9.2

Question 1

Describe the fetal circulation before birth.

Answer 1

Oxygenated blood enters fetus via umbilical vein from placenta.
Bypasses liver by ductus venous
IVC (mixed with desaturated)
From RA -> LA by foramen ovale (some to RV via crusta dividens)
LA–> LV
RV–>From pulmonary artery –> aorta via ductus arteriosus
Aorta –> Deoxygenated blood leaves via 2 umbilical arteries.

Question 2

Why does blood not enter the lungs?

Answer 2

Resistance is very high due to hypoxic pulmonary vasoconstriction

Question 3

What happens when an infant takes its first breath?

Answer 3

Hypoxic pulmonary vasoconstriction removed.
Resistance of lungs reduced.

Greater venous return to LA
Pressure in LA > RA
Closure of foramen ovale

Increased oxygen saturation of blood and removed prostaglandins (placenta removed) results in:
Constriction of ductus arterioles
Contraction of umbilical artery

Stasis of blood in umbilical vein and ductus venous leads to clotting of blood and closure due to fibrosis.

Question 4

What is the function of amniotic fluid?

Answer 4

Surrounds fetus providing mechanical protection - shock absorption, and a moist environment so the fetus does not dehydrate.

Question 5

How much amniotic fluid is there at 8 weeks, 38 weeks and 42 weeks

Answer 5

10 ml at 8 weeks
1 L at 38 weeks
Falls to 300 ml at 42 weeks

Question 6

How much urine is produced at 25 weeks and at term? How much do adults?

Answer 6

25 weeks - 100 ml per day
Term 500 ml per day
Adults 1L per day

Question 7

What does amniotic fluid contain?

Answer 7

98% water
Electrolytes
Creatinine
Cells from fetus/amnion --> amniocentesis and karyotyping
Urea
Bile pigments
Renin
Glucose
Hormones

Question 8

What happens to swallowed amniotic fluid?

Answer 8

Fetus absorbs water and electrolytes.
Debris accumulates in fetal gut
Fluid washes this away forming meconium together.
This is debris from amniotic fluid plus intestinal secretions including bile

Question 9

What happens to fetal bilirubin?

Answer 9

Fetus cannot conjugate bilirubin due to immaturity of the liver and intestinal processes for metabolism, conjugation and excretion
Bilirubin crosses the placenta after accumulating
Excreted by the mother.

Question 10

After birth, what stimulates neonate liver to begin conjugation?
What if this does not establish quickly?

Answer 10

Light exposure (Phototherapy)
Jaundice

Question 11

How long are fetal oxygen stores

Answer 11

Very low - 2 mins

Question 12

What is the haemomonochorial

Answer 12

Thin barrier that oxygen diffuses across in the placenta from the maternal blood.

Question 13

What is the driving factor for this diffusion

Answer 13

Gradient of partial pressure between maternal and umbilical blood.

Question 14

Why is the placenta well adapted for exchange?

Answer 14

Lass SA and low resistance to diffusion

Question 15

What is used to indicate fetal oxygen saturation

Answer 15

Fetal heart rate

Question 16

What is fetal pO2 and normal adult pO2 in arteries

Answer 16

Fetal 4kPa

Adult arterial 13.3kPa

Question 17

How is fetal Hb adapted to work better at low pO2

Answer 17

Different Hb without beta chains and with gamma chains.
Doesn’t readily bind 2,3-BPG
Higher affinity for O2
70% saturated at 4kPa
Adult is 45% saturated at 4kPa
Means that fetal blood contains 7.5mM/L O2, simlar to adult blood.
More Hb (higher haematocrit)

Question 18

What is the double Bohr effect

Answer 18

Increase in pCO2 or [H+] results in Hb losing affinity and releasing more O2. Happens in both mother and fetus

Question 19

How does the mother blow off extra CO2 from the baby?

Answer 19

Hyperventilation stimulated by progesterone

Question 20

Why ultrasound scan at 20 weeks?

Answer 20

Organs big enough to see but early enough to abort

Question 21

Why should pregnant women take folic acid

Answer 21

Important for neural tube development

Question 22

Fetal heart rate at term

Answer 22

140-160 bpm

Question 23

What is fetal abdominal circumference assessing?

Answer 23

Growth of liver and amount of subcutaneous fat

Question 24

What does meconium in the amniotic fluid indicate

Answer 24

Fetal distress

Question 25

What determines O2 transport rate

Answer 25

Umbilical arter pO2

Question 26

Significance of CO2 in fetus

Answer 26

Cannot tolerate higher pCO2 than mother –> acid base problems
Placental transfer of CO2 facilitated by lower maternal pCO2 through hyperventilation

Question 27

How is amniotic fluid formed

Answer 27

Early in pregnancy - from maternal fluids and fetal extracellular fluid by diffusion across skin
Late - fetal turnover