15. Fetal Circulation
Persistent fetal circulation
A condition wherein newborns regress to fetal circulation patterns. This can be caused by stimuli that cause the pulmonary arterioles to constrict (ex: hypoxia, acidosis, cold) thereby increasing pulmonary resistance. PVR favors right to left shunting through foramen ovale and ductus arteriosus; but this time there is no placenta to provide oxygenation and the baby experiences trouble breathing and pulmonary hypertension.
Why is fetal circulation necessary?
A fetus's blood circulation is underdeveloped to oxygenate its own blood. Therefore, the placenta is a feto-maternal organ that oxygenates the blood. Fetal circulation is modified at birth and transitions into neonatal circulation.
fetal hemoglobin
A placental mammal fetus has fetal hemoglobin with higher affinity for oxygen than the mother's hemoglobin in the placenta. This is driven by the difference in oxygen partial pressures between fetus and mother. Notice the mother has higher partial pressure for oxygen, which drives oxygen to be unloaded into fetal tissues to and from the mom. Fetal hemoglobin can carry up to 30% more oxygen than adult hemoglobin.
Ductus arteriosus
Also divert blood from pulmonary to systemic circuit. Most of the blood enters the descending aorta via ductus arteriosus (ductus botalli), which connects the pulmonary trunk to the aorta (below the level of the subclavian). Blood in aorta provides lower levels of oxygen to caudal part of body and then enters umbilical arteries to be reoxygenated in placenta. This shunt arises from the distal part of the left 6th aortic arch passing to dorsal aorta.
What if the placental membrane is compromised?
An incomplete barrier only prevents large molecules from passing thru like bacteria, viruses, drugs (Heparin) and hormones. If compromised the rubella, toxoplasma gondii (HIV), treponema pallidum (Syphilis), plasmodium falciparum can be transferred from mother to fetus.
What happens at birth?
At birth, shunts have closed to allow proper neonatal circulation. When the baby takes their first breath, the lungs inflate causing the pulmonary vascular resistance to fall and there is increased blood flow to the lungs. The left atrial pressure increases and closes the foramen ovale, leaving the fossa ovalis. The ductus arteriosus also closes due to increase oxygen and decreased prostaglandin E.
eustachian valve (valve of the IVC)
At the lower posterior part of the right atrium and is the opening to the inferior vena cava. This valve is continuous with the limbus of the fossa ovalis. During prenatal life it directs more oxygenated blood from IVC towards and through foramen ovale.
Why are there fetal shunts?
Blood from the placenta is 80-85% oxygenated; however, mixture with less oxygenated blood at various points during fetal circulation reduces oxygen content. The lungs receive blood to keep the tissues alive, not for gas exchange because fetal lungs don't work.
How is fetal lung fluid expelled at birth?
By contractions thru the birth canal that squeezes the lungs, absorption of fluid by the lymphatic system and clearance of fluid by pulmonary capillaries.
How is the umbilical cord formed?
By the connecting stalk, yolk sac, and amnion between days 28-40. Blood from the embryo flows to the placenta through two umbilical arteries and returns via one umbilical vein for exchange of nutrients and wastes between mother and embryo.
Patent ductus arteriosus
Can lead to left-right shunting of blood; this is more common in girls, predisposed heart problems, down syndrome, maternal rubella infections and infants with neonatal distress syndrome.
The opening of pulmonary circulation and clamping of umbilical flow create what?
Changes in pressure and flow which cause the ductus arteriosus to constrict and the foramen ovale to close.
Ductus venosus
Connects the umbilical vein to the inferior vena cava, A slender "trumpet like" shunt that connects the intraabdominal umbilical vein to the inferior vena cava. A sphincter in the umbilical vein regulates umbilical blood flow thru liver sinusoids. Blood that flows to the liver passes thru sinusoids of the liver and enters the inferior vena cava via hepatic veins. Fetal cardiovascular system is adapted so that most highly oxygenated blood is delivered to myocardium and brain. In the inferior vena cava, notice oxygenated blood from ductus venosus flows separately from deoxygenated blood from lower extremities. The narrow diameter of the ductus venosus accelerated blood flow thru the IVC.
prostaglandin E
Decreased levels of this hormone are also associated with ductus arteriosus closure within 96 hours after birth.
Foramen ovale
Divert blood from pulmonary to systemic circuit. a one-way shunt for right-left oxygenated blood flow.
Lumen of umbilical vein
Does not disappear completely and can be cannulated in adults for the injection of contrast medium or chemotherapeutic drugs.
How is the internal iliac artery formed?
During the 5th week umbilical arteries are obliterated, and arteries form a new connection with the 5th lumbar intersegmental artery
Pre-eclampsia
Faulty remodeling of marternal arteries will lead to parts of placenta to degenerate and enter maternal blood stream. The baby may have low birth weight and the mother will suffer from maternal high blood pressure and proteinuria; this can progress to eclampsia which causes convulsions and death.
Chronic villi
Fiinger-like projections made of connective tissue derived from extraembryonic mesoderm surrounded by cytotrophoblasts and syncytiotrophoblasts. After week 3, the mesenchyme differentiated into blood vessels and cells forming a fetal arteriocapillary network that fuses with placental and umbilical vessels.
C section and lungs
If a C-section is performed, the lungs may not expand fully due to improper fluid removal and pulmonary disease may occur a couple days after birth.
When does placenta development begin?
Implantation of blastocysts around day 7-13.
paradoxical embolism
In adults, if right atrial pressure rise with pulmonary hypertension the foramen may open and allow a thrombus to go from right to left. A thrombus arising from venous circulation appears in the systemic circulation.
Ductus venosus closure
Loss of the umbilical blood supply causes the sphincter in the ductus venosus to constrict thereby diverting blood to the hepatic sinusoids. Closure of the ductus venosus becomes permanent after two to three weeks. Since the umbilical vein remains for a while in early infancy, it may be used for transfusion.
Placental membrane
Made by chronic villi, separating fetal and maternal blood so they don't mix. The fetal capillary endothelium and the underlying basement membrane are present in all mammals, for gas/nutrient exchange.
Patent foramen ovale
Neonates have a persistent foramen ovale; this is more common in females than males and often doesn't create a problem.
Chorion
Outermost layer of the two membranes surrounding the embryo; it forms the embryonic part of the placenta.
What occurs if there's folic acid deficiency, retinoic acid, and trisomy 18?
Persistent R umbilical vein might occur if the L umbilical becomes occluded. This can be detected with ultrasound and other anomalies.
What causes closure of foramen ovale?
Postnatal lung expansion and initiation of the pulmonary circulation reverses the atrial pressure gradient. The increase in pulmonary blood flow leads to a rise in pulmonary return to the LA. LA pressure becomes greater than RA pressure and forces the foramen ovale shut, leaving the fossa ovalis in the atrial septum.
How to treat patent ductus arteriosus?
Prostaglandin inhibitors such as Indomethacin close the ductus as well as acetylcholine, histamine and catecholamines are given to induce closure of the shunt.
Why does the lungs expand?
Pulmonary resistance is lowered (because nitric oxide dilates the vessels) leading to pressure changes. Also, increased partial pressure of oxygen when breathing of the neonate allows more blood flow to pulmonary circulation.
Vitelline vein
Returns deoxygenated blood from the yolk sac
Blood flowing thru pulmonary trunk
Some blood remains in R atrium and mixes with desaturated blood from superior vena cava (SVC). Blood coming from SVC is directed primarily through tricuspid valve into R ventricle. Blood from R ventricle flows through pulmonary trunk and a small quantity goes to the fetal lungs to feed the tissue, NOT for gas exchange.
Variation in umbilical arteries
Some fetuses have a single umbilical artery instead of two, most often the left umbilical artery is absent. This may indicate cardiovascular arteries.
What happens to the veins from week 5?
The R umbilical vein, L vitelline vein and L common cardinal vein are obliterated
How is the syncytiotrophoblast invisible from the mother's immune system?
The layer lacks usual cell surface antigens; this is good so that the mother does not attack the placenta when the syncytiotrophoblast cells are producing proteolytic enzymes to invade the uterine wall.
decidual reaction
The maternal placenta has this reaction wherein stromal cells of endometrium (inner layer of uterus) swell and accumulate glycogen and lipids; they adhere tightly to each other to form a cellular matrix to constrain the embryonic invasiveness.
Maternal and Paternal contributions to Placenta Development
The maternal sides is divided into lobes called decidua basalis and the fetal portion is the chronic villi. Male genes favor placental growth, female restrict growth.
What two layers form in the trophoblast?
The outer layer is the syncytiotrophoblast and the inner layer is the cytotrophoblast.
How happens to the venous system during liver development
The proximal L umbilical vein regresses and the distal L umbilical vein persists; the distal part of the L umbilical vein takes over the entire blood supply.
crista dividens
The thickening edges of the septum secundum around what becomes the limbus of the fossa ovalis in the interatrial septum after birth.
By the 5th week, what is the role of the umbilical cord?
The umbilical cord replaces the yolk sac as source of nutrients for the embryo.
How does syncytiotrophoblast cells help control blood flow?
They normally colonize and remodel maternal arteries.
When nitric synthase levels increase in pulmonary vasculature at birth, what happens?
This increases the potential to generate nitric oxide and dilate these vessels. Opening of pulmonary vessels is thought to be direct response to oxygen, because hypoxia in newborns can cause pulmonary vessels to constrict. At the same time, spontaneous constriction (or obstetrical clamping) of umbilical vessels cuts off flow from placenta.
Cesarean section
When a woman has a C section, she is liable to lose more blood than a woman who has a vaginal delivery, as the blood from the intervillous space is not pushed back toward her body during such a delivery.
When are maternal pools of blood (lacunae) created?
When the trophoblasts invade the maternal endometrium. As a result, the invasion opens up maternal spiral arteries and now intervillous spaces form and are filled with maternal blood, from which the fetus gains nutrition. Maternal arteries and veins directly enter the intervillous space after 8 weeks estation, and the intervillous space will contain about a unit of blood (400- 500 mL). Much of this blood is returned to the mother with normal uterine contractions
septum secundum
a division between RA and LA
yolk sac
becomes separated from cytotrophoblast by chorionic cavity
Umbilical vein
delivers oxygenated blood from the placenta to the fetus
Umbilical arteries
develop in 4th week and arise from paired dorsal aortae.
Fetal shunts
foramen ovale, ductus arteriosus, and ductus venosus
What hormone does the chorion produce?
human chorionic gonadotropin (hCG), which to sustain pregnancy by stimulating the ovaries to produce estrogen and progesterone
How does closing of the ductus arteriosus contributes to higher blood flow to lungs?
less blood is diverted from pulmonary trunk to aorta
Location of chronic villi
may be free or anchored to the decidua basalis
Cotyledon
one main stem villus forms a unit of the placenta
What separates the cotyledon from the placenta?
placenta septum
sinus venosus
receives 3 veins that drain the placenta.
ligamentum arteriosum
remnant of ductus arteriosus
ligamentum venosum
remnant of ductus venosus
ligamentum teres
remnant of the umbilical vein
common cardinal vein
returns deoxygenated blood from the wall of the embryo
What connects the placenta to the fetus?
umbilical cord
What veins supply the placenta?
umbilical vein, vitelline vein, common cardinal vein
