2020A05 Describe the mechanisms that facilitate oxygen and carbon dioxide exchange in the placenta.

 

List:

·         Intro

·         Maternal adaptations

·         Foetal adaptations

·         Haemoglobin adaptations: Bohr & Haldane effects

 

Intro:

Mechanism of transfer

·           Diffusion down partial pressure gradient

Problem

·           Area 15m2 (cf. lung 80m2)

·           Thickness 3.5μm (cf. lung 0.5μm)

·           Hence slow rate of transfer cf. lung

Solution

·           Maternal adaptations: ↑CO, ↑MV

·           Foetal adaptations: ↑Hb affinity for O2, ↑Hb concentration

·           Haemoglobin design: Bohr and Haldane effects

 

Maternal adaptations:

↑Cardiac output

·           ↑50% at term, with 600mL/min to placenta

·           Cause: a) ↑metabolic demand -> ↑preload b) progesterone-induced ↓SVR

·           Effect: ↑DO2 to placenta, ↑O2 uptake, ↑CO2 excretion

↑Alveolar ventilation

·           ↑50% at term (≈ ↑RR 10% + ↑TV 35%)

·           Cause: a) ↑metabolic demand b) progesterone sensitises central chemoreceptors

·           Effect on CO2: ↓PaCO2 -> enhanced double Bohr effect

·           Effect on O2: ↑PaO2 -> ↑concentration gradient and ↑DO2

 

Foetal adaptations:

HbF

·           Left shift OHDC with p50 19mmHg (cf.26mmHg)

·           Cause: serine substitution -> ↓affinity for 2,3-DPG

·           Effect: umbilical venous SvO2 80% despite pO2 30mmHg

Polycythaemia

·           Hb 180g/L cf. 140g/L

·           Cause: hypoxia -> ↑erythropoiesis

·           Effect: umbilical venous CaO2 16mL/100mL despite SvO2 80%

 

Bohr effect:

Description

·           If ↓pH, ↑pCO2:

o  ↓Affinity of Hb for O2

o  Right shift oxyhaemoglobin dissociation curve (OHDC)

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/H+ in maternal lacuna -> ↓HbA affinity for O2 -> UNLOAD O2

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

 

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+

Double Haldane effect

·           As HbA unloads O2:

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

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

->Amplification of Bohr effect -> ↑foetal PaO2

(these values are currently in dispute)

 

 

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