Physiological buffers are chemicals used by the body to prevent large changes in the ##”pH”## of a bodily fluid.
The four physiological buffers are the bicarbonate, phosphate, hemoglobin, and protein systems.
The ##”pH”## of a buffer is determined by the Henderson-Hasselbalch equation:
##”pH” = “p”K_a + log([“A”^-]/[“HA”])##
The buffer is best able to resist changes in ##”pH”## when the ##”pH”## of the buffer is close to the ##”pH”## of blood (7.37 to 7.42), so the ##”p”K_a## of the acid should be close to 7.4.
Phosphate Buffer
The phosphate buffer system consists of ##”H”_2″PO”_4^(-)## and ##”HPO”_4^(2-)## ions.
The equilibrium is
##”H”_2″PO”_4^(-)(“aq”) + “H”_2″O” ⇌ “H”_3″O”^+(“aq”) + “HPO”_4^(2-)(“aq”)##; ##”p”K_a = 7.21##
The phosphate buffer can easily maintain a ##”pH”## of 7.4.
Carbonate Buffer
The equilibrium is
##”H”_2″CO”_3(“aq”) + “H”_2″O(l)” ⇌ “HCO”_3^(-)(“aq”) + “H”_3″O”^+(aq)##; ##”p”K_a = 6.1##
This buffer functions in exactly the same way as the phosphate buffer, but it is not ideal because its ##”p”K_a## is too far from ##”pH”## 7.4.
Perhaps more importantly, the enzyme carbonic anhydrase converts ##”H”_2″CO”_3## into ##”CO”_2## that is dissolved in the blood and is then exhaled as ##”CO”_2 ##gas.
Hemoglobin
The general equation is:
##”HHb”^+ + “O”_2 + “H”_2″O” ⇌ “HbO”_2 + “H”_3″O”^+##; ##”p”Ka = 6.8##
It shows that oxygenation of ##”Hb”## promotes the formation of ##”H”_3″O”^+##.
This shifts the bicarbonate buffer equilibrium towards ##”CO”_2## formation, and ##”CO”_2## is released from the red blood cells.
Proteins
A protein is a long chain of amino acid residues, but this long chain still has free carboxylate groups ##”COO”^(-)## and free amino groups ##”NH”_2##.
We could write the equation for a protein buffer system as
##”H”_3stackrel(+)(“N”)—”R—COO”^(-) + “H”_2″O” ⇌ “H”_2″N—R—COO”^(-) + “H”_3″O”^+##
The protein can then act as a buffer.