8. Fetal and Neonatal Physiology Flashcards
What are the steps with oxygen in the fetus, at birth, and as the neonate?
Fetus- placenta provides O2 and nutrients
At birth- lungs become functional and redirection of circulation from fetal to adult
Neonate (newborn)- becomes accustomed to extrauterine life
What is the different between the circulatory system of the fetus and that of the infant/adult?
Presence of the placenta
2 distinct circulations:
Fetoplacental
Uteroplacental
What is the placenta?
Almost all materials needed for fetal growth and development move from maternal circulation to fetal circulation across the placenta (by passive diffusion or active transport)
Produces essential hormones (progestins, estrogens, chorionic gonadotropins)
Performs a number of vital functions: Gas exchange (O2) Nutrient transport Fluid balance Waste removal (through amniotic fluid)
What is the physiology of the placenta?
Fetal blood flow, chorionic villi
Large fraction of fetal blood flow from the aorta via the 2 umbilical arteries to the fetoplacental circulation within chorionic villi
Chorionic villi- fetal tissue protruding into the maternal blood
Slides 9-10 Oct 29
What is the path of maternal blood flow?
Maternal arterial blood is discharged from ~120 spiral arteries (arteries have multiple openings)
Blood enters pulsatile spurts through wall of uterus then moves in discrete streams into the intervillous space towards chorionic plate
Small lies of blood near the plate dissipate the force of the arterial spurts and reduce blood velocity
Maternal blood spreads laterally, reverses direction and bathes chorionic villi (blood slows even more for time to exchange)
The maternal blood then drains through venous orifices and enters larger maternal placental veins
Ultimately flows into uterine and other pelvic veins
Flows into intervillous space
Slide 11 oct 29
What are the 3 principal factors that regulate flow of maternal blood in the intervillous space?
- Maternal arterial blood pressure
- Intrauterine pressure
- Pattern of uterine contraction
What is the path of fetal blood flow?
Fetal blood flow originates from 2 umbilical arteries (these carry deoxygenated blood)
As umbilical arteries near placenta they branch repeatedly beneath the amnion
Penetrate chorionic plate
Branch again within the chorionic villi forming a capillary network
Blood with higher O2 and nutrients returns to the fetus from the placenta through the umbilical vein
Slide 12
What do the umbilical arteries and umbilical veins carry?
Umbilical artery- deoxygenated blood away from fetus
Umbilical vein- oxygenated blood to the fetus
What is amniotic fluid?
Fills amniotic cavity
2 functions:
- mechanical buffer that protects the fetus from external physical insults
- mechanism by which the fetus excretes many waste products
Water in amniotic fluid turns over atleast once a day
What is the gas composition of the maternal blood coming into the intervillous space?
Similar to that of systemic arterial blood:
Partial pressure of oxygen (PO2) of ~100mmHg
PCO2 of ~40mmHg
pH of 7.40
Diffusion of O2 from the maternal blood into chorionic villi of fetus causes PO2 of blood in intervillous space to fall
Intervillous space PO2 is ~30-35mmHg
What is the difference between fetal and maternal hemoglobin?
Fetal hemoglobin has a much higher affinity (~85%) for O2 that does maternal Hb (~65%) to make up for the low PO2 of maternal blood in the intervillous space
Fetal also had ~50% more hemoglobin
Slides 15-18 oct 29
What is umbilical veins PO2?
~30mmHg
What are the 4 unique pathways the fetal circulation uses to facilitate nutrient and gas exchange?
Placenta- fetal blood entering placenta has low PO2 that facilitates the diffusion until fetal blood has higher PO2
Ductus venosus- the fetal first shunt, umbilical venous blood returns to lower body from placenta but bypasses largely nonfunctional liver by shunting through this
Foramen ovale- second major shunt is blood entering right atrium and crossing this foramen (oval hole between 2 flaps of tissue in septum that divides atria) right-to-left shunt
Ductus arteriosus- third major stunt also right to left shunt that directs blood from the pulmonary artery to the aorta via this (large vessel with smooth muscle in its wall)
What is the fundamental difference between the fetal and postnatal circulations?
