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   ↑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:

·     a)↑pO2 in capillary -> ↓HbF affinity for CO2 -> unload CO2

·     b)↓pO2 in lacuna -> ↑HbA affinity for CO2 -> load CO2

·     >Amplification of Bohr effect -> ↑foetal PaO2

 

 

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