6. Carbon dioxide in the blood Flashcards
What is the normal pH of arterial blood?
7.35-7.45
How is amount CO2 dissolved in plasma calculated?
Solubility x pCO2
What is the solubility if CO2?
0.23 at 37 degrees
why is there a high co2 conc in arterial blood?
- CO2 has a major role in controlling blood pH
* Chemical reactant in the major pH buffering system of blood
What does dissolved CO2 react with to form?
Reacts with water to form Carbonic acid which very quickly dissociates into H+ and HCO3-
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
Why is there more dissolved CO2 even though lower pCO2 c/w pO2 ?
because of markedly increased solubility c/w oxygen
WHat does pH of plasma depend on?
Depends on how much CO2 reacts to form H+
- which depends on dissolved [CO2] and [HCO3-]
And the ratio of [HCO3-]:[CO2]
What does the amount of CO2 dissolved depend on?
Partial pressure of CO2
What happens to pH if arterial pCO2 increases?
Plasma pH will fall, more H+ ions produced
What happens to pH if arterial pCO2 decreases?
Plasma pH will rise, H+ ions react with HCO3- (to form CO2)
what determines arterial pCO2?
The pCO2 of alveoli
how is Alveolar, and hence arterial pCO2 controlled
by altering the rate of breathing
What is the Henderson-Hasselbalch equation?
pH = pK + log [HCO3-] / (pCO2 x 0.23)
What is the normal amount of bicarbonate dissolved in arterial plasma?
25 mmol/L
How is high [HCO3-] established?
CO2 in RBCs react with water rapidly forming H+ and HCO3-. H+ are bound to haemoglobin which has a large buffering capacity. Reaction is therefore drawn towards production of HCO3-.
Why is the reaction of CO2 with water in RBCs rapid?
• Reaction is speeded up by the enzyme carbonic anhydrase (CA) enzyme present in RBCs but not present in plasma
How is bicarbonate transported out of RBC?
The HCO3- is transported out of the RBC chloride:bicarbonate exchanger
What happens to the H+ produced?
the H+ is bound to haemoglobin
What does the amount of HCO3- produced in RBCs depend on?
Depends primarily upon the buffering effects of haemoglobin (binding of H+ top Hb)
What controls concentration of HCO3- in the blood?
although RBC makes the HCO3- it does not CONTROL
HCO3- concentration - HCO3- concentration controlled by KIDNEY - varying excretion– also can create more HCO3-
what is pH dependent on?
– How much CO2 is present (controlled by rate and depth of breathing)
– And how much HCO3
- is present (controlled by kidneys)
describe how Bicarbonate buffers extra acid?
• As part of normal metabolism body produces acids
– Lactic acid, keto acids, sulphuric acid
• Acids react with HCO3- to produce CO2
– Reaction pushed to the left CO2 + H2O ← H+ + HCO3-
– CO2 levels increase
– The extra CO2 produced is removed by breathing and pH changes are minimised (buffered)
– More HCO3 needs to be produced to replenish
What is the value of venous pCO2, why is it higher than arterial?
6.0kPa, comes from metabolically active tissues
What determines the amount of H+ bound to Hb?
The state of the Hb:
-– If more O2 binds Hb → R-state and less H+ ions bind (HbO2, at lungs)
If less O2 binds Hb → T-state and more H+ ions bind (Hb, as tissues)
