Respiratory Acid/Base Balancemacleod/bioen/be6000/prevnotes/L14-acid-base.pdf · Acid-Base Balance...
Transcript of Respiratory Acid/Base Balancemacleod/bioen/be6000/prevnotes/L14-acid-base.pdf · Acid-Base Balance...
Acid-Base Balance Bioengineering 6000 CV Physiology
Respiratory Acid/Base Balance
Bioengineering 6000 CV PhysiologyAcid-Base Balance
Gas Transport, pH, and Erythrocytes
LungsTissue
Bioengineering 6000 CV PhysiologyAcid-Base Balance
Role of Hb in CO2/pH Management
H+ release and uptake drive CO2 conversion
(Lungs)
Bioengineering 6000 CV PhysiologyAcid-Base Balance
pH Regulation
• Normal plasma pH is 7.4 = 40 nM (i.e., low H+ conc.)• Mammals can tolerate a range of 7.0-7.8 (100-16 nM
H+) • Largest source of H+ ions is production of CO2 (and
bicarb)
• Imbalances in production and excretion of CO2 alter pH
• Meat produces net acid, plants net base, overall net acid; kidneys (in mammals) manage net changes
• Balance between metabolic and respiratory mechanisms
CO2 + H2O HCO3- + H+
pK = � log10(K)
Bioengineering 6000 CV PhysiologyAcid-Base Balance
Henderson-Hasselbalch Equation
Dissociation equation for weak acids
We can define a dissociation constant K
and as with pH, derive a logarithmic scale for K
K =[H+][A�]
[HA]
log10(K) = log10([H+]) + log10
[A�][HA]
K =[H+][A�]
[HA]
Bioengineering 6000 CV PhysiologyAcid-Base Balance
Henderson-Hasselbalch Equation
and taking the log of both sides, we can write
which we can rearrange as
With, after substituting yields
Starting from the dissociation constant K
pH = pK + log[A�][HA]
� log10([H+]) = � log10(K) + log10
[A�][HA]
Bioengineering 6000 CV PhysiologyAcid-Base Balance
Henderson-Hasselbalch EquationSo the final equation is
Which we can also write more generally as
If pH = pK then half the acid is bound and the other half dissociated.
If pH - pK = 1, dissociated exceeds by factor of 10.
pH = pK + log[proton acceptor][proton donor]
pH = pK + log[A�][HA]
pH = pK0 + log[HCO�
3 ][CO2]
Bioengineering 6000 CV PhysiologyAcid-Base Balance
Henderson-Hasselbalch Equation
Now if we apply the HH Equation to the CO2/HCO3 pair (noting that this is the result of two interactions and hence not strictly speaking, a dissociation):
Or substituting for pK’ (=6.1 in blood at 37C) and PCO2 with α the solubility of CO2
pH = 6.1 + log[HCO�3 ]↵PCO2
CO2 + H2O H2CO3 H+ + HCO3-
Bioengineering 6000 CV PhysiologyAcid-Base Balance
Role of respiration in Acid Balance
• pH depends on PCO2 (respiration) and HCO3-
(metabolism/kidneys)• Respiratory acidosis: PCO2 rises and pH drops• Metabolic acidosis: loss of HCO3
- and pH drops• Changes in respiration can change PCO2 and thus pH
levels in the blood • Charge balance is necessary:
– e.g. drop in Cl- leads to drop in HCO3- and acidosis
pH = 6.1 + log[HCO�3 ]↵PCO2
Bioengineering 6000 CV PhysiologyAcid-Base Balance
Blood Buffers
• Best buffers are those with pK = pH• The CO2/HCO3- system is not a major chemical buffer
(pK is too low) but it is the largest functional buffer because of regulation of HCO3- in the kidneys
• Proteins (hemoglobin) and phosphates are the real chemical buffers
• To change blood pH from 7.4 to 7.0 requires 28 mM H+
– 18 mM by conversion HCO3- to CO2
– 8 mM by hemoglobin– 2 mM by blood proteins and phosphates
• To achieve the same pH change in aqueous solution requires 60 nM H+