Obstetrics - Maternal and Neonatal Physiology

on 1.2.08 with 1 comment



  • The foetus requires an increasing oxygen supply as it grows. How are these increased oxygen needs met?
The factors involved are:
  1. Increased maternal blood supply to the placenta (Uterine flow increases 20 fold during pregnancy)
  2. Increased foetal blood supply to the placenta
  3. Presence of foetal haemoglobin which has a higher oxygen affinity than maternal HbA
  4. Higher [Hb] concentration in the foetus (40% higher than in adult)
  5. The Double Bohr Effect.


  • What is the uterine blood flow at term?
About 500-750 mls/min and 85% of this goes to the placenta. Note that the uterine supply to the placenta is not autoregulated and flow is pressure dependent.

  • What vessels supply foetal deoxygenated blood to the placenta?
Umbilical arteries.

  • What is the umbilical artery flow at term?
The foetal cardiac output at term is about 1,000 mls/min: from 25 to 55 percent of this goes
to the placenta.

  • What is the special feature of foetal haemoglobin? How does it assist the foetus?
It has a lower P50 (18 to 20 mmHg) than adult Hb (26.6 mmHg). This means foetal
haemoglobin has a higher oxygen affinity and this assists it to load oxygen in the placenta
while maternal haemoglobin is unloading oxygen. It has a higher saturation at a given pO2
than adult haemoglobin eg foetal Hb has a saturation of 80% at a pO2 of 30 mmHg.

  • Why does foetal haemoglobin have such a low P50?
  1. The higher P50 of adult haemoglobin in red cells is due to the right shift that occurs in the presence of high levels of 2,3 DPG in the red cell.
  2. The 2,3 DPG binds to the β-chains of HbA (especially deoxy HbA) to cause this effect.
  3. Several amino acids are involved in the binding and each tetramer of adult haemoglobin binds one molecule of 2,3 DPG.
  4. Foetal Hb is a tetramer: α2γ2.
  5. There are no β-chains so HbF is insensitive to a shift due to 2,3 DPG binding.
  • How long does foetal haemoglobin persist after birth?
At birth about 80% of the neonate’s haemoglobin is HbF. This decreases rapidly so that by the age of 6 months less than 5% of the baby’s haemoglobin is HbF. Only very small amounts of HbF are normally present in adults (<1% style="text-align: left;">
  • What is the ‘double Bohr effect’? How does this affect oxygen uptake by the foetus?
  1. The term double Bohr effect refers to the situation in the placenta where the Bohr effect is operative in both the maternal and foetal circulations.
  2. The increase in pCO2 in the maternal intervillous sinuses assists oxygen unloading.
  3. The decrease in pCO2 on the foetal side of the circulation assists oxygen loading.
  4. The Bohr effect facilitates the reciprocal exchange of oxygen for carbon dioxide.
  5. The double Bohr effect means that the oxygen dissociation curves for maternal HbA and foetal HbF move apart (ie in opposite directions).


  • What are the special factors which assist carbon dioxide transfer across the placenta?
The important factors relevant to the placenta are:
• Maternal hyperventilation results in a low maternal pCO2 which increases the gradient favouring CO2 transfer
• A double Haldane effect occurs and this is unique to the placenta.


  • What is the [Hb] at birth?
This is typically high: about 17 to 18 g/dl.

  • In what ways does the haemoglobin change in the first year after birth?
The major changes are:
  1. Decrease in [Hb] (physiological anaemia of infancy)
  2. Rapid decrease then elimination of HbF (as discussed above)
  3. Increased production of adult haemoglobin (HbA) to replace HbF.
  4. After birth, the arterial pO2 (and Hb saturation) in the neonate is much higher and erythropoietin levels consequently drop to undetectable levels.
  5. Red cell production is markedly decreased and [Hb] falls.
  6. The decrease in oxygen delivery is partially compensated by a shift of the haemoglobin oxygen dissociation curve to the right as HbF is replaced by HbA and red cell 2,3 DPG levels rise.
  7. Finally, [Hb] levels fall far enough to cause increased levels of erythropoietin and red cell production increases.
  8. The physiological anaemia of infancy lasts for about 6 months.

  • Can you tell me what are the pO2, SO2 and pCO2 values in the uterine vessels and in the umbilical vessels?
Typical values are:
Maternal circulation
• Uterine artery: pO2 100mmHg (SO2 98%); pCO2 32mmHg
• Uterine vein: pO2 40mmHg (SO2 75%); pCO2 45mmHg

Foetal circulation
• Umbilical artery: pO2 18mmHg (SO2 45%); pCO2 55mmHg
• Umbilical vein: pO2 28mmHg (SO2 70%); pCO2 40mmHg



Tags: , , , , , ,

Category: Obstetrics Notes

POST COMMENT

1 comments:

Anonymous said...
June 20, 2010 at 11:42 AM

These questions and answers are a sample section from chapter 11 of “The Physiology Viva” by Kerry Brandis
Details on: Http://www.AnaesthesiaMCQ.com

Post a Comment

Newer Post
Older Post
Subscribe to: Post Comments (Atom)