Question #13933

Extracellular fluid, or ECF, is being kept at a pH of approximately #7.4#, which means that the concentration of hydrogen ions, #"H"^(+)#, is

#["H"^(+)] = 10^(-"pH")#

#["H"^(+)] = 10^(-7.4) = color(green)(4.0 * 10^(-8)"M")#

In the context of extracellular liquid, it's useful to express this value in milimoles per liter, or milimolar

#4.0 * 10^(-8)color(red)(cancel(color(black)("moles")))/"L" * (10^3"mmoles")/(1color(red)(cancel(color(black)("mole")))) = 4.0 * 10^(-5)"mM"#

By comparison, the concentration of sodium cations, #"Na"^(+)#, is in the range

#["Na"^(+)] = "136-145 mEq/L"#

and the concentration of bicarbonate anions, #"HCO"_3^(-)#, is in the range

#["HCO"_3^(-)] = "22-28 mM"#

For example, the concentration of the bicarbonate anions is

#(28color(red)(cancel(color(black)("mM"))))/(4.0 * 10^(-5)color(red)(cancel(color(black)("mM")))) = 7 * 10^5#

higher than the concentration of hydrogen ions.

As a cool fact, concentrations of hydrogen ions that either exceed #12.0 * 10^(-5)"mM"# or fall below #2.0 * 10^(-5)"mM"# are not compatible with life.