S7.1 Foetal Physiology

Question 1

How is foetal o2 content increased in pregnancy?

Answer 1

Foetal Hb - predominant from week 12, has greater affinity for O2 as doesn’t bind 2,3-DPG effectively.
Higher Hb concentration.
More maternal production of 2,3-DPG

Question 2

What is the double Bohr effect

Answer 2

When CO2 enters intervillous blood get lower pH so reduced affinity of Hb for O2;
As CO2 is lost, pH increases so Bohr effect increases affinity of Hb for O2.
This produces a left shift on the Hb curve

Question 3

What controls CO2 transfer in pregnancy?

Answer 3

Progesterone-driven hyperventilation, hence lower pCO2 in maternal blood
Double Haldane effect – foetus gives up and accepts O2/ CO2, so no alterations in local pH.

Question 4

Summarise foetal circulation

Answer 4

Receive oxygenated blood from mother via placenta in umbilical vein
Return to placenta via the umbilical arteries.
Small amount of blood still goes from RA to RV so muscles don’t atrophy, and also to lungs for their growth and development.

Question 5

What occurs at the ductus venosus?

Answer 5

Blood bypasses liver into IVC then RA, to maintain saturation.

Question 6

What occurs at the ductus arteriosus

Answer 6

Blood goes from pulmonary trunk to aorta to avoid non-functional lungs

Question 7

What occurs at the foramen ovale

Answer 7

RA pressure>LA pressure, so blood goes into LA (after birth LA pressure higher so FA shuts).

Question 8

How does the foetus respond to hypoxia?

Answer 8

Redistribution of flow to protect heart and brain (so less to GI, kidneys)
Slowing of HR to decrease O2 demand (low PO2 or high pCO2 stimulates foetal chemoreceptors; vagus stimulation activates bradycardia - unlike in adults when low O2 causes tachycardia)

Question 9

Describe the effects on the foetus of poor nutrition in pregnancy

Answer 9

Growth restrictions.

Can be symmetrical (generalised, proportional) or asymmetrical (abdomen growth lags but head normal)

Question 10

Describe the formation of amniotic fluid

Answer 10

Pre W8 is made by the transudation of fluid across the amnion and foetal skin
Inhalation of AF develops lungs, while GI absorbs essential water + electrolytes.
Later is mostly urine which begins to be made in W9.
Composition: 98% water, also has urea, electrolytes, creatinine, glucose.

Question 11

Describe bilirubin metabolism in the foetus

Answer 11

During gestation clearance of fetal bilirubin is handled by the placenta.
Foetus cannot conjugate bilirubin due to immaturity of liver and intestinal processes, so physiological jaundice is common.