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Henderson Hasselbalch equation

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HCO3: meq/L
PCO2: mmHg

Background:

PCO2 is equivalent to PaCO2

Normal values:
HCO3: 22-26 meq/L
PCO2: 35 - 45 mmHg
pH: 7.35 - 7.45

The Henderson-Hasselbalch equation describes the relationship of pH as a measure of acidity with the acid dissociation constant (pKa), in biological and chemical systems. The equation is especially useful for estimating the pH of a buffer solution and finding the equilibrium pH in acid-base reactions.

Quick derivation:
Carbonic acid (H2CO3) equilibrium:
[H+] x [HCO3-] <---> [H2CO3] <---> [CO2] x [H2O]

[H+] x [HCO3-] = K x [CO2] x [H2O]
K = equilibrium constant

pH = pK + log ( [HCO3-] / [CO2] )

pK = -log K
-log H+ = -log pK -log (0.03 x PCO2)/ HCO3-
pH = pK -log (0.03 x PCO2)/ HCO3-
pH = pK +log HCO3-/ (0.03 x PCO2)
pK = 6.1 (bicarbonate buffer system)

Final:
pH = 6.1 +log (HCO3-/ (0.03 x PCO2))
Henderson Hasselbalch equation