Lecture 7 - CO2 Transport in Blood Flashcards
What are the 3 forms in which CO2 exists in blood as?
1) Dissolved CO2 - CO2 more soluble than oxygen
2) Bicarbonate ions - CO2 reacts with H2O to form HCO3-
3) Carbamino-Hb compounds - CO2 reacts with Hb
Total O2 content in arterial blood is 8.9mmol/L, but total CO2 content in arterial blood is 21mmol/L - why is there so much CO2 going to the tissues?
CO2 in arterial blood not primarily a waste product. CO2 is a major buffer in acid-base balance of blood (can bind or release H+ to dampen swings in pH).
Describe how the bicarbonate buffer system works in the body to control pH
- The reaction is fully reversible. H2CO3 is a fleeting intermediate that quickly breaks down into H+ & HCO3- so sometimes not even written.
- Net reaction is slightly to the left as there is more dissolved HCO3- (25mmol/L) than dissolved CO2 (1.2mmol/L).
- Concentration of reactants & products determines direction, e.g.: in hypoventilation, CO2 increases, so reaction swings to right and pH increased leading to respiratory acidosis.
What does the Henderson Hasselbalch equation calculate?
Apply this to the bicarb buffer system to work out body pH?
- Calculates pH involving a weak acid if you know the concentration of the acid and conjugate base
- Conjugate base is HCO3-, weak acid is CO2 in the bicarb buffering system
- 6.1 + Log (25/1.2) = 7.4 pH
According to the Henderson Hasselbalch equation, what does the pH in the blood depends on?
- Concentration of dissolved CO2 + HCO3-
- If CO2 increases, reaction swings to right, increases pH, vice versa for HCO3-
- In body, because there’s more dissolved HCO3-, reaction favoured to left which is why pH is slightly alkaline
- Ratio of [HCO3-]:[CO2] should be 20:1
How do primary respiratory acidosis + alkalosis occur?
How is the rate of arterial PCO2 controlled?
Primary respiratory acidosis = PCO2 rises (hypoventilation), reaction swings to right, plasma pH falls
Primary respiratory alkalosis = PCO2 falls (hyperventilation), reaction swings to left, plasma pH rises
- Arterial PCO2 determined by Alveolar PCO2, therefore controlled by altering rate of breathing (expelling CO2)
Dissolved HCO3- in plasma (25mmol/L) is high, however very small amount comes from the CO2 reaction (as there’s much less dissolved CO2), where does the majority come from?
What is HCO3- concentration controlled by?
- Produced by RBC’s in tissues. CO2 from tissues enter RBC
- CO2 combined with H2O, catalysed by C.A to form HCO3- & H+
- HCO3- removed via chloride:bicarbonate co-transporter
- Hb binds H+ to buffer them, so reaction can stay to right
- HCO3- concentration controlled by the kidney
Therefore, what is the role of the kidneys and the lungs in control of pH
- Kidney alters HCO3- levels by varying amount of excretion
- Lungs alter CO2 levels by varying rate/depth of breathing
- pH depends on HCO3- and CO2 levels
PCO2 is higher in venous blood as it comes from metabolically active tissues, how is all this CO2 delivered to the lungs to be eliminated?
- Hb binds H+ from RBC’s. When oxygen is low (PCO2 higher), Hb is in T-state and binds more H+ ions (e.g.: at the tissues - vice versa for lungs)
- This drives reaction to the right, producing more HCO3-, which is transported out of RBC in venous blood plasma
- Therefore, increasing amount of CO2 transported in blood as bicarbonate ions
What happens to venous blood when it arrives at the lungs so that CO2 is breathed out?
- Hb picks up oxygen in lungs, goes into R-state (co-operativity effect)
- This means it gives up H+ it took on in tissues, H+ reacts with HCO3- to form CO2
- Reaction is pushed to left (opposite to tissues), excess CO2 moves into alveolus and is exhaled
How are carbamino compounds formed and how do they contribute to CO2 transport?
What is the Haldane effect?
- CO2 binds directly to amine group of Hb. More carbamino compounds formed at tissues at PCO2 is higher and unloading of O2 from Hb facilitates CO2 binding (T-state Hb binds CO2 better)
- CO2 given up at lungs as Hb becomes oxygen rich. Haldane effect = oxygenation of Hb in lungs displaced CO2, increasing removal of CO2
Of the 3 forms of CO2 that can exist in blood, how much of CO2 do each account for in blood?
1) Dissolved CO2 - 10%
2) Bicarbonate - 60%
3) Carbamino compounds - 30%
What are the steps for analysing arterial blood gas (ABG) values if we have an acidosis (pH <7.35)
1) Look at pH - if <7.35 we have an acidosis
2) Look at PCO2 - if elevated it is respiratory acidosis, if normal its not respiratory
3) Look at HCO3 - if decreased it is a metabolic acidosis
4) If respiratory acidosis - If HCO3- is elevated there is compensation, if pH 7.35-7.39 it is full compensation, if pH <7.35 it is partial compensation
5) If metabolic acidosis - if PCO2 has decreased, there is compensation, if pH 7..35-7.39 it is full compensation, if pH <7.35 it is partial compensation
What are the steps for analysing an ABG when there is an alkalosis (pH >7.45)
1) Look at pH, if >7.45 then it is an alkalosis
2) Look at PCO2, if its low then it is a respiratory alkalosis, if normal or high it is not
3) Look at HCO3-, if its high then its metabolic alkalosis
4) If respiratory alkalosis, look at HCO3-. If decreased there is compensation, if pH 7.4-7.45 then full compensation, if pH >7.45 then partial compensation
5) If metabolic alkalosis, look at PCO2, if increases there is compensation, if pH 7.4-7.45 then full compensation, if >7.45 then partial compensation
Explain the acid-base status using the ABG results below
1) pH is 7.48 so we have an alkalosis
2) PCO2 is low, so we have a respiratory alkalosis
3) HCO3- is normal, so we have no compensation
Therefore, we have a respiratory alkalosis with no compensation, most likely due to hyperventilation (too much removal of CO2)
More questions on slide 31-36 good for revision
