F8iii / 20A01 / 18B05 / 15A13: Describe how CO2 is carried in the blood

20A01: Exam Report

Describe the carriage of carbon dioxide in blood

68% of candidates passed this question.

A detailed understanding of the carriage of carbon dioxide (CO2) in the blood is essential to the practice of intensive care medicine.

Comprehensive answers classified and quantified the mechanisms of CO2 carriage in the blood and highlighted the differences between the arterial and venous systems.

An explanation of the physiological principles surrounding these differences and the factors which may affect them was expected.

The changes that occur at the alveolar and peripheral tissue interfaces with a similar explanation of process was also required.

Candidate answers were often at the depth of knowledge required for an ‘outline question’ and a more detailed explanation was required to score well.

18B05: Exam Report

Describe the carriage of carbon dioxide (CO2) in the blood.

65% of candidates passed this question.

A definition of arterial and venous CO2 content (mls and partial pressure) and an outline of the 3 forms of CO2 in the blood and their contribution to the AV difference, followed by a detailed explanation of each form of carriage was required for this question. A good answer included a table of the contribution of each form of carriage to arterial and venous content and the AV difference; explained the concepts of chloride shift when describing carriage as HCO3-; detailed the Haldane effect and its contribution to carbamino carriage and referenced Henry’s law when describing dissolved CO2.

West’s Chapter 6 on gas transport details the key information to score well on this question.

15A13: Exam Report

Describe how carbon dioxide (CO2) is carried in the blood.

79% of candidates passed this question.

It was expected answers would describe each of the main categories of how CO2 is carried: Dissolved (10%), Plasma Bicarbonate (70%) and conjunction with plasma proteins and Hb as Carbamino Hb (20%). An opening statement quantifying the amount of CO2 dissolved in arterial (48mL/100mL) and venous blood (52mL/100mL) (4mL/100mL) and how this compares with Oxygen was expected (20 times more soluble).

For dissolved CO2, the application and description of Henry’s Law was awarded marks. A description of the consequences of the Haldane effect: difference in CO2 carriage of oxygenated and deoxygenated blood was  expected. A diagram of pCO2 v CO2 content was helpful

F8iii 20A01 / 18B05 / 15A13: Describe how CO2 is carried in the blood

Definition

CO2 is a colourless, odourless gas composed of one carbon and two oxygen atoms present in the atmosphere and formed during respiration

CO2 production in the body is a result of aerobic metabolism in mitochondria. Normally 200mL/min

Carriage in Blood

Arterial

Mixed Venous

V-A Difference

Carbamino

Arterial

0.0017mmol/L

Mixed Venous

0.002 mmol/L

V-A Difference

30%

Dissolved

Arterial

1.2 mmol/L

Mixed Venous

1.4 mmol/L

V-A Difference

10%

HCO3–

Arterial

24.4 mmol/L

Mixed Venous

26.2 mmol/L

V-A Difference

60%

PCO2

Arterial

40mmHg

Mixed Venous

45mmHg

V-A Difference

mL/dL

Arterial

48mL/dL

Mixed Venous

52mL/dL

V-A Difference

Carriage in Blood

Straight Outta West’s:

The first column shows the proportions of the total CO2 concentration in arterial blood.  The second column shows the proportions that make up the venous-arterial difference

Carbamino Compounds

  • Carbamino compound = CO2 + terminal amine group of a protein (Hb)
  • Hb is the most abundant protein (15g/dL)
  • ∴Its terminal amine group is the most important for CARBAMINO formation
  • Hb is a tetramer → ∴4 terminal amine groups
  • Carbamino compounds form when CO2 binds with terminal amine of the α-chain
  • DeoxyHb forms carbamino compounds 3.5 x more readily than OxyHb
  • 70% of HALDANE EFFECT: for any given PCO2, deoxygenated blood is able to carry more CO2
  • Due to conformational ∆
    • Unbinding of O2 causes β-chain conformational ∆
    • Affinity for CO2 ↑
    • DeoxyHb 3.5 x better at forming carbamino compounds than OxyHb

Dissolved CO2

  • Obeys HENRY’S LAW: the [ ] of a gas in liquid is ∝ to its partial pressure
  • CO2 is x 20 more soluble than O­2
    • 1mmHg CO2 = 0.06mL/100mL of blood
    • 1mmHg O2 = 0.003mL/100mL of blood
  • ∴Solubility Coefficient = 0.03mmol/L/mmHg
  • ∴Dissolved CO2 (mL) = 0.03 x pCO2

HCO3 –

HCO3 –
  • Occurs in RBC
  • HCO3– diffuses out, H+ cannot
  • Cl– is exchanged for HCO3– to maintain electroneutrality & continue moving the equation forward KA ‘CHLORIDE SHIFT’
  • Deoxygenated Hb is less acidic than OxyHb
  • Accepts a H+ & also drives equation forward
  • 30% of HALDALE EFFECT: for any given PCO2, deoxygenated blood is able to carry more CO2

CO2 Dissociation Curve

  • CO2 dissociation curve shows relationship between blood PCO2 and CO2 content per dL
  • Linear ODC → lacks +ve COOPERATIVITY
  • Steeper than ODC → due to Haldane Effect, when there is ↑CO2 the blood can carry more
  • Haldane Effect is responsible for difference between Arterial Pt & Venous Pt
  • Arterial Point = PCO2 40mmHg, 48mL CO2/dL
  • Venous Point = PCO2 46mmHg, 52mL CO2/dL