7.1: 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 resistance
4. 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 the O2 saturation of blood as it travels from the umbilical vein to and throughout the fetus?

What is the O2 saturation when blood reaches the carotids?

What is the resulting appearance of a fetus when born due to this?

Answer 8

Saturation drops slightly every time blood mixes with small amounts of deoxygenated blood, therefore neonatal O2 is LOW and there is an appearance of cyanosis at birth

1. Drops from 70 to 65% as it mixes with ascending blood in the IVC (not a major problem as the fetus’s lower body is small and not very metabolically active).
2. Drops to 60% as it mixes with the pulmonary venous flow (lungs don’t utilize lots of O2 and most is diverted)
3. From the aorta, blood reaches the carotids 60% saturated

Question 9

What is the role of the crista dividens?

Answer 9

Crista dividens is a division in the R atria that can direct bloodflow into the 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 the SVC back to the R ventricle (instead of going back through foramen ovale). It’s then pumped into the pulmonary artery where a small amount will continue to the lungs but most is shuttled through ductus arteriosus to the descending aorta

Question 11

Where does meconium come from and what is its role? What happens if there are excessive amounts and why might this happen?

Answer 11

Meconium is initially produced in the fetal intestine and helps pass the fetus’s ‘earliest stool’, which is composed of materials ingested by the fetus (including any debris that’s accumulated in the fetal gut). Excessive amounts are released when the baby is stressed (such as in hypoxic conditions), and even though meconium is sterile it can increase the chances of bacterial infection. It can also hinder oxygen transfer in the lungs (as in excessive amounts it may be aspirated in with the amniotic fluid)

Question 12

What is amniotic fluid composed of?

Describe how it’s volume changes normally, how can its volume be assessed?

Answer 12

Maternal fluids and fetal extracellular fluid (that’s diffused across the fetus’ non-keratinized skin). This includes cells from the fetus and amnion and a variety of proteins. Later in the pregnancy, a large portion of its volume comes from fetal urine

It reaches max volume around 38 weeks but may fall as labour nears, the volume can be assessed using an ultrasound

Question 13

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

Answer 13

It can be diagnosed when maternal bile acids are high even though all screens for infection and LFT come back negative. Since the baby relies on the mother’s liver to remove bile acids from the blood, the elevated bile acids can cross the placenta and cause stress on the baby’s liver. They can also cause peristalsis causing excessive meconium to pass, which can be aspirated by the fetus causing a stillbirth

Question 14

Why (and when) might amniocentesis be performed?

Answer 14

1. Check for chromosomal abnormalities (typically before 20 weeks)
2. Neural tube defects

Question 15

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

Answer 15

1. Early: risk of miscarriage or vascular damage if the needle penetrates the umbilical vessels
2. Late: risk of infection

Question 16

What happens to the fetal bilirubin? What happens if the mother is having problems conjugating bilirubin?

Answer 16

Since the fetus cannot conjugate bilirubin it crosses the placenta, is conjugated and excreted via maternal bile. The neonate may get jaundiced if conjugation isn’t established quickly

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

From constantly swallowing amniotic fluid

Question 20

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

Answer 20

1. Placental progesterone promotes fetal corticosteroid production (especially near term) - this steroid is especially vital for CVS function
2. Thyroid hormones (active from week 12) mediate the development of the nervous system and bone and hair growth

Question 21

Describe how fetal circulation adapts once the baby is been born

Answer 21

1. Trauma and cold temperatures induce the neonate to take its first breath
2. The lungs inflate causing a fall in pulmonary vascular resistance, so more blood passes through the pulmonary vessels
3. L atrial pressure rises>R atria (now it’s receiving much more blood from the lungs) so foramen ovale closes
4. Smooth muscle in the wall of ductus arteriosus is sensitive to high pO2 and prostaglandins and contracts
5. A sphincter in ductus venosus constricts after birth due to the lack of flow from the cord clamping so blood can go to liver sinusoids.

Question 22

What is rhesus disease and what causes it?

Answer 22

Rhesus disease occurs when the mother is rhesus negative and the baby is positive. The mother’s body responds by producing antibodies against the foreign rhesus positive blood, these antibodies can cross the placenta causing rhesus disease in the fetus and attacking the baby RBCs.

Question 23

What can be done to prevent rhesus disease, how does it work and is it always effective?

Answer 23

As part of antenatal screening, all women are offered blood tests to determine whether their blood is Rh-positive or negative, if found to be negative she’s offered anti-D immunoglobulin injections which removes RhD fetal blood cells before they can cause sensitization.

However, if a woman has made anti-D antibodies in a previous pregnancy she’s already been sensitized and the injections won’t work. The pregnancy and baby will be monitored closely.

Question 24

Name three treatment methods for a baby born with rhesus disease

Answer 24

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

Question 25

Define cholestasis

Answer 25

The decrease in bile flow due to impaired secretion by hepatocytes or to obstruction of bile flow through intra-or extrahepatic bile ducts

Question 26

Why is it essential that the umbilical vein pO2 be low?

Answer 26

So that oxygen will diffuse (high-low concentrations) from the maternal pO2 (which has risen) into the umbilical vein

Question 27

Name four factors that can influence the flow of blood in the umbilical artery and vein

Answer 27

Blood pressure, medication, hormones, uterine contractions

Question 28

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

Answer 28

Shifts left because of the nature of fetal Hb and the Bohr effect: As CO2 crosses the placenta from fetus-mother it creates an area of local acidity, this lowers the maternal Hb’s affinity for O2 and increases it’s diffusion across the placenta to the fetus. The higher fetal Hb level and Hb affinity for O2 helps facilitate O2 uptake from the placenta to the fetus.

Question 29

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

Answer 29

1. Oxygenated blood enters circulation via placental transfer
2. Pulmonary blood flow accounts for <20% of total cardiac output due to high pulmonary vascular resistance
3. Fetal vascular structures exist to direct blood flow

Question 30

Name the adult structures that arise from the following embryological ones:

a) foramen ovale
b) ductus arteriosus
c) ductus venosus
d) umbilical vein
e) umbilical arteries

Answer 30

a) foramen ovalis
b) ligamentum arteriosum
c) ligamentum venosum
d) ligamentum teres
e) medial umbilical ligaments (and superior vesical arteries to urinary bladder)

Question 31

How much time passes after birth for the following shunts to undergo functional and anatomical closure?

a) ductus arteriosus
b) foramen ovale
c) ductus venosus

Answer 31

a) ductus arteriosus
Functional: 10-96 hrs
Anatomical: 2-3 weeks

b) foramen ovale
Functional: several mins
Anatomical: 1 year

c) ductus venosus
Functional: several mins
Anatomical: 3-7 days

Question 32

Describe the process of the ‘trauma’ that initiates the newborn to begin breathing

Answer 32

1. With cutting of the cord, O2 supply is removed
2. Asphyxia occurs: where the baby’s body is deprived of O2 and acidosis occurs
3. The acidotic state stimulates the respiratory centre in the medulla and the chemoreceptors in the carotid to initiate breathing