What do you mean by physiological buffers? What are the different types of buffers found in the blood? If possible, please state examples.

1 Answer
Jul 10, 2015

Answer:

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.

Explanation:

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.