fetal and maternal blood are improved by maternal hyperventilation.

Explain oxygen uptake by the fetus

In spite of the low fetal blood PO2, the fetal blood has a reasonable oxygen concentration due to HbF having a lower P50 than HbA (HbF has a higher affinity for O2) and the fetus being polycythemic (increased RBC's).


*on the O2 equilibrium curve, the fetal curve is shifted to the left of the maternal curve because of the higher affinity of HbF

What are the roles of the three different shunts?

*The fetus has three shunts that basically ensure that the more oxygenated blood goes to the head and the less oxygenated blood goes to the trunk.


*Ductus venosus: brings oxygenated blood from placenta to heart, bypassing the liver.


*Foramen ovale: shunts oxygenated placental blood from inferior vena cava through the right atrium into the left atrium, then from left ventricle and aorta towards the head.


*Ductus arteriosus: Less oxygenated blood from the superior vena cava enters the right atrium, right ventricle and pulmonary artery and bypasses the lungs through the ductus arteriosus. This less oxygenated blood then enters the aorta and supplies the trunk.

Explain the sequence of fetal circulation

*umbilical vein -> ductus venous -> inferior vena cava or

portal vein -> inferior vena cava


inferior vena cava -> RA -> LA through the foramen ovale


LA -> LV -> ascending aorta



*superior vena cava -> RA -> RV -> pulmonary artery -> descending aorta through the ductus arteriosus - bypasses the lungs so the blood to the trunk is less oxygenated

What are the transitions that occur at birth?

*The major factors that convert the cardiorespiratory system from a fetal one to a newborn one involve converting the venous (right) side of the circulatory system into a low pressure system and the arterial (left) side into a high pressure system.


*Clamping the umbilical cord increases arterial vascular resistance and decreases venous return.


*Inflating the lungs decreases pulmonary arterial resistance and pressure, making the right ventricule a lower pressure pump than the left ventricle.


*To convert to the newborn pattern the shunts must close.

Explain the transition that occurs at the first breath

*The first breath involves a very significant negative pressure, expanding the lungs and lowering the pulmonary vascular resistance to 20% of fetal value.


*Many factors act as stimuli for the first breath, such as low PO2, high PCO2, catecholamines, increased chemoreceptors sensitivity, cooling, etc.

Explain the closure of the shunts after birth

*Ductus venosus: Closure in 1-3 hours, closure of smooth muscle - mechanism poorly understood.


*Foramen ovale: Decreased pulmonary vascular resistance lowers right atrial pressure and blood returning from the lungs raises pressure in left atrium, closing valve-like foramen ovale.


*Ductus arteriosus: Muscle contracts at birth, closure in 1-8 days, should be morphologically completely closed within 1-4 months. Rise in PaO2 at birth may be involved. Bradykinin constricts umbilical vessels, dilates pulmonary vessels and constricts the ductus arteriosus. Prostacyclin keeps ductus arteriosus open prior to birth - role in preventing pulmonary hypertension.

Explain patent ductus arteriosus

*left to right shunt - blood shunted from descending aorta to the pulmonary artery


*this increases the amount of blood that goes to lung and the left side of the heart, thus causing


a) pulmonary hypertension

b) pulmonary edema

c) hypertrophy of LA and LV


*the pulmonary hypertension leads to increased pressure in the right side of the heart and causes Eisenmerger syndrome


*Eisenmenger's syndrome is the reversal of a L to R shunt to a R to L shunt due to the change in right side pressure

-it is a cyanotic condition (blue skin b/c of deoxygenated blood)



Explain atrial septal defect

* foramen ovale does not close


*left to right shunt (LA to RA)


*acyanotic condition



a) hypertrophy of the RA and RV

b) pulmonary hypertension

c) Eisenmenger's syndrome (over time, reversal of the shunt causes cyanosis)

Explain the ventricular septal defect

*most common defect


*Left to right shunt (LV to RV)


*overloads RV then pulmonary circulation then LA and LV



a) pulmonary hypertension

b) hypertrophy of RV, LA, and LV

c) Eisenmenger's Syndrome - (over time shunt reverses to R to L and causes cyanosis)

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