7.2: Fetal Physiology

Question 1

What do the physiological functions of the baby depend on?

Answer 1

Maternal systems

Question 2

Describe how oxygen reaches the fetus from maternal arteries and is circulated in the fetus!

Answer 2

Diffuses across placenta -> umbilical vein -> bypasses liver via ductus venosus -> IVC -> R atria-> Some to R ventricle and pulmonary trunk, SMALL amount into lungs due to high vascular pressure -> Most -> ductus arteriosus -> aorta (lower vascular resistance) OR R->L atria via foramen ovale -> L ventricle -> aorta (meets with blood entering from ductus arteriosus)Aorta-> systemic distribution->umbilical artery -> placenta

Question 3

Name 4 factors that contribute to increasing oxygen saturation in fetal blood.

Answer 3

1. Fetal hb has higher O2 affinity (no beta chains (only 2 alpha and 2 gamma)) 2. More hb 3. Low diffusion resistance4. Double Bohr effect (in maternal and fetal blood)

Question 4

Why is even a short interruption to bloodflow possibly detrimental to the fetus and when can this be a problem in labour?

Answer 4

Fetus only stores O2 for ~2min. Frequent contractions in labor may constrict available bloodflow without giving much recovery time between

Question 5

What should you do if you notice fetal deceleration?

Answer 5

Take blood sample from fetus head to check O2 *fetal deceleration is a decrease in fetal HR below the fetal baseline HR, measured with CTG.

Question 6

How might you determine whether the fetus has acidosis?

Answer 6

Take lactate level

Question 7

What enables the fetus to have a relatively normal pCO2?

Answer 7

Maternal CO2 levels are low due to physiological hyperventilation (progesterone)

Question 8

What happens to O2 saturation as it travels from the umbilical vein to and throughout the fetus?

What is the O2 saturation when blood reaches the carotids?

Answer 8

Saturation drops slightly when blood mixes with deoxygenated blood (therefore neonatal O2 is low)

Drops when it mixes with

• 70-65% IVC
• to 60% pulmonary venous flow
• > reaches carotids 60% saturated

Question 9

What is the role of the crista dividens?

Answer 9

Division in R atria that can direct bloodflow into foramen ovale

Question 10

From the aorta blood is sent to the fetal brain, arms and heart muscle. Where does it go after?

Answer 10

Down SVC back to R ventricle -> pulmonary artery -> some to lungs but most to descending aorta via ductus arteriosus

Question 11

What produces meconium and what is its role?

What happens if there are excessive amounts and why might this happen?

Answer 11

Fetal intestines, initially helps pass ‘earliest stool’

Excessive amounts released with stress (i.e hypoxia), can lead to

• > increased chances of bacterial infection (even though its sterile)
• > aspirated into lungs -> hinder O2 transfer

Question 12

What is amniotic fluid composed of? WHen does it reach its max volume and how can its volume be assessed?

Answer 12

Maternal fluids and fetal extracellular fluid (that diffused across fetus’ non-keratinized skin).

Reaches max volume close to term, can be assessed with USS

Question 13

How would you diagnose obstetric cholestasis and how might this affect the baby?

Answer 13

If maternal bile acids are high even though all infection screens and LFT come back negative.

Baby relies on mother’s liver to remove
bile acids from blood, if it doesn’t
-> crosses placenta -> stresses fetal liver
-> cause peristalsis -> excessive meconium passes -> aspiration -> stillbirth

Question 14

Why might amniocentesis be performed?

Answer 14

Check for chromosomal abnormalities and neural tube defects

Question 15

What are the risks of performing amniocentesis early and later in the pregnancy?

Answer 15

Early: miscarriage or vascular damage (if needle penetrates umbilical vessels)
Late: infection

Question 16

How does the mother’s inability to conjugate bilirubin affect the fetus?

Answer 16

Jaundice (Fetus cannot conjugate bilirubin)

Question 17

The placenta replaces the function of which fetal organs?

Answer 17

Fetal gut, lungs and kidneys

Question 18

When does fetal insulin secretion begin?

Answer 18

Week 10

Question 19

How does the fetal gut absorb water and electrolytes?

Answer 19

Constantly swallowing amniotic fluid

Question 20

Describe when and how the fetal endocrine system plays a part in fetal development

Answer 20

1. Placental progesterone -> promotes fetal corticosteroid production -> vital for CVS function
2. Thyroid hormones -> development of nervous system, bone & hair growth

Question 21

Describe how fetal circulation adapts once the baby is been born
*include the disappearance of fetal structures

Answer 21

1. Trauma and cold temperatures -> first breath -> Lungs inflate -> fall in pulmonary vascular resistance -> increased bloodflow to pulmonary vessels -> L atrial pressure>R atria -> foramen ovale closes

Smooth muscle in ductus arteriosus is sensitive to high pO2 and prostaglandins -> contracts

Cord clamping -> lack of blood flow -> constriction in ductus venosus sphincter -> blood goes to liver sinusoids.

Question 22

How is rhesus disease managed and when is this strategy INeffective?

Answer 22

Anti-D IgG injections remove RhD fetal blood cells before they cause sensitization.

Won’t work if she’s made anti-D IgG in previous pregnancy -> close monitoring of pregnancy and baby

Question 23

Name three treatment methods for a baby born with rhesus disease

Answer 23

1. Phototherapy
2. Blood transfusions
3. IV immunoglobulins (antibodies) to prevent RBCs from being destroyed

Question 24

Define cholestasis, what causes it? (2)

Answer 24

Decrease in bile flow

1. Impaired secretion by hepatocytes
2. Obstruction

Question 25

Which direction does the fetal O2 dissociation curve shift and why?

Answer 25

Left:
1. Bohr effect: CO2 crossing the placenta from fetus-mother creates an area of local acidity -> lowers maternal Hb’s O2 affinity -> increases diffusion across placenta

2. Higher fetal Hb level and O2 affinity facilitates uptake from placenta-fetus.

Question 26

Name three major ways in which fetal circulation differs from normal circulation

Answer 26

1. Oxygenated blood enters circulation via placental transfer
2. Pulmonary blood flow accounts for <20% of total CO
3. Fetal vascular structures exist to direct blood flow

Question 27

Name the adult structures that arise from the following embryological ones:
a) foramen ovaleb) ductus arteriosusc) ductus venosusd) umbilical veine) umbilical arteries

Answer 27

a) foramen ovalisb) ligamentum arteriosumc) ligamentum venosumd) ligamentum terese) medial umbilical ligaments (and superior vesical arteries to urinary bladder)

Question 28

How much time passes after birth for the following shunts to undergo functional and anatomical closure? a) ductus arteriosusb) foramen ovalec) ductus venosus

Answer 28

a) ductus arteriosusFunctional: 10-96 hrs Anatomical: 2-3 weeks b) foramen ovaleFunctional: several mins Anatomical: 1 year c) ductus venosusFunctional: several mins Anatomical: 3-7 days

Question 29

Describe the ‘trauma’ process that initiates breathing in a newborn

Answer 29

Cord is cut -> O2 drop -> asphyxia -> acidotic state stimulates respiratory centre in medulla and chemoreceptors in the carotid to initiate breathing