9. Fetal Physiology

Question 1

What are the maternal and fetal pO2?

Answer 1

Maternal pO2 only increases marginally

Therefore to make the gradient work, fetal pO2 must be lower than maternal pO2

Question 2

What factors increase fetal O2 content?

Answer 2

Fetal haemoglobin variant

Fetal haematocrit is increased over that in the adult

Question 3

What is fetal haemoglobin?

Answer 3

HbF
2 alpha subunits plus 2 gamma subunits
Greater affinity for oxygen because it doesn’t bind 2,3-DPG as effectively as HbA

Question 4

What is the double Bohr affect?

Answer 4

Speed up process of O2 transfer
As CO2 passes into intervillous blood, pH decreases
Bohr effect
Decreasing affinity of Hb for O2
At same time on fetal side, as CO2 is lost, pH rises
Bohr effect
Increasing affinity of Hb for O2 on fetal side

Question 5

What is the change in CO2 levels in the mother?

Answer 5

Progesterone-driven hyperventilation
Hence lower pCO2 in maternal blood
Creates concentration gradient across placenta

Question 6

Describe the double haldane effect

Answer 6

As Hb gives up O2, it can accept increasing amounts of CO2
Fetus gives up CO2 as O2 is accepted
No alterations in local pCO2

Question 7

What helps the blood to bypass the liver in the fetus?

Answer 7

Ductus venosus

Question 8

What helps blood bypass the lungs and right ventricle in the fetus?

Answer 8

Foramen ovale

Question 9

What helps blood bypass the lungs in the fetus?

Answer 9

Ductus arteriosus

Question 10

What does the ductus venosus ensure?

Answer 10

Bypasses liver, saturation is mostly maintained - drops from 70% to 65%

Question 11

What is the role of the foramen ovale and how does it work?

Answer 11

Stops blood flowing into the right ventricle, therefore stopping blood going to the lungs
Right atrial pressure is greater than in left atrium, forces leaves of FO apart and blood flows into LA

Question 12

What is the crista dividens?

Answer 12

Free border of septum secundum forms a crest next to foramen ovale
Creates 2 streams of blood flow
Majority flows to LA
Minor proportion flows to RB, mixing with blood from SVC

Question 13

Why does some blood need to flow into the RV?

Answer 13

So that the RV has something to work against, preventing muscle wastage

Question 14

What happens in the left atrium of the fetus?

Answer 14

Small amount of pulmonary venous return - deoxygenated
Blood reaching left atrium has saturation approx. 60%
Pumped by LV to aorta
Ensures heart and brain get large share of oxygen

Question 15

What is the role of the ductus arteriosus?

Answer 15

Shunts blood from RV to PT to aorta
Joins aorta distal to the supply to the head
Minimising drop in O2 saturation

Question 16

What are the fetal responses to hypoxia?

Answer 16

HbF and increased [Hb]
Redistribution of flow to protect supply to heart and brain
Fetal heart rate slows in response to reduce O2 demand

Question 17

How does the fetal heart rate slow in hypoxia?

Answer 17

Fetal chemoreceptors detecting decreased pO2 or increased pCO2
Vagal stimulation leading to bradycardia

Question 18

What is the result in the fetus of chronic hypoxaemia?

Answer 18

Growth restriction

Behavioural changes - impact on development

Question 19

What are the effects of nutrition on fetal growth during pregnancy?

Answer 19

Malnutrition can cause symmetrical or asymmetrical growth restriction
Can influence health in later life

Question 20

What is the role of amniotic fluid?

Answer 20

Amniotic sac loses embryo/fetus in amniotic fluid
For protection
Also contributes to development of lungs
Volume - 10ml at 8 weeks, 1 litre at 38 weeks, falls away post-expected delivery date

Question 21

How is amniotic fluid produced/recycled?

Answer 21

Fetal urinary tract - urine production by 9 weeks
Fetal lungs
Fetal GI tract
Placenta and fetal membranes

Question 22

What is the composition of amniotic fluid?

Answer 22

98% water
Electrolytes, creatinine, urea, bile pigments, renin, glucose, hormones, fetal cells, lanugo, vernix caseosa
Debris accumulates in gut - meconium

Question 23

What is amniocentesis?

Answer 23

Sampling of amniotic fluid
Allows for collection of fetal cells
Useful diagnostic test - fetal karyotyping

Question 24

Describe fetal bilirubin metabolism

Answer 24

During gestation, clearance of feta bilirubin is handles efficiently by placenta
Fetus cannot conjugate bilirubin
Immaturity of liver and intestinal processes for metabolism, conjugation and excretion
Physiological jaundice common