2004B09 Describe how carbon dioxide is produced in the body.
How does it move from the site of production to the pulmonary capillary?

 

List:

·         Production

·         Transfer

·         Carriage

·         Variable CO2 affinity

 

Production:

Site

·   Mitochondrion

Means

·   Krebs cycle, aerobic respiration

·   From carbohydrate, fat, amino acid metabolites

Rate

·   ~200mL/min

·   Variable respiratory quotient (VCO2:VO2)

o Average: 0.8

o Carbohydrate: 1.0

o Fat: 0.8

o Protein: 0.7

o Ketones: 0.7

·   ↑VCO2: exercise, sepsis

·   ↓VCO2: hypothermia

 

Transfer from cell -> capillary blood:

Fick’s law

·   Hence factors increasing CO2 transfer: ↑P1, ↓P2, ↑area, ↓thickness

↑P1

·   ↑metabolic rate -> ↑VCO2

↓P2

·   ↑[Hb] -> ↑capacity for uptake

·   ↑volume of capillary blood -> ↑capacity for uptake

·   ↑cardiac output -> ↑rate of removal

↑Area

·   ↑Capillarisation (e.g. adaptation to aerobic exercise)

·   ↑Recruitment and distension (e.g. exercise)

↓Thickness

·   ↑Capillarisation -> ↓diffusion distance

 

Carriage in venous blood:

Total

·   CvO2 52mL/100mL venous blood

·   PvCO2 46mmHg

As HCO3-

(62%)

·   Location:

o Haemoglobin (140g/L, 39 imidazoles per molecule)

o Plasma proteins (70g/L, 13 imidazoles per molecule)

·   Purpose:

o Maintain partial pressure gradient for CO2 from tissues into capillary blood

·   Mechanism:

o CO2 + H2O < -> H+ + HCO3- (carbonic anhydrase in RBC)

o KHb + H+ <-> HHb + K+ (buffering by imidazole groups of histidine residues)

o HCO3- exchanged for Cl- at membrane (Hamburger effect, facilitates above reactions)

As carbamino compound

(28%)

·   Location: same

·   Purpose: same

·   Mechanism: NH2 + CO2 <-> NHCOO- + H+

As dissolved CO2

(10%

·   Amount dissolved = PaCO2 x k (Henry’s law)

·   Solubility coefficient k: 0.03mmol/L/mmHg at 37°C

·   Note ↑temp -> ↓solubility -> ↑PaCO2:CaCO2

Illustration

 

Variable CO2 affinity:

Haldane effect

·   Higher uptake of CO2 and H+ in HHb cf. HbO2:

o 70%: due to 3.5x ↑carbamino formation

o 30%: due to ↑buffering (imidazole pKa 6.6 -> 8.2)

Isohydric buffering

·   For each mol O2 unloaded by Hb, 0.7mol H+ can be added without ↓ plasma pH

·   Due to Haldane effect

 

 

Feedback welcome at ketaminenightmares@gmail.com