Lecture 16: Fetal physiology

Question 1

What are the adaptations for gas exchange at the placenta?

Answer 1

• diffusion barrier is small and decreases as pregnancy proceeds
• there is a gradient of partial pressures
• maternal oxygen increases, therfore we must ensure fetal oxygen is lower than maternal oxygen (fetus lives in slightly hypoxic environment)

Question 2

What are the factors that increase fetal oxygen content?

Answer 2

• fetal Hb variant
• fetal haematocrit is increased compared to adults
• increased maternal production of 2,3 DPG causing a decrease in affinity for her, for oxygen

Question 3

What is fetal Hb?

Answer 3

HbF is the predominant form from 12 weeks

• 2 alpha subunits, 2 gamma subunits
• greater affinity for oxygen, doesn’t bind 2,3DPG as effectively (so can absorb more oxygen from the maternal blood)

Question 4

What is the double bohr effect?

Answer 4

Speeds up process of oxygen transfer

• as CO2 passes into intervillous blood, pH decreases, decreasing the affinity of maternal Hb for oxygen
• at the same time, as CO2 is lost, pH rises, increasing the affinity for fetal Hb for oxygen

Question 5

Why is there a lower pCO2 in the maternal blood?

Answer 5

Progesterone driven hyperventilation

-allows for concentration gradient, so there is more CO2 on the fetal side

Question 6

What is the double haldane effect?

Answer 6

• as Hb gives up oxygen it can receive increasing amounts of CO2
• fetus gives up CO2 as O2 is accepted

Question 7

What is the fetal circulation?

Answer 7

• receives oxygenated blood from mother via placenta in umbilical vein
• lungs are non-functional so the blood bypasses the lungs
• returns to the placenta via umbilical arteries

Question 8

What are the fetal cirulatory shunts?

Answer 8

Ductus venosus: blood bypasses the liver and enters the IVC (as liver is highly metabolically active- want to keep oxygen saturation of the blood maintained)
Foramen ovale: hole in interatrial septum between right and left atrium (pressure in RA is higher than LA)
Ductus ateriosus: connects the pulmonary artery to the aorta

Question 9

What is the crista dividens?

Answer 9

Free border of septum secundum forms a ‘crest’, creating 2 streams of blood flow

• majority flows to LA
• minor proportion flows to RV and mixes with deoxygenated blood from the SVC (to prevent atrophy of the muscle in the RV)

Question 10

What is the oxygen saturation of blood reaching the LA?

Answer 10

60%: we want to ensure the heart abd brain get the majority of the oxygen
-small amount of pulmonary venous return (deoxygenated as had to supply developing lungs)

Question 11

Where does the ductus arteriosus join the aorta?

Answer 11

Distal to the vessels to the head and neck branching off the aorta

Question 12

What is the fetal response to hypoxia?

Answer 12

• HbF and increased Hb conc
• redistribution of flow to protect supply to the heart and brain (reducing supply to the GIT, kidneys, and limbs)
• fetal HR slows to reduce oxygen demand

Question 13

What can chronic hypoxaemia lead to?

Answer 13

• growth restriction
• behavioural changes
• impact on development

Question 14

What hormones are necessary for fetal growth?

Answer 14

-insulin
-leptin (placental production)
-IGF 1 and 2
(all generated from fetal liver, unless indicated otherwise)

Question 15

What are the types of cellular growth during pregnancy?

Answer 15

0-20 wks: hyperplasia
20-28 wks: hyperplasia and hypertrophy
28-term: hypertrophy

Question 16

How does nutrition effect fetal growth?

Answer 16

Malnutrition can cause symmetrical (whole fetus) or asymmetrical growth restriction (parts of the fetus)
-depending on where malnutrition occurs in the mother, it may have a distinct influence of the type of growth restrictions we could see

(nutritional and hormonal status during fetal life can influence health in later life)

Question 17

What is the function of amniotic fluid?

Answer 17

Amniotic sac encloses the embryo/fetus in amniotic fluid

• protection
• contributes to development of the lungs

Question 18

What is the volume of amniotic fluid?

Answer 18

Increases over time
10ml at 8 weeks
1 litre at 38 weeks (optimal)

Question 19

How is amniotic fluid produced?

Answer 19

Fetal urinary tract: urine production by 9 weeks

some fluid is produced by the fetal lungs and the fetal GI tract

Question 20

What is the composition of the amniotic fluid?

Answer 20

98% water
-plus electrolytes, creatinine, urea, bile pigments, renin, glucose, hormones and fetal cells, lanugo, vernix caseosa (waxy protective substance)

It gets recycled as we swallow it- allowing water and electrolytes to be absorbed

Question 21

What is meconium?

Answer 21

Debris that accumulates in the gut (debris from amniotic fluid and intestinal secretions e.g. bile:green)
-passed after delivery

Question 22

How would you sample amniotic fluid?

Answer 22

Amniocentesis

• allows for collection of fetal cells
• useful diagnostic test e.g. fetal karyotype

(carries risk of miscarriage)

Question 23

What is bilirubin metabolism in a fetus?

Answer 23

Fetal bilirubin is handled by the placenta
-fetus can’t conjugate bilirubin due to the immaturity of the liver and intestinal processes for metabolism, conjugation and excretion
=physiological jaundice is common