2015B11 Explains the mechanisms whereby oxygen transfer is facilitated at the placenta.

 

List:

·         Intro

·         Foetal Hb

·         Double Bohr

·         Double Haldane

·         Maternal hyperventilation

 

Intro:

Placental gas exchange

·  Supplies O2 and removes CO2 instead of lungs

o Foetal VO2 17mL/min

o Placental VO2 6mL/min

·  Net diffusion down partial pressure gradients

Placenta cf. lung

·  Area:                  15m2 (cf. 80m2)

·  Thickness:        3.5μm (cf. 0.5μm)

·  i.e. significantly slower diffusion rate

Favourable adaptations

·  Foetal haemoglobin

·  Double Bohr effect

·  Double Haldane effect

·  Maternal hyperventilation

 

Foetal vs. adult haemoglobin:

 

Foetal

Adult

Globin chains

α2γ2

α2β2 >> α2δ2

Concentration

18g/dL

15g/dL (12g/dL pregnant!)

2,3-DPG affinity

Low

Neutrally charged serine at position 143

High

Positively charged histidine at position 143

p50

19mmHg

26mmHg

Implication

Very high CaO2 for given PaO2

Increased partial pressure gradient

 

 

Bohr effect:

Description

·  If ↓pH, ↑pCO2:

o ↓Affinity of Hb for O2

o Right shift oxyhaemoglobin dissociation curve (OHDC)

·  Note ↑temp and ↑2,3-DPG also cause right shift

·  Note opposite changes cause left shift

Mechanism

·  Allosteric interactions between haems:

o ↓pH, ↑pCO2 -> tense (T) state -> ↓affinity for O2

o ↑pH, ↓pCO2 -> relaxed (R) state -> ↑affinity for O2

Double Bohr effect

·  As HbF unloads CO2:

o a)↑pCO2 in maternal lacuna -> ↓HbA affinity for O2 -> UNLOAD

o b)↓pCO2 in foetal capillary -> ↑HbF affinity for O2 -> LOAD

 

Haldane effect:

Description

·  HHb has higher affinity for CO2 than HbO2

Mechanism

·  70%: because HHb is 3.5x better at forming carbaminoHb

·  30%: because ↑pKa of imidazoles to 8.2 enhances buffering of H+ formed by H2CO3 dissociation

Double Haldane effect

·  As HbA unloads O2:

o a)↑pO2 in capillary -> ↓HbF affinity for CO2 (UNLOAD)

o b)↓pO2 in lacuna -> ↑HbA affinity for CO2 (LOAD)

·  This amplifies the double Bohr effect, further improving foetal oxygenation

(these values are currently in dispute)

 

Maternal hyperventilation:

Description

·  RR ↑10%

·  VT ↑35%

·  MV and VA ↑50%

·  Changes present from early T1

Mechanism

·  Sensitisation of central chemoreceptors by progesterone

·  (but note progesterone level normal in early pregnancy; ? enhanced sensitivity to progesterone)

Effect

·  ↑VA -> ↓PaCO2 -> ↑partial pressure for CO2 transfer

·  Amplifies double Bohr effect -> improves foetal oxygenation

 

 

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