Metabolic acidosis: Difference between revisions

The [[Henderson-Hasselbalch equation]] mathematically describes the relationship between blood pH and the components of the bicarbonate buffering system: <math chem display=block>p\ce{H}=pK_\text{a}+\operatorname{\mathrm{Log}}\frac{\left[\ce{HCO3^-}\right]}{\left[\ce{CO2}\right]}\text{,}</math> where {{math|''pK''_a&nbsp;&approx;&nbsp; 6.1}}. In clinical practice, the {{CO2}} concentration is usually determined via [[Henry's law]] from {{math|''P''_a{{CO2}}}}, the {{CO2}} partial pressure in arterial blood: <math chem display=block>[\ce{CO2}] = (0.03\text{ L}^{-1}/\text{mmHg})\times P_{\text{a}\ce{CO2}}\text{.}</math>

For example, blood gas machines usually determine bicarbonate concentrations from measured ''p''H and {{math|''P''_a{{CO2}}}} values. Mathematically, the algorithm [[substitution (algebra)|substitutes]] the Henry's law formula into the Henderson-Hasselbach equation and then rearranges: <math chem display=block>\left[\ce{HCO3^-}\right]=0.03\cdot P_{\text{a}\ce{CO2}}\cdot 10^{p\ce{H}-pK_\text{a}}</math> At [[sea level]], normal numbers might be {{math|''p''H&nbsp;&approx;&nbsp; 7.4}} and {{math|''P''_a{{CO2}}&nbsp;&approx; 40&nbsp;40mmHg}}; these then imply <math chem display=block>\begin{align}

\left[\ce{HCO3^-}\right]&=(0.03\cdot40text{ L}^{-1}/\text{mmHg})(40\text{ mmHg})\cdot10^{7.4-6.1} \\

&=24\text{ L}^{-1}