V1iii: Gas exchange across placenta & Double Bohr & Haldane effects

Definition: Placenta = organ which is the interface between mother + foetus during pregnancy

  • Transfer of CO2 + O2 is by DIFFUSION down partial pressure gradients, influenced by:
    1. FICK’S LAW
    2. DOUBLE BOHR + HALDANE EFFECT
    3. BLOOD FLOW
    4. Hb AFFINITY/TYPE OF Hb

Fick’s Law

Fick's Law
  • SA = large 15m2
  • Thickness = v. thin 3.5µm (lung 160m2; 0.3µm)
  • Gas solubility/MW = both gases v. small
    • CO2 x 20 more soluble cf. O2
    • ∴diffuses easier
  • Gases move down partial pressure gradients
Fick's Law2

Double Bohr + Haldane Effects

  • Explains that foetal ODC shifts L) & maternal ODC shifts R)
  • Foetal Hb releases CO2 to maternal blood
  • HbF undergoes L) shift (becomes alkalotic)
  • Gives itself greater O2 affinity
  • CO2 reaches maternal blood
  • ↑H+ → maternal Hb ODC shifts R)
  • Offloads O2 easily

Double Haldane

  • Maternal blood releases O2
  • Blood can carry more CO2 as HCO3–/carbamino compounds
  • Without ↑CO2 tension
  • Foetal blood takes up O2
  • ∴less capacity

Type of Hb

• HbF

  • [Hb] = 17g/dL → ∴↑capacity for O2 carriage
  • pSO 20mmHg → ∴ high O2 affinity
  • 2α + 2γ chains → as β chains

• Maternal Hb

  • 2α + 2β → 2,3 DPG bonds β-chain ) & facilitates O2 offloading
  • [Hb] = 12g/dL
  • P50 = 27mmHg

Blood Flow

Blood Flow
  • High maternal BF constantly replenishing [ ] gradients for movement of substances
Uteroplacental BF
  • ∴affected by:
    • ↓uterine a. P
    • ↑uterine v. P
    • ↑uterine vasc. resistance