F8i: Compare & contrast the carriage of O2 & CO2 in blood

Content of Arterial Blood

mL/dL

mmHg

CO2

48mL

40mmHg

O2

21mL

100mmHg

Content of Venous Blood

mL/dL

mmHg

CO2

52mL

46mmHg

O2

15mL

40mmHg

Mechanism of O2 Transport & ODC

CaO2 = Dissolved O2 + OxyHb

Dissolved O2

  • Solubility co-efficient 0.003
  • Obeys Henry’s Law → the amount carried dissolved in solution is directly ∝ to the partial pressure
  • ∴pO2 x 0.003 = O2 mL/dL

Oxyhaemoglobin

  • O2 binds reversibly to Hb: Hb + O2 ⮂ HbO2
  • HUFFNER’S CONSTANT = 1.34mL O2 per 1g Hb
  • O2 binds to heme portion of Hb
  • 4 x heme → ∴1Hb can bind up to 4 O2
  • Displays +ve COOPERATIVITY → affinity for O2 is lowest at first O2 binding because DeoxyHb Is in TENSE configuration
  • With each subsequent O2 binding heme is ↑O2 affinity

ODC

  • This is what gives ODC a sigmoid shape
Sigmoid Shape
  • P50 = partial pressure oxygen in blood where Hb is 50% saturated with O2 → it is a measure of O2 affinity
  • ODC can be shifted to give Hb altered affinity for O2 & this is compared with P50

R) Shift ODC

  • ↑CO2
  • ↑H+
  • ↑Temp
  • ↑2, 3 DPG
  • ↓pH

= ↓affinity = offloads O2

L) Shift ODC

  • ↓CO2
  • ↓H+
  • ↓Temp
  • ↓2,3 DPG
  • ↑pH

= ↑affinity = binds O2 tighter

CO2 Carriage in Blood & CO2 Dissociation Curve

Carbamino Compounds + Dissolved + HCO3–

Carbamino Compounds

  • Carbamino compound = CO2 + terminal amine group of a protein (i.e. Hb)
  • Hb most abundant protein → 15g/dL
  • ∴its terminal amine group most important for CO2 carriage
  • 4 terminal amine groups
  • CO2 binds to α-chain
  • DeoxyHb forms carbamino compounds 3.5 x more readily than OxyHb

NB: 70% HALDANE EFFECT → for any given PCO2, deoxygenated blood is able to carry more CO2

Dissolved

  • Obeys HENRY’S LAW → the [ ] of a gas in liquid is ∝ to its partial pressure
  • CO2 is x 20 more soluble cf. O2
  • Solubility co-efficient 0.03
  • ∴ Dissolved CO2 (mL) = 0.03 x pCO2

HCO3–

  • Occurs in RBC
  • HCO3– diffuses out, H+ cannot
  • Cl – is exchanged for HCO3– to maintain electroneutrality
  • DeoxyHb is less acidic cf. oxyHb
  • Accepts H+ to drive equation forward → 30% HALDANE EFFECT

CO2 Dissociation Curve

CO2 Dissociation Curve
  • CO2 is much more linear
  • Lacks Positive Cooperativity
  • Steeper because of greater content per mL
  • HALDANE EFFECT responsible for A – V difference