Respiratory Acid/Base Balance
Acid-Base Balance
Bioengineering 6000 CV Physiology
Gas Transport, pH, and Erythrocytes
Lungs
Tissue
Acid-Base Balance
Bioengineering 6000 CV Physiology
Role of Hb in CO2/pH Management
(Lungs)
H+ release and uptake drive CO2 conversion
Acid-Base Balance
Bioengineering 6000 CV Physiology
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)
+
CO2 + H2O
HCO3 + H
• 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
Acid-Base Balance
Bioengineering 6000 CV Physiology
Henderson-Hasselbalch Equation
Dissociation equation for weak acids
We can define a dissociation constant K
[H+ ][A ]
K=
[HA]
and as with pH, derive a logarithmic scale for K
pK =
log10 (K)
Acid-Base Balance
Bioengineering 6000 CV Physiology
Henderson-Hasselbalch Equation
Starting from the dissociation
constant K
[H+ ][A ]
K=
[HA]
and taking the log of both sides, we can write
log10 (K) = log10 ([H ]) + log10
+
which we can rearrange as
log10 ([H+ ]) =
log10 (K) + log10
With, after substituting yields
pH = pK + log
Acid-Base Balance
[A ]
[HA]
[A ]
[HA]
[A ]
[HA]
Bioengineering 6000 CV Physiology
Henderson-Hasselbalch Equation
So the final equation is
pH = pK + log
[A ]
[HA]
Which we can also write more generally as
pH = pK + log
[proton acceptor]
[proton donor]
If pH = pK then half the acid is bound and the other half
dissociated.
If pH - pK = 1, dissociated exceeds by factor of 10.
Acid-Base Balance
Bioengineering 6000 CV Physiology
Henderson-Hasselbalch Equation
CO2 + H2O
H2CO3
H+ + HCO3-
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):
pH = pK0 + log
[HCO3 ]
[CO2 ]
Or substituting for pK’ (=6.1 in blood at 37C) and PCO2 with α the
solubility of CO2
pH = 6.1 + log
Acid-Base Balance
[HCO3 ]
↵PCO2
Bioengineering 6000 CV Physiology
Role of respiration in Acid Balance
pH = 6.1 + log
[HCO3 ]
↵PCO2
• 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
Acid-Base Balance
Bioengineering 6000 CV Physiology
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+
Acid-Base Balance
Bioengineering 6000 CV Physiology