(Placentas jobs)
Placenta performs functions that are performed by 4 organ systems in extrauterine life: The lungs (gas exchange) The GI tract (nutrition) The liver (nutrition, waste removal) The kidneys (fluid/electrolyte balance, waste removal)
This is possible because of 3 shunts that allow blood to bypass future postnatal routes and instead direct larger fraction of deoxygenated blood to the placenta
What is the combined cardiac output (CCO)?
Sum of the outputs of the right and left ventricles
Left and right largely pump in parallel rather than in series
The inputs and outputs of these 2 sides mix
Slide 21-22 oct 29
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What triggers newborns first breath?
Mild hypoxia, hypercapnia, tactile stimuli, cold skin (hypothermia), hypoglycemia
It is entering a cold new world
Triggers circulatory changes that convert the fetal pattern of blood flow to the adult pattern
Loss of placenta circulation requires the newborn to breathe on its own
Vasoconstriction of umbilical arteries terminated O2 delivery immediately upon birth (stretches umbilical arteries and raises systemic arterial PO2 in newborn)
Slides 23-25 oct 29
Why is your first breath the most difficult in your life?
Large negative pressure necessary to overcome surface tension
Transpulmonary pressure (PTP) of 60cm H2O
Adult PTP in tidal breathing is ~2.5cm H2O
Adult level of lung compliance ~1h after birth
Slide 26 oct 29
What does the removal of the placenta at birth trigger?
Increased systemic vascular resistance
Doubles total peripheral resistance (placental circulation receives 50% of CCO which accounts for low vascular resistance of fetal systemic circulation so when this goes away resistance doubles because blood flow through the descending aorta is unchanged and aortic and left ventricular systolic pressure must increase
What does lung expansion and breathing lead to after birth?
Lung expansion decreases pulmonary vascular resistance > 5 fold
Increase alveolar PO2
Decrease alveolar PCO2
Pulmonary vasculature flow increased by ~4 fold at birth
Pulmonary arterial and right ventricular pressure fall abruptly
Pressure continues to decline over a few months
What is the closure of the ductus venosus?
Majority of portal blood continues to flow through DV after birth even tho blood flow through umbilical vein ceases soon after birth
Immediately after birth the portal blood flow to liver remains low
Within a few days after birth, constriction of the vascular smooth muscle within the ductus venous causes functional closure of this shunt pathway
Causes pressure in portal vein to increase and divert blood into the liver
What is the closure of the foramen cycle?
At birth, decrease pulmonary vascular resistance increases blood flow through lungs increased venous return to left atrium and increase left atrial pressure
Increase in pulmonary blood flow closed ductus arteriosis decreases venous return and decreases right atrial pressure
Increase in LAP and decrease in RAP reverses pressure gradient and pushes the flap of tissue back against the septum which closes foramen ovale
Slides 31-32 oct 29
What is the closure of the ductus arteriosus?
At birth, begins ti shunt left to right since blood flows path least resistance cause the increase systemic resistance (removal of placenta) and decrease pulmonary resistance (expansion of lungs)
Few hours after birth the ductus arteriosus muscular wall contracts which reduces or eliminates flow which closes ductus arteriosus
Slide 33 oct 29
What is surfactant?
What produces it?
Increases lung compliance and thereby reduced the effort of inspiration
Surfactant synthesis starts during the saccular period
Cortisol (and the other hormones) stimulate production of surfactant
2/3 fetal, 1/3 mother
If an infant is born premature it doesn’t have sufficient surfactant need excessive work to create adequate tidal volume
Slide 34 oct 29
Slide 37 oct 29
What is infant respiratory distress syndrome?
Usually caused by a deficiency of pulmonary surfactant owing to prematurity
Increased work if breathing
Impaired gas exchange
Refractions (collapsed fluid filled alveoli, less compliant lung, distorted chest wall caving in between above or beneath ribs)
Slide 36 oct 29
Review at birth slide 38 oct 29
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