11. Fetal Physiology

Question 1

Is oxygenated blood carried via the umbilical vein or arteries, and so what is deoxygenated blood carried in?

Answer 1

Oxygenated blood - umbilical vein.

Deoxygenated blood - umbilical arteries.

Question 2

Fetal blood has a low pO2 so that O2 passes from the mothers blood to the fatal blood at the placenta, because the maternal pO2 only increases slightly in pregnancy. So how does the fetus still obtain enough O2?

Answer 2

Fetal haemoglobin - predominant form is HbF.
Fetal haematocrit is higher than in adults.
Fetal haemoglobin is high.
Increased maternal production of 2,3 DPG (so O2 is given up more easily).
Double Bohr effect.

Question 3

Why does HbF have a greater affinity for O2 than HbA?

Answer 3

Doesn’t bind 2,3-DPG as effectively as HbA.

Question 4

What are the subunits of HbF?

Answer 4

2 alpha, 2 gamma.

Question 5

What is the double Bohr effect?

Answer 5

At the placenta, CO2 passes from the fetus into the intervillous blood, decreasing the pH in the uterine vein, and therefore decreasing the affinity of Hb for O2, so O2 is given up to the fetus.
At the same time, as the CO2 is lost, the pH rises in the umbilical vein, increasing the affinity of Hb for O2, and more O2 can be taken in by the fetal blood.

Question 6

Progesterone-driven hyperventilation causes a lower pCO2 in the maternal blood, increasing the pCO2 concentration gradient at the placenta. But why are there no alterations in local pCO2?

Answer 6

Double Haldane effect - as Hb gives up O2, it can accept increasing amounts of CO2. The fetus gives up CO2 as O2 is accepted and so there are no alterations in local pCO2.

Question 7

What fetal circulatory shunt divers blood from the placenta around the liver to the inferior vena cava?

Answer 7

Ductus venosus.

Question 8

What fetal circulatory shunt divers blood from the right atrium to the left atrium, bypassing the lungs?

Answer 8

Foramen ovale.

Question 9

A small amount of blood needs to pass from the right atrium to the right ventricle and pulmonary trunk, to prevent atrophy, rather than diverting straight to the left atrium. What feral circulatory shunt takes the blood from the pulmonary trunk to the aorta, bypassing the lungs and left side of the heart?

Answer 9

Ductus arteriosus.

Question 10

Why is blood shunted around the liver?

Answer 10

To maintain O2 saturation of the blood.

Question 11

Why is most blood shunted from the right atrium to the left atrium, bypassing the right ventricle?

Answer 11

Avoids mixing with deoxygenated blood from the superior vena cava in the right ventricle. Blood enters aorta from left ventricle, and lots is pumped to brain and heart, ensuring adequate O2 supply.

Question 12

Why is the small of amount of blood that passes into the right ventricle and pulmonary trunk shunted around the lungs to the aorta?

Answer 12

Joins aorta distal to the supply to the head and hear, so minimises drop in O2 saturation for blood for the rest of the body.

Question 13

How does a does a fetus respond to hypoxia eg in preeclampsia?

Answer 13

Increased HbF and [Hb].
Redistribution of blood flow to protect the supply to the hear and brain, reducing flow to the GI, limbs and then kidneys.
Fetal heart rate slows to reduce demand for O2 (fetal chemoreceptors detect decreased pO2 or increased pCO2, and fetal stimulation leads to bradycardia).

Question 14

What 2 impacts does chronic hypoxaemia have on a fetus?

Answer 14

Growth restriction.

Behavioural changes - eg moving less and affecting the sleeping patterns - impacts on development.

Question 15

Name 3 hormones necessary for fetal growth.

Answer 15

Insulin.
IGFI.
IGFII.
Leptin.
EGF.
TGFalpha.

Question 16

In what trimester of fetal development in insulin-like growth factor II dominant and why?

Answer 16

First trimester, as is nutrient independent.

Question 17

In what trimester of fetal development is insulin-like growth factor I dominant and why?

Answer 17

Trimesters 2 and 3, is nutrient dependant.

Question 18

Where is the fetal growth hormone lepton produced in pregnancy?

Answer 18

From the placenta.

Question 19

What is the dominant cellular growth mechanism in weeks 0-20, 20-28 and 28-term of pregnancy?

Answer 19

0-20 - hyperplasia.
20-28 - hyperplasia and hypertrophy.
28-term - hypertrophy.

Question 20

What are the two types of growth restriction caused by malnutrition in pregnancy?

Answer 20

Symmetrical.

Asymmetrical - head sparing.

Question 21

What produces amniotic fluid?

Answer 21

Fetal urinary tract.

Fetal lungs.

Question 22

When does urine production begin in the foetus?

Answer 22

9 weeks.

Question 23

When does production of amniotic fluid peak?

Answer 23

38 weeks.

Question 24

What is the composition of amniotic fluid?

Answer 24

98% water.

Electrolytes, creatinine, urea, bile pigments, renin, glucose, hormones and fetal cells, lanugo, vernix caseosa.

Question 25

What is manconium?

Answer 25

First bowel movement after birth, formed of intestinal secretions and debris from amniotic fluid that has accumulated in the gut due to the fetus swallowing it.

Question 26

Why is physiological jaundice common at birth?

Answer 26

During gestation, clearance of fetal bilirubin is handled efficiently by the placenta, as the fetus cannot conjugate bilirubin. If there is still immaturity of the liver and intestinal processes for metabolism, conjugation and excretion of bilirubin at birth, this leads to jaundice.

Question 27

How is amniotic fluid recycled by the foetus?

Answer 27

GI tract.
Placenta and fatal membranes (intramembranous pathway).
Some produced by fetal lungs retained.