1.24.1.16 - Vasculature of the Neonate
why is fetal circulation different?
fetal blood is oxygen-poor
fetus swims in amniotic fluid
fetus breathes amniotic fluid
fetus is fed parenterally, not orally
therefore pulmonary artery doesn't carry oxygen
this is normal but haemoglbin is different
stable temperature
water helpds protect bumps
therefore hepatic portal vein is not needed
placenta replaces many functions
fetus is a symbolic parasite
fetal lungs and livers: net recievers during fetal life to help rowth, in adults lungs liver are net providers or oxygen and nutrients
fetal lungs
collapsed, unnecessary, high resistance
fetal liver - not required to excerete waste, becomes important over late gestation
developing organs, tissues are fragile therefore most blood kept away and pressure low to prevent tissue damage
how is the fetal vasculature different
foramen ovale
ductus arteriosus
ductus venosus
shunts
a passage or anastomosis between two natural channels such as blood vessels
shunts
the fetus has to survive birth
during physiologic stress then remianing blood volume is distributed to organs essential for immediate survival
3 shunts help this happen
shunt 1: ductus venosus
creates low resistance pathway through liver
from umbilical vein to caudal VC
umbilical arteries go to placenta
umblicial vein goes to the fetus
DV allows 50% blood to bypass liver
DV streams blood to formaen ovale
shunt 2: foramen ovale
shunts oxygenated blood straight throuhg RA to LA
pressure difference across FO keeps it open in fetus
patency maintained by high blood flow
shunt 3: ductus arteriosus
pressure differential between lungs and lower body streams flow back to placenta
allows equivalent ventricular function in fetus - helps develops ventricular musculature and vasculature
shunts blood in pulmonary artery straight to descending aorta
at birth
fetus must now take over all functions of placenta
fetal lungs are collapsed
fetal circulation now a closed system
fetus has no oral intake of nutrients - digestion begins
fetus has no need to regulate body temp - thermoregulation
ftal kidneys do not need to process waste - rid waste
fetal circulation must immediately become more efficient like adult circulation. process is not immediately efficient, takes time to mature
fate shunt 1
umbilical cord is ligated as placenta detaches
no blood in umbilical vein
therefore DV sphincter constrcits
blood diverted through liver (hepatic portal vein)
remnant of DV called ligamentum venosum
fetal circulation has very high PGE2 levels
no ventilation = no clearance
rapidly disappears at birth as blood oxygenated
neonatal ventilation = pulmonary clearance
PGE2 is avasodilator
during birth, vasoconstrictor levels high
fate shunt 2: foramen ovale
lungs open = low pulmonary resistance
placenta gone = high systemic pressure
blood diverted to lungs via right ventricle
remnant is called the fossa ovalis
fate shunt 3: ductus arteriosis
patency maintained by low pO2 and High PGE2 and PGE3-5 fold higher in fetus vs neonate
first breath increases pO2, DA constricts
DA is a big muscular conduit
blood flow in opened lungs
pressure lower in lung
pressure higher in body
also helps to close the DA
but can take a while for full closure
remnant called ligamentum arteriosum
summary of mechanisms
loss of placenta
birth is a huge challenge to fetus
most changes secondary to lung inflation
thyroid glucocorticoid catecholamine
all potential mechanisms
high hormone stimulation helps
C section risks
vascular resistance in body increases
increased PaO2
decreased vasodilatory prostaglandins, PGE2
closure of shunts
1
closure of DV in minutes
improved pulomnary clearance
no umbilical blood supply
anatomic closure within days
2
3
functional closure is relatively quick (i.e. hours)
anatomic closure is slow - weeks/years
hole in heart
failure to close results in atrial septal defect
small opening present in about 25% mammals
anatomic closure 2-7 days
if closure does not occur, ventricular setal defect = patent ductus arteriosus
functional closure in mins-hours
non life threatening
relatively common condition
reduces efficiency of heart
exertional incompetence
anatomical remnants
- ductus venosus become ligamentum venosum
formane ovale becomes fossa ovalis
internal umbilical vein redundant - become round ligament o the liver
ductus arteriosus become ligamentum arterosum
internal umbilical arterial atrophy - become round ligaments of the bladder
urachus as the way of draining fetal urine