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QUESTION

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

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.

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