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 CvO2 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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