Resp7 - Carbon Dioxide in the Blood

Question 1

6 features of control of plasma pH

Main Reaction/Equation
Controlling Dissolved CO2
Maintaining High [HCO3-]
Controlling [HCO3-]
HCO3- as a Buffer
Effect of pH Disturbances

Answer 1

1. ) Dissolved CO2 reacts with water to form H+ ions and HCO3- ions in a reversible reaction
   - slow in plasma due to no carbonic anhydrase (CA)
   - CO2 + H2O —> H(+) + HCO3(-)
   - high CO2 favours forwards reaction –> increase in H+ ions –> decrease in pH
   - high HCO3- favours backwards reaction –> decrease in H+ ions –> increase in pH

2.) Dissolved CO2 - depends on the alveolar p(CO2) which is altered by the rate of ventilation

3. ) High [HCO3-] - maintained due to RBC production
   - RBCs have carbonic anhydrase (CA) which naturally speeds up the reaction of dissolved CO2 with water
   - haemoglobin binds to H+ ions which decreases the conc of the H+ ions to favour the production of HCO3-
   - Cl-HCO3 exchanger transports HCO3- out of the RBC and into the plasma to increase plasma conc of HCO3-
4. ) Control of [HCO3-] - kidneys controls amount of HCO3- by varying its excretion
   - pCO2 doesn’t really affect the [HCO3-] because the concentration of dissolved pCO2 is so much lower
5. ) HCO3- as a Buffer - body produces acids which react with HCO3- to produce CO2
   - CO2 produced is removed by breathing so pH changes are minimised
6. ) Effect of pH Disturbances
   - if pH falls below 7, enzymes denature
   - if pH rises above 7.6, free Ca conc drops –> tetany

Question 2

Henderson-Hasselbalch Equation to work out pH

Full Equation
Calculating Dissolved CO2
Overall pH

Answer 2

1. ) pH = pK + Log([HCO3-] / dissolved CO2)
   - pK is a constant and = 6.1 at 37 degrees
   - [HCO3-] is 25 mmol/l in normal conditions
2. ) Dissolved CO2 = p(CO2) x solubility of CO2
   - solubility coefficient of CO2 = 0.23
   - p(CO2) is 5.3 kPa in alveoli in normal condition
   - dissolved CO2 = 5.3 x 0.23 = 1.2 mmol/l
3. ) pH = 6.1 + Log(25/1.2) = 6.1 + 1.3 = 7.4
   - it is slightly alkaline due to higher conc of HCO3- than dissolved CO2

Question 3

2 features of carbamino compounds

Function
Carbamino Compounds at the Tissues

Answer 3

1. ) CO2 Transport - CO2 can bind to amine groups on Hb to form carbamino haemoglobin
   - this is NOT part of the acid-base balance
2. ) Carbamino Compounds at the Tissues
   - more formed due to higher p(CO2)
   - more formed due to unloading of O2
   - this CO2 is given up and breathed out at the lungs

Question 4

2 features of transporting carbon dioxide

3 Methods (and %s)
Amount Transported

Answer 4

1. ) 3 Methods
   - HCO3- (60%)
   - Carbamino Compounds (30%)
   - Dissolved CO2 (10%)
2. ) Amount Transported - only 8% is transported at rest
   - Total in Venous Blood = 23.3 mmol/l
   - Total in Arterial Blood = 21.5 mmol/l
   - 23.3 - 21.5 = 1.8 mmol/l, therefore, only 8% of the total is transported (1.8/22.4)
   - the rest of the CO2 is there as part of the pH buffering system