Lecture 7 - CO2 in blood Flashcards
What does CO2 react with
Water - ECF
Hb - at a different site to O2
Purpose of CO2 in blood
Maintains pH of ECF via acid - base balance
Arterial blood pH
7.35 - 7.45
Dissolved CO2
Reacts with water to form H2CO3 - carbonic acid using carbonic anhydrase
(slow reaction as little carbonic anhydrase in plasma)
Carbonic acid quickly dissociates into H+ and HCO3-
What determines pH
Extent of dissociation of H+ in plasma and ECF (acid base balance)
Dependent on:
Amount of CO2 dissolved - rate of breathing
Concentration of HCO3- in blood
Therefore the ratio between CO2 and HCO3-
Increase in CO2
More H+ is produced
pH falls - acidosis
Increase in HCO3-
The increased HCO3- reacts with H+ to form CO2
CO2 is breathed off
pH increases - alkalosis
Henderson - Hasselbach equation
pH = pK + log ([HCO3-]/(pCO2 x 0.23)
pK - constant (6.1 at 37 degrees)
0.23 = CO2 solubility coefficient
Calculate pH at body temperature
pH = 6.1 + log (20) pH= 6.1 + 1.3 = 7.4
Log 20 as 20x more HCO3- than the amount of CO2 dissolved
Ratio of HCO3- to CO2
20:1
What determines pCO2 in arteries
Alveolar pCO2 which is controlled by the rate of ventilation
Hydrogen carbonate production in RBCs
In red blood cells
CO2 + H20 –> H2CO3- —> H+ + HCO3-
H+ Hb- —> HbH
H+ binds to Hb which are negatively charged which drives the reaction in the forwards direction
HCO3- is transported out of the cell via a chloride bicarbonate exchanger into the plasma where it is dissolved
Reaction speeded up by carbonic anhydrase in RBCs
HCO3- plasma concentration
25 mmol/L
H+ binding to Hb
Hb has a large capacity to bind H+
Controls amount of HCO3- produced by the erythrocyte
What controls HCO3- concentration in the blood?
Kidneys by varying HCO3- excretion
Why is pCO2 higher in venous blood?
CO2 produced by metabolically active tissues in mitochondria and dissolved in blood
More HCO3- is produced in RBCs due to T state of Hb therefore can produce CO2
Venous blood
In tissues, Hb is less saturated with O2 as unloaded more. (T state)
Therefore it is a better buffer as H+ more readily binds.
Increased drive for forward reaction producing HCO3- so more CO2 is created
Venous blood at lungs
- Higher pO2 therefore Hb have a higher affinity for O2 and load more (R state)
- Hb gives up extra H+
- H+ reacts with HCO3- to form CO2
- CO2 is breathed out
Carbamino compounds
CO2 can directly bind to the amine group of globin in Hb
For CO2 transport
More carbamino compounds are formed in tissues due to a higher pCO2 and the unloading of O2 facilitates the binding of CO2 to Hb
This CO2 is given up at the lungs
How is CO2 transported
Dissolved CO2
Hydrogen carbonate - reacted form
Carbamino compounds
What percentage of CO2 is a waste product
Venous [CO2] - 23.3
Arterial [CO2] - 21.5
23.3 - 21.5 = 1.8 mmol/L
8% = waste
92% used in the pH buffer system
