Lec 8 -Carbon Dioxide in the blood Flashcards
What is the concentration of carbon dioxide in the arterial blood?
- Total content CO2 in arterial blood is 21 mmol.I-1
- Total content O2 in arterial blood is 8.9 mmol.I-1
How is blood pH controlled?
- CO2 has a major role in controlling blood pH.
- Arterial blood pH ,must be kept within a narrow range (pH 7.35 - pH 7.45)
What is the equation for the dissolved concentration of CO2?
[CO2] dissolved = solubility x pCO2.
What does dissolved CO2 react with?
It reacts with water in plasma and in red blood cells.
What is the solubility factor for CO2 at 37 degrees celsius?
0.23
How much water dissolves at pCO2 of 5.3 kPa?
At pCO2 of 5.3 kPa water dissolves 1.2 mmol.I-1 CO2
What is formed when dissolved CO2 reacts with water?
Dissolved CO2 reacts with water to form carbonic acid (H2CO3)
- In other words dissolved CO2 reacts with water to form hydrogen ions (H+) and hydrogen carbonate ions (HCO3-)
What does carbonic acid dissociate to?
- Carbonic acid dissociates to hydrogen ions (H+) and hydrogen carbonate ions (HCO3-)
What does the pH of plasma depend on?
- depends on how much CO2 reacts to form H+.
- –> depends on [CO2] dissolved which pushes the reaction to the right.
- –> depends on [HCO3-] which pushes the reaction to the left. - Depends on dissolved CO2
- And depends on the concentration of hydrogen carbonate.
What does the amount of CO2 depend on?
It depends directly on the partial pressure of CO2.
What happens to plasma pH when pCO2 rises?
plasma pH will fall
- becomes more acidic
What happens to plasma pH when pCO2 falls?
plasma pH will rise
- becomes more alkaline
What happens when there is a high [HCO3-]?
- A high [HCO3-] prevents nearly all dissolved CO2 from reacting.
- The pH of plasma is alkaline.
- The high HCO3 cannot come from CO2 in plasma.
What is the concentration of HCO3- in plasma and what cation is associated with this?
- plasma contains 25 mmol.I-1 HCO3-
- The cation associated with this is mostly Na+ not H+.
What is the Henderson- Hasselbalch equation?
- it provides a way of calculating pH from pCO2 and [HCO3-]
- pH = pK + Log ([HCO3-]/ (pCO2 x 0.23))
- pK is a constant, pK = 6.1 at 37 degrees celsius.
Describe hydrogen carbonate production in red blood cells?
- H+ ions bind to the negatively charged Hb inside RBCs.
- Chloride-bicarbonate exchanger transports HCO3- out of RBCs.
- This creates a plasma concentration of 25 mmol.1-1 HCO3-.
How is the reaction for hydrogen carbonate production in red blood cells sped up?
- The reaction is sped up by the enzyme carbonic anhydrase in red blood cells.
- CO2 + H2O —> H+ + HCO3-
- —> Carbonic anhydrase is above the arrow.
- The reaction proceeds in the forward direction because the products are mopped up in the RBC.
What does the binding of H+ to haemoglobin dictate?
- The amount of HCO3- that erythrocytes produce depends on the binding of H+ to haemoglobin.
- –> Note that erythrocytes produce HCO3- but they don’t control the concentration of HCO3- in plasma.
What is the role of the kidney in controlling [HCO3-]?
- The kidney controls the amount of HCO3- by varying excretion.
What is pH dependent on?
- pH is dependent on how much CO2 is present (controlled by rate of breathing) and how much bicarbonate is present (controlled by kidneys).
How does hydrogen carbonate buffers extra acid?
- The body produces acids such as lactic acid, kept acids and sulphuric acid.
- Acids react with HCO3- to produce CO2.
- Therefore [HCO3-] goes down.
- The CO2 produced is removed by breathing and pH changes are minimised (buffered).
What does the buffering of H+ by Hb depend on?
Buffering of H+ by Hb depends on level of oxygenation.
—> Always H+ ions bound to Hb but amount depends on the state of the Hb molecule.
What happens if more O2 binds Hb?
- R state
- Less H+ ions bind
- happens at lungs
What happens if less O2 binds Hb?
- T state
- More H+ ions bind
- happens at tissues
