pk 7 - Carriage of CO2 and pH Flashcards
In what 3 ways is CO2 carried in the blood?
How much CO2 is carried each way?
- dissolved in plasma 5%
- as bicarbonate ion 5%
- bound to proteins as carbamino compounds 90%
Explain the diagram that shows the movement of CO2 between tissues and blood.
- CO2 is produced by metabolism in the tissues, it diffuses along in conc gradient into the plasma and then in RBC’s.
- The CO2 reacts with water to form carbonic acid which then dissociates into H+ and bicarbonate. this reaction is catalysed by carbonic anhydrase in RBC’s
- The bicarbonate in RBC’s diffuses along its conc grad into the plasma, chloride shift maintains electrical neutrality
- In the plasma and RBC’s CO2 is carried as carbamino compounds (bound to Hb in RBC’s), these can as buffers if H+ rises, so reduces pH change.
- The whole process is shifted right as the H+ produced from carbonic acid in RBC’s is removed by the buffering by carbamino compounds, otherwise reaction would naturally come to a halt
Describe three differences between the CO2 and O2 dissociation curves. Draw them.
- The amount of each gas held in the body, CO2 content is much greater
- The shape of the curves, essentially linear vs sigmoid
- Haldane effect on CO2 curve vs Bohr effect on O2 curve
Explain the difference between the CO2 dissociation curve in arterial and venous blood.
The dissociation curve in venous blood is upwards shifted, allows a greater carriage of CO2 in venous blood.
This is the Haldane effect.
What 2 factors account for the Haldane effect?
How does this affect O2 and CO2 carriage?
What is the difference in CO2 carriage in ml/L-1?
How much O2 is displaced?
What is the significance of these values?
- deoxy Hb is more effective at binding Co2 than oxy Hb
- deoxy binds H+ more avidly than oxy Hb so CO2 movement is pulled right
40ml/L-1
50ml/L-1
40/50 = 0.8 which is RQ so matches metabolism
What is normal blood pH?
What range is required for life?
- 4
6. 8-7.8
What does the Henderson-Hasselbach equation show?
the pH of a solution is determined by the pk of the buffer system in operation and the ratio of its ionized to unionised forms
- for a given buffer system, the ratio of HA and A- define a unique pH
Which buffer system is most important one for blood? Why?
CO2/HCO3-
- high concs easy to measure
- metabolism is always adding CO2
- both can be regulated by lungs and kidneys
What can be used as a proxy for carbonic acid conc and why?
PaCO2 because HA readily dissociates into CO2 and water
Use the CO2/HCO3 buffer system and the movement of CO2 between tissues and blood to explain why hypochloremia is a consequence of repsiratory failure.
High CO2 in respiratory failure
Combines with H20 to produce carbonic acid.
Dissociates into bicarbonate ions and hydrogen ions.
High conc of bicarbonate that diffuse out of RBC’s into plasma. Cl- move in to maintain electrical neutrality
What is the henderson hasslebach equation?
Use these values of normal plasma to determine the pH
PCO2 = 5kPa
HCO3- = 24mM
pH = pK + log(A-/HA) pH = pK + log(HCO3-/CO2) pH = 6.1 + log(24/0.23x5) pH = 7.4
What is the pH, PaCO2 and HCO3 in:
- respiratory acidosis
- respiratory alkalosis
- metabolic acidosis
- metabloc alkalosis
Respiratory acidosis - low pH, high PaCO2, normal HCO3 Respiratory alkalosis - high pH, low PaCO2, normal HCO3 Metabolic acidosis - low pH, normal PaCO2, low HCO3 Metabolic alkalosis - high pH, normal PaCO2, high HCO3
What are the possible causes of:
- respiratory acidosis
- respiratory alkalosis
Respiratory acidosis
- hypoventilation
Respiratory alkalosis
- hyperventiliation
- PE
- panic attacks, anxiety
What are the possible causes of:
- metabolic acidosis
- metabolic alkalosis
Metabolic acidosis
- DKA
- diarrhoea
- renal failure
- shock
Metabolic alkalosis
- vomiting
- burns
- hyperkalaemia
