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

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.