BProteins

- Imidazole and a-amino groups on proteins have pKs that are within the physiologic pH range.

- Hemoglobin is a major intracellular buffer.

- In the physiologic pH range, deoxyhemoglobin is a better buffer than oxyhemoglobin.

3. Using the Henderson-Hasselbalch equation to calculate pH

where:

pH = -logio [H+] (pH units) pK = -log10 equilibrium constant (pH units) [A"] = base form of buffer (raM) [HA] = acid form of buffer (mM)

- A", the base form of the buffer, is the H+ acceptor.

- HA, the acid form of the buffer, is the H+ donor.

- When the concentrations of A" and HA are equal, the pH of the solution equals the pK of the buffer, as calculated by the Henderson-Hasselbalch equation.

- Example: The pK of the H2P04/HP04-2 buffer pair is 6.8. What are the relative concentrations of H2P04~ and HP04~2 in a urine sample that has a pH of 4.8?

hpo4-2

h2pof h2p04

h2po4-hpo4-2 h2po4-h2po4-

For this buffer pair, HP04~2 is Ar and H2P04~ is HA. Thus, the Henderson-Hasselbalch equation can be used to calculate that the concentration of H2P(Xf is 100 times that of HP04 2 in a urine sample of pH 4.8.

4. Titration curves (Figure 5-17)

- describe how the pH of a buffered solution changes as H+ ions are added to it or removed from it.

- As H+ ions are added to the solution, the HA form is produced; as H+ ions are removed, the A" form is produced.

- A buffer is most effective in the linear portion of the titration curve, where the addition or removal of H+ causes little change in pH.

- According to the Henderson-Hasselbalch equation, when the pH of the solution equals the pK, the concentrations of HA and A" are equal.

C. Renal acid-base

1. Reabsorption of filtered HC03" (Figure 5-18)

- occurs primarily in the proximal tubule.

a. Key features of reabsorption of filtered HC03~

(1) H+ and HCO:. are produced in the proximal tubule cells from C02 and H20. C02 and H20 combine to form H2C03, catalyzed by intracellular carbonic anhydrase; H2C03 dissociates into H+ and HCO;. . H+ is secreted into the lumen via the Na+-H+ exchange mechanism in the luminal membrane. The HCO;. is reabsorbed.

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Figure 5-17. Titration curve for a weak acid (HA) and its conjugate base (A ).

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Figure 5-17. Titration curve for a weak acid (HA) and its conjugate base (A ).

Lumen

Cell

Blood

Lumen

Cell

(filtered) H2C03

(filtered) H2C03

Filtered HC03 is reabsorbed

H2C03

H2C03

Figure 5-18. Mechanism for reabsorption of filtered HCO, in the proximal tubule. CA = carbonic anhydrase.

(2) In the lumen, the secreted H+ combines with filtered HC03" to form H2C03, which dissociates into C02 and H20, catalyzed by brush border carbonic anhydrase. C02 and H20 diffuse into the cell to start the cycle again.

(3) The process results in net reabsorption of filtered HC03". However, it does not result in net secretion of H+.

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