12.3 Acid-Base Physiology & Compensation

12.3 Acid-Base Physiology & Compensation

Quick Answer: Use Henderson–Hasselbalch: pH rises when HCO3−/PaCO2 rises. Classify primary metabolic (HCO3−) vs respiratory (PaCO2) disorders, compute the anion gap, verify compensation (Winter’s formula for metabolic acidosis), and localize renal defects to RTA types 1, 2, and 4. Kidneys eliminate daily acid as titratable acid and NH4+.

Acid–base items reward a fixed sequence: (1) acidemia or alkalemia, (2) metabolic vs respiratory primary process, (3) anion gap if metabolic acidosis, (4) expected compensation vs mixed disorder, (5) clinical cause and renal handling when relevant.

Henderson–Hasselbalch and the Bicarbonate Buffer

The clinically used relationship is:

$\mathrm{pH} = 6.1 + \log_{10}\left(\frac{[\mathrm{HCO}_3^-]}{0.03 \times P_a\mathrm{CO}_2}\right)$

Normal arterial values (teaching targets):

CO2 is the respiratory acid (volatile); HCO3− is the metabolic base. Lungs adjust PaCO2 in minutes; kidneys adjust HCO3− over hours to days by reabsorbing filtered HCO3− and generating “new” HCO3− via net acid excretion.

Primary Disorders at a Glance

Acute vs chronic respiratory rules of thumb (common teaching deltas):

• Acute respiratory acidosis: HCO3− rises ~1 mEq/L per 10 mmHg PaCO2 rise; chronic ~3–4 mEq/L per 10 mmHg.
• Acute respiratory alkalosis: HCO3− falls ~2 mEq/L per 10 mmHg PaCO2 fall; chronic ~5 mEq/L per 10 mmHg.

These numbers exist so you can spot a second primary disorder when HCO3− is outside the expected band.

Anion-Gap Metabolic Acidosis

$\mathrm{AG} = \mathrm{Na}^+ - (\mathrm{Cl}^- + \mathrm{HCO}_3^-)$

(Optionally correct AG upward ~2.5 mEq/L for every 1 g/dL albumin below 4 g/dL—hypoalbuminemia hides gap acidosis.)

A high AG means unmeasured anions have replaced HCO3−. Teaching mnemonics (either is acceptable if you know the contents):

MUDPILES (classic): Methanol, Uremia, Diabetic/alcoholic/starvation ketoacidosis, Paraldehyde/phenformin (historical), Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates.

GOLDMARK (updated): Glycols (ethylene/propylene), Oxoproline (chronic acetaminophen), L-lactate, D-lactate, Methanol, Aspirin, Renal failure, Ketoacidosis.

Osmolar gap = measured Posm − calculated Posm; elevated early in toxic alcohol ingestion before the parent alcohol is fully metabolized to anions (gap acidosis rises as osmolar gap falls).

Non–Anion-Gap (Hyperchloremic) Metabolic Acidosis

HCO3− loss or impaired renal net acid excretion with Cl− retention keeps AG normal.

HARDUP-style teaching set: Hyperalimentation, Acetazolamide, Renal tubular acidosis, Diarrhea, Uretero-sigmoidostomy, Pancreatic fistula. Also: normal saline resuscitation (Cl− load), early renal failure sometimes, and toluene (hippurate excreted rapidly → may appear non-gap).

Urine anion gap (Na+ + K+ − Cl−) is a bedside surrogate for NH4+ excretion: negative UAG suggests appropriate NH4+ (e.g., diarrhea); positive UAG suggests impaired NH4+ excretion (RTA). It is a teaching tool with caveats (ketones, drugs), but CBSE loves the diarrhea vs RTA contrast.

Winter’s Formula and Compensation Checks

For metabolic acidosis, expected PaCO2 (Winter’s formula):

$P_a\mathrm{CO}_2 \approx 1.5 \times [\mathrm{HCO}_3^-] + 8 \; (\pm 2)$

Example: HCO3− = 10 mEq/L → expected PaCO2 ≈ 1.5×10 + 8 = 23 ± 2 mmHg.
If measured PaCO2 is much higher than expected → coexisting respiratory acidosis (e.g., fatigue in DKA, opioid suppression).
If PaCO2 is much lower than expected → coexisting respiratory alkalosis (e.g., salicylate, sepsis, liver failure).

Other common checks:

• Metabolic alkalosis: expected PaCO2 rises roughly 0.7 mmHg per 1 mEq/L HCO3− rise (limited).
• Always interpret ABG with the clinical story; pure rules fail when three processes coexist (classic salicylate: primary respiratory alkalosis + AG metabolic acidosis).

Mixed Disorders: Recognition Pattern

Suspect a mixed disorder when:

1. Compensation is outside the predicted range (Winter’s or chronic respiratory rules).
2. pH is near normal but both PaCO2 and HCO3− are abnormal in “same direction” extremes (e.g., very high HCO3− and very high PaCO2 with near-normal pH → chronic respiratory acidosis plus metabolic alkalosis, or fully compensated single process—use history).
3. AG metabolic acidosis coexists with metabolic alkalosis: AG rises more than HCO3− falls (ΔAG vs ΔHCO3− comparison / “excess gap”).

CBSE loves salicylates, vomiting plus lactic acidosis, and COPD patient who also receives diuretics (respiratory acidosis + metabolic alkalosis).

Renal Acid Excretion: Titratable Acid and Ammonium

Daily nonvolatile acid production is roughly 50–100 mEq/day (diet-dependent). Filtered HCO3− must be nearly completely reabsorbed (mostly PCT via NHE3 + carbonic anhydrase) or the animal would rapidly lose base.

Net acid excretion (NAE) ≈ titratable acid + NH4+ − urinary HCO3−.

Chronic acidosis increases glutamine metabolism and ammoniagenesis, