Acid–Base Regulation
by the Kidney

Normal arterial blood pH is maintained between \(7.35\) and \(7.45\). Even small deviations disrupt enzyme function, oxygen delivery, and cardiac rhythm. The kidneys are the slow but powerful arm of pH homeostasis: they cannot correct in seconds like the lungs, but they can generate new bicarbonate and permanently eliminate acid.

$$ \text{pH} = 6.1 + \log \frac{[\text{HCO}_3^-]}{0.03 \times P_{\text{CO}_2}} $$

When all filtered bicarbonate has already been reclaimed, any additional \(\text{H}^+\) secreted into the tubule must be buffered by other molecules. This is how the kidney generates brand-new bicarbonate:

A patient with uncontrolled diabetes produces excess ketoacids. Lab values: pH = 7.28, \([\text{HCO}_3^-]\) = \(14 \text{ mEq/L}\) (normal: 24), \(P_{\text{CO}_2}\) = \(30 \text{ mmHg}\) (normal: 40).

A patient with chronic obstructive pulmonary disease retains \(\text{CO}_2\). Lab values: pH = 7.33, \(P_{\text{CO}_2}\) = \(58 \text{ mmHg}\), \([\text{HCO}_3^-]\) = \(32 \text{ mEq/L}\).

A patient vomits repeatedly, losing gastric HCl. Lab values: pH = 7.52, \([\text{HCO}_3^-]\) = \(38 \text{ mEq/L}\), \(P_{\text{CO}_2}\) = \(48 \text{ mmHg}\).

In the proximal convoluted tubule, filtered \(\text{HCO}_3^-\) is reclaimed by first combining with which ion in the lumen?

Which process allows the kidney to generate brand-new bicarbonate (not merely reclaim filtered bicarbonate)?

A patient has: pH = 7.30, \(P_{\text{CO}_2}\) = 60 mmHg, \([\text{HCO}_3^-]\) = 29 mEq/L. What is the primary disorder?

During metabolic alkalosis, the kidney compensates primarily by:

In the Henderson–Hasselbalch equation \(\text{pH} = 6.1 + \log \frac{[\text{HCO}_3^-]}{0.03 \times P_{\text{CO}_2}}\), the kidneys regulate which variable?