Download L06 - Fetal Circulation

L06 - Fetal Circulation
Thursday, April 16, 2015
11:17 AM
Fetal circulation is modified to
- Maximizes gas/nutrient exchange between mother and fetus VIA PLACENTA
- Provides more O2/Nutrients to developing organs with high metabolic need and bypassing low
metabolic need VIA SHUNTS
○ Heart/Brian vs Lungs/kidneys
- By early 4th week, embryo's future heart cells becomes contractile to move blood
Circulation moves from higher pressure to lower pressure
- In adults, HIGH Pressure is Left side of heart (Systemic circulation) and LOW pressure is right side
(Pulmonary)
- In fetus, HIGH pressure is pulmonary because collapsed lungs with fluid built up causing
constricted arterioles => high resistance
○ LOW pressure is systemic as blood goes from aorta -> placenta
Prenate vs Neonatal circulation
- Differences are due to 3 factors
○ Gas exchange in fetus occurs at Placenta, NOT lungs
○ Fetus obtains nutrients via Placenta, NOT GI Tract
○ Fluid excretion and ion regulation via Placenta, NOT kidneys
- Primary circulatory organs in fetus are Lungs, Heart, Liver, and Umbilical vessels
○ Umbilical cord has 3 vessels: 2 umbilical arteries and 1 Umbilical Vein
 Arteries because DEOXYGENATED blood AWAY from fetal heart
 Vein because OXYGENATED blood TOWARDS fetal heart from placenta
 Wharton's Jelly composes space between vessels
 Torsion of umbilical cord may compromise blood flow
- Ductus Arteriosus
○ Fetal artery that carries blood from pulmonary trunk to arch of aorta
○ 90% of blood passes through FO and DA
○ Pulmonary Hypoxic Vasoconstriction = hypoxic conditions causes constriction of pulmonary
arterioles -> increases resistance of flow into lungs => forces blood into Ductus Arteriosus
- Foramen Ovale
○ Opening between right and left atria (interatrial septum) to get to aorta faster so that the
partially oxygenated blood can get to rest of body
○ 90% of fetal blood passes through FO and DA
- Ductus venosus
○ Shunt that carries blood from umbilical vein to inferior vena cava and mostly bypassing liver
○ Causes mixing of Oxygenated and Deoxygenated blood
Placenta
- Placenta contains 3 general portions
○ Chorionic Plate (Fetal)
○ Basal plate (maternal)
○ Intervilus space (combined)
- Deoxygenated/Nutrient poor blood travels via Umbilical artery to Fetal Capillaries within Chorionic
villi -> Intervillus space -> UTERINE Vein
- Oxygenated/Nutrient rich blood travels from Maternal UTERINE Artery into Intervilus space
immediately next to Fetal Capillaries -> Umbilical Vein
Remember that fetal Hb has higher affinity for O2 than Adult Hb
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○ Remember that fetal Hb has higher affinity for O2 than Adult Hb
Parturition (Birth)
- Several stimuli causes first breath in newborn
○ Physical stress of birth
○ Sudden exposure to lower temp
○ Post-birth hypoxia and hypercapnia due to severed connection with placenta => Stimulates
CNS breathing center
- Changes in Pulmonary resistance
○ First breath
 Lungs take in air -> pushes fluid out of alveoli -> arterioles dilate -> decrease
resistance
○ Placenta expelled
 Leads to Decrease PG concentration => Causes DUCTUS ARTERIOSIS CLOSURE
- Changes in Systemic Resistance
○ Placenta Expelled
 Leads to Increased resistance in Umbilical artery causing sudden increase in
aortic/systemic pressure
○ Left Atrial pressure increase at birth causes closure of Foramen Ovale and forms Fossa
Ovalis
 Right atrium has less pressure and blood cannot flow directly from Right atrium to Left
atrium
○ Cutting/Closure of Umbilical vessels causes systemic FLOW to decrease => Increases
systemic RESISTANCE => increases pressure in left side of heart
- Anatomical Changes
○ Ductus Arteriosus closes
 Closure of DA due to O2 increase and PG decrease leads to formation of Ligamentum
Arteriosum
○ Foramen Ovale closes due to increase in Left Atrial Pressure -> Fossa Ovalis formed
○ Ductus venosus closes forming Ligamentum Venosum
○ Umbilical vein closes to form ligamentum teres hepatis
○ Umbilical artery closes to form medial umbilical ligaments
○ Due to temperature change, Wharton's Jelly contracts and "clamps" umbilical structures ~5
minutes after birth
Umbilical Cord Pathologies
- Nuchal Cord = 1+ loops of umbilical cord are wrapped around fetal neck
- True Umbilical Cord Knot
- Umbilical cord pseudoknot = not true knot, just exaggerated loop of umbilical artery because it is
longer than vein
Patent Ductus Arteriosus
- Occurs when DA fails to close at birth -> Blood flows through PDA and INCREASES Pulmonary
pressure while allowing Oxygenated and Deoxygenated blood to mix
- Presentation
○ No early symptoms
○ Aortic pressure increases with age resulting in greater flow through PDA into pulmonary
vessels
 Pressure increase due to body/heart not getting enough O2
○ Cyanosis
○ Worsens as PDA increases in diameter
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