Session 4 - Carbon dioxide in blood + chemical control of breathing Flashcards
How much Carbon dioxide is in arterial blood compared to oxygen? Why?
21 mmol/L
Almost 2.5x as much Co2. (oxygen = 8.9mmol/L)
Co2 is more soluble than oxygen.
Why is control of Co2 important in arterial blood?
Control of Co2 is important as concentration of Co2 determines pH.
How does some CO2 react with water in plasma?
CO2 + H20 > H2CO3 > H+ + HCO3-
Forms carbonic acid (H2CO3) which quickly dissociated, forming protons and bicarbonate ions.
Reversible!
Which factors affect the pH of plasma?
1) How much CO2 reacts to form H+
2) How much CO2 is dissolved in it
3) The concentration of hydrogen carbonate
What affects how much CO2 forms H+?
- More dissolved CO2, pushed reaction to the right, more protons.
- Less dissolved CO2, pushes reaction the left, less protons.
How does dissolved CO2 change the pH? What is the main control of this?
- pCO2 Rises - plasma pH FALLS.
- pCO2 falls - plasma pH RISES.
- pCO2 of alveoli is the determining factor, as the amount of CO2 dissolved is directly dependent of partial pressure of CO2.
Roughly how much hydrogen carbonate is present in the blood? Which cation is associated with it?
Around 25mmol/L of HCO3-
Main cation is Na+, not H+
Therefore this high bicarbonate conc. is not from CO2 in plasma.
Why does most of the dissolved CO2 not react in the plasma?
Due to the high concentration of bicarbonate ions.
Means the equilibrium is pushed to the left, where most CO2 does not react.
Why is the pH of blood slightly alkaline?
Due to the dissolved bicarbonate ions.
Which equation can be used to calculate the pH of blood from CO2 and bicarbonate concentration?
Henderson Hasselbalch.
pH = pK + Log ([HCO3-]/(pCO2 x 0.23))
pK is a constant = 6.1 at 37C
(0.23 is CO2 solubility constant)
Which main factors then are responsible for the pH of arterial blood? What controls these factors?
Ratio of [HCO3-] and pCO2.
Breathing rate changes pCO2.
Kidneys vary excretion of HCO3-
What determines the pCO2 of arterial blood?
The rate of ventilation in the lungs.
How is hydrogen carbonate produced in the red blood cells?
Carbonic Anhydrase
Reaction is forward, as H+ binds to Haemoglobin, and bicarbonate is removed by the chloride bicarbonate exchanger.
(1 bicarbonate out, 1 chloride in)
This allows bicarbonate to be made more readily in the RBC.
Creates plasma level of 25mmol/L of bicarbonate.
What determines the amount of bicarbonate that erythrocytes produce?
The amount of H+ that binds to negative sites in haemoglobin.
What is the relationship between plasma [HCO3-] and pCO2?
[HCO3-] doesnt change much with pCO2.
How is the amount of HCO3 in the blood controlled?
Kidneys vary the amount by varying secretion.