S4) Carbon Dioxide in the Blood Flashcards
Describe 3 chemical properties of carbon dioxide in the blood
- CO2 is more soluble than oxygen
- CO2 reacts chemically with water
- CO2 also reacts with Hb (different site from O2)
Compare and contrast the content of O2 and CO2 in arterial blood
- Total content CO2 in arterial blood ≈ 21 mmol.l-1
- Total content O2 in arterial blood ≈ 8.9 mmol.l-1
What is the normal range of arterial blood pH?
pH 7.35 – pH 7.45
What is the most important substance in controlling blood pH?
CO2
What is the solubility factor for CO2 at 37oC?
0.23
How does one calculate the [CO2]dissolved in blood?
[CO2]dissolved = solubility factor x pCO2
What is the role of dissolved CO2 in plasma and in red blood cells?
- Not a waste product
- Reacts with water to act as a buffer
What is the result of dissolved carbon dioxide reacting with water?
- Dissolved CO2 reacts with water to form carbonic acid
- Carbonic acid very quickly dissociates to hydrogen ions and hydrogen carbonate ions
pH of blood plasma depends on how much CO2 reacts to form H+.
What determines this?
- [CO2]dissolved (pushes reaction to right)
- [HCO3- ] (pushes reaction to left)
Explain the relationship between the plasma pH and pCO2 of the alveoli
[CO2]dissolved depends directly on pCO2:
- If pCO2 rises = plasma pH falls
- If pCO2 falls = plasma pH rises
Using the Hendersson-Hasselbach equation, what is the formula to calculate pH from pCO2 and [HCO3-] ?
How does one calculate the pH of blood plasma using the Hendersson-Hasselbach equation?
- pK is a constant (6.1 at 37oC)
- 20x more HCO3- than dissolved CO2 (log20 = 1.3)
pH = 6.1 + 1.3 = 7.4
Which values determine the pH of arterial blood?
- Ratio of [HCO3-] and pCO2 determine pH
- pCO2 determined by alveolar pCO2
- Alveolar pCO2 is determined by rate of ventilation
How is the plasma concentration of hydrogen carbonate established
Hydrogen carbonate production in red blood cells:
– C02 can recat with water inside RBC
- H+ ions bind to Hb(-) inside RBCs
- Chloride-bicarbonate exchanger transports HCO3- out of RBCs in exchange for Cland creates a plasma concentration of 25mmol.l-1 HCO3-
Which enzyme catalyses the production of hydrogen carbonate in red blood cells?
Carbonic anhydrase
this is a rapid reaction unlike in plasma as there is a high conc of it
C02 + H20 → H + Hco3
Describe the role of haemoglobin in the production of hydrogen carbonate ions by red blood cells
- Haemoglobin has a large capacity for binding H+ ions
- The amount of HCO3- that erythrocytes produce depends on the binding of H+ to haemoglobin
Explain the role of the kidney in [HCO3-]
- Kidney controls the amount of HCO3- by varying excretion
- Hence, pH is dependent on how much CO2 and HCO3- is present
Hydogen carbonate buffers extra acids.
Identify 3 of these
- Lactic acid
- Keto acids
- Sulphuric acid
Explain how hydrogen carbonate buffers extra acids
- Acids react with HCO3- to produce CO2
- [HCO3-] decreases
- CO2 is produced & removed through ventilation
- pH changes are buffered
Why is pCO2 is higher in venous blood?
More CO2 from metabolically active tissues
Explain how the buffering of H+ by Hb depends on level of oxygenation
- If more O2 binds Hb → R-state and less H+ ions bind (lungs)
- If less O2 binds Hb → T-state and more H+ ions bind (tissues)
Explain the processes occurring in venous blood at the tissues
- Hb has lost O2 and so binds more H+
- More HCO3- forms & is exported to the plasma
Explain the processes occurring in venous blood at the lungs
- Hb picks up O2 and goes into R-state
- Hb gives up the extra H+ (from tissues)
- H+ reacts with HCO3- to form CO2
- CO2 is breathed out
How are carbamino compounds formed?
- CO2 binds directly to amine groups on globin of Hb
- This contributes to CO2 transport
Why are more carbamino compounds are formed at the tissues?
- pCO2 is higher
- Unloading of O2 facilitates binding of CO2 to Hb
- CO2 is then given up at the lungs
Identify the 3 forms in which carbon dioxide is transported
- Dissolved CO2 (10%)
- Hydrogen carbonate (60%)
- Carbamino compounds (30%)
How much carbon dioxide is actually transported?
Transported CO2
= CO2 venous blood – CO2 arterial blood
= 23.3 – 21.5 mmol.l-1
= 1.8 mmol.l-1 (8% of the total is transported)
What is the role of the CO2 in the blood that is not transported?
Remaining CO2 is part of the pH buffering system
what is the range of Ph that shows acidotic blood and what range shows alkalotic
- 35-7.4 → acidotic
- 4-7.45 → alkalotic
what is the pH of bodily fluids determined between the relationship of
- amount of CO2 dissolved in the plasma
- amount of HC03 formed from c02 in the red blood cell involving haemoglobin
- as long as there is 20x as much HCO3 dissolved as Co2 in plasma then pH will be 7.4
what organ controls the levels of HCO3
kidney
what is pCo2 at venous blood give 3 reasons for this
6 pKa
more co2 in venous blood as:
- tissue produce c02, increasing pco2, more dissolves
- oxygen is removed from heat at tissue so more c02 can attach and recat in RBC
- 10% more co2 reacts in venous bloods
what is the relationship between Pco2 and the content of `co2 in the blood
- linear
- so with hyperventilation both will reduce
what is the difference between the speed of the lungs and kidneys
lungs can make changes in co2 much faster
kidney takes much longer to alter HCo3 levels
what the body response to increase Co2
hypoventilate
