Foetal and neonatal physiology

Question 1

Explain how gas exchange in the foetus is optimised during pregnancy

Answer 1

1. Progesterone levels
   - They rise which leads to physiological hyperventilation, excreting more CO2 than normal, which creates a concentration gradient for exchange at the placenta, whereby foetal pCO2 arriving at the placenta will always be higher than the pCO2 in the mother’s blood
   - Maternal pO2 increases only marginally so the diffusion gradient is not available
2. Relative hypoxia in the foetus
   - A state of hypoxia in the foetus gives us a diffusion gradient to encourage exchange

Question 2

Give some factors which promote gas exchange in the foetus

Answer 2

1. HbF variant
2. Increased foetal haematocrit
3. Increased maternal production of 2,3-BPG
4. Double Bohr and Haldane effects at the materno-foetal interface

Question 3

What is the foetal response to hypoxia?

Answer 3

1. Adaptations to manage transient decreases in O2
2. HbF and [Hb] increased
3. Redistribution of flow to protect supply to the heart and brain (reducing supply to GIT, kidneys and limbs)
4. Foetal heart rate SLOWS in response to hypoxia to reduce O2 demand
5. Foetal chemoreceptors detecting decreased pO2 or increased pCO2
   - Vagal stimulation leading to bradycardia
6. Chronic hypoxaemia
   - Growth restriction
   - Behavioural changes

Question 4

Why are shunts important in the foetus?

Answer 4

In the foetus, the lungs dont work, oxygenation and removal of CO2 occur at the placenta and oxygenated blood arrives at the RIGHT side of the heart, so shunts are required to maintain foetal life since we need oxygenated blood in the LEFT side of the heart to be pumped to the body

Question 5

What is the role of ductus venosus? What does it connect?

Answer 5

It is the shunt that helps us avoid the liver

• It connects the umbilical vein carrying oxygenated blood to the IVC, blood enters the RA.
• By ensuring shunting of blood around the liver, saturation is mostly maintained

Question 6

What is the foramen ovale?

Answer 6

• It is the shunt that connects the RA to the LA
• RA pressure > LA pressure due to the huge volume of blood going to the RA via the umbilical vein. So the >RA pressure forces the leaves of the foramen ovale apart, allowing blood to flow into the LA

Question 7

What is the role of ductus arteriosus?

Answer 7

It shunts blood from the RV and PT to the aorta

Question 8

What is atrial septation?

Answer 8

Formation of the wall between the left and right atrium, building a reversible shunt

Question 9

Why is formation of the intraventricular septum key?

Answer 9

It allows blood from the right side (which has come from the umbilical vein) to shift to the left atrium

Question 10

How do we go from the foramen ovale to the fossa ovalis?

Answer 10

1. During intrauterine life, right sided pressure > left sided pressure, allowing blood flow through the foramen ovale
2. At birth, the pulmonary circulation begins and we get a venous return from the lungs which inverts the pressure relationship between the two chambers, now LA > RA which forces the leaves of the inter-atrial septum together
3. The foramen ovale now meets the septum primum and blood cant get through, over time they become knitted together and permanently closed

Question 11

How does the ductus arteriosus shunt close?

Answer 11

1. Physiological at first - spasm of vascular smooth muscle in the ductus in response to increased pO2
2. Then anatomical - transformation to a fibrous cord, not the course of the L recurrent laryngeal nerve

Question 12

How does the ductus venosus shunt close?

Answer 12

• Flow in DV stops at removal of placental support