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+