2020A Question 05
Describe the mechanisms that facilitate oxygen and carbon dioxide exchange in the placenta.
Examiner Report
The domains covered were transfer of oxygen and carbon dioxide, diffusion, and other factors.
Main points expected were: Description of how gases diffuse across the placenta with relevance to Fick’s Law and in comparison to the lung, quantification of partial pressure differences for oxygen & carbon dioxide between the vessels involved, and definitions with explanations for the Bohr & Haldane effects applied to the placental barrier. Graphical demonstrations could show the effect of these mechanisms on diffusion gradients.
Additional credit was awarded for explaining the mechanism & significance of the Bohr & Haldane effects, understanding other factors which alter the partial pressure gradients, maternal changes by trimester and the effects of blood flow on diffusion.
Common errors included: Misconception that the gradient for oxygen transfer was derived from the uterine artery PO2, attributing the Bohr effect solely to CO2 binding to Hb, and quoted values for uterine & umbilical vessel gas partial pressures were frequently inaccurate as were graphs, and there seemed to be many “typos” e.g. Writing O2 when CO2 was correct or umbilical instead of uterine.
A good source for this material is Nunn’s Respiratory Physiology and any of the general medical physiology recommended texts.
Model Answer
Structure:
- Introduction
- Maternal adaptations
- Foetal adaptations
- Haemoglobin adaptations: Bohr & Haldane effects
Introduction
| Factor | Detail |
|---|---|
| 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
| Factor | Detail |
|---|---|
| ↑ Cardiac output | - ↑ 50% at term, with 600mL.min-1 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
| Factor | Detail |
|---|---|
| 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-1 despite SvO2 80% |
Bohr Effect
| Element | Detail |
|---|---|
| Description | - If ↓ pH, ↑ pCO2: - ↓ Affinity of Hb for O2 - Right shift oxyhaemoglobin dissociation curve (OHDC) |
| Mechanism | - Allosteric interactions between haems: - ↓ PH, ↑ pCO2 → Tense (T) state → ↓ Affinity for O2 - ↑ PH, ↓ pCO2 → Relaxed (R) state → ↑ Affinity for O2 |
| Double Bohr effect | - As HbF unloads CO2: - a)↑ pCO2/H+ in maternal lacuna → ↓ HbA affinity for O2 → Unload O2 - b)↓ pCO2/H+ in foetal capillary → ↑ HbF affinity for O2 → Load O2
|
Haldane Effect
| Element | Detail |
|---|---|
| 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
|
