24A11: Exam Report

Compare and contrast the carriage of oxygen in the blood with the carriage of carbon dioxide

55% of candidates passed this question.

A good response included normal values of the partial pressures of both oxygen and carbon dioxide including the arterial and venous contents.

Methods of carriage (haemoglobin, dissolved and other forms), haemoglobin binding characterisitcs (co-operative binding and affinities); and impact on loading/off loading at tissues and lungs for both oxygen and carbon dioxide was included.

Correctly labelled diagrams with a brief accompanying explanation could be used to convey some of these concepts.

F8i / 24A11: 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
Oxygen saturation curve with key points labeled.
  • 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–

Carbonic acid formation reaction with chemical equation
  • Occurs in RBC
  • HCO3 diffuses out, H+ cannot
  • Clis exchanged for HCO3to maintain electroneutrality
  • DeoxyHb is less acidic cf. oxyHb
  • Accepts H+ to drive equation forward → 30% HALDANE EFFECT

CO2 Dissociation Curve

CO2 dissociation curve illustrating Haldane effect.
  • CO2 is much more linear
  • Lacks Positive Cooperativity
  • Steeper because of greater content per mL
  • HALDANE EFFECT responsible for A – V difference