8. Fetal and Neonatal Physiology

Question 1

What are the steps with oxygen in the fetus, at birth, and as the neonate?

Answer 1

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

Question 2

What is the different between the circulatory system of the fetus and that of the infant/adult?

Answer 2

Presence of the placenta

2 distinct circulations:
Fetoplacental
Uteroplacental

Question 3

What is the placenta?

Answer 3

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)

Question 4

What is the physiology of the placenta?

Fetal blood flow, chorionic villi

Answer 4

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

Question 5

What is the path of maternal blood flow?

Answer 5

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

Question 6

What are the 3 principal factors that regulate flow of maternal blood in the intervillous space?

Answer 6

1. Maternal arterial blood pressure
2. Intrauterine pressure
3. Pattern of uterine contraction

Question 7

What is the path of fetal blood flow?

Answer 7

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

Question 8

What do the umbilical arteries and umbilical veins carry?

Answer 8

Umbilical artery- deoxygenated blood away from fetus

Umbilical vein- oxygenated blood to the fetus

Question 9

What is amniotic fluid?

Answer 9

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

Question 10

What is the gas composition of the maternal blood coming into the intervillous space?

Answer 10

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

Question 11

What is the difference between fetal and maternal hemoglobin?

Answer 11

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

Question 12

What is umbilical veins PO2?

Answer 12

~30mmHg

Question 13

What are the 4 unique pathways the fetal circulation uses to facilitate nutrient and gas exchange?

Answer 13

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)

Question 14

What is the fundamental difference between the fetal and postnatal circulations?
(Placentas jobs)

Answer 14

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

Question 15

What is the combined cardiac output (CCO)?

Answer 15

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
Page 1160

Question 16

What triggers newborns first breath?

Answer 16

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

Question 17

Why is your first breath the most difficult in your life?

Answer 17

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

Question 18

What does the removal of the placenta at birth trigger?

Answer 18

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

Question 19

What does lung expansion and breathing lead to after birth?

Answer 19

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

Question 20

What is the closure of the ductus venosus?

Answer 20

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

Question 21

What is the closure of the foramen cycle?

Answer 21

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

Question 22

What is the closure of the ductus arteriosus?

Answer 22

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

Question 23

What is surfactant?

What produces it?

Answer 23

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

Question 24

What is infant respiratory distress syndrome?

Answer 24

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

Question 25

Review at birth slide 38 oct 29

Answer 25

Okay