Part 4 of the urinary lab. The lungs hold the fast dial and the kidney holds the slow one — but only the kidney can make new base.
Module A · The 20-to-1 ratio
pH = 6.1 + log ( [HCO₃⁻] ÷ 0.03 × PCO₂ )
Module B · What the kidney does about it
Two different moves, and students blur them together every year. Pick one.
Module C · Net acid excretion
NAE = (titratable acid + NH₄⁺) − HCO₃⁻ excreted
mEq/day — mostly H⁺ stuck onto phosphate
mEq/day — made from glutamine, and the part that can really scale up
mEq/day — base lost in the urine
The floor at pH 4.5
The pumps in a type A intercalated cell can build a gradient of about a thousandfold, and no more. That puts a hard floor under urine pH at roughly 4.5.
Free hydrogen ions are a tiny part of the job — at pH 4.5 the urine carries well under a milliequivalent of them. Almost everything excreted is hydrogen already stuck onto a buffer: phosphate, which we count as titratable acid, and ammonia, which leaves as ammonium. Without the buffers the urine would hit its floor in minutes and the kidney would be finished for the day.
Which is why ammonium is the part that scales. In a sustained acidosis the kidney can push ammonium production up several-fold over a couple of days, and every ammonium that leaves takes an acid with it and leaves a new bicarbonate behind in the blood.
Module D · The frame
| Started in the kidney (metabolic) | Started in the lungs (respiratory) | |
|---|---|---|
| Acidosis | HCO₃⁻ is low. Lungs compensate in minutes by blowing CO₂ off; the kidney makes new base over days. | Lecture. |
| Alkalosis | HCO₃⁻ is high. Breathing slows a little; the kidney dumps base over days. | Lecture. |
You have now done the kidney column. When the lungs arrive in lecture, the only new question is which organ caused the problem and which one is cleaning it up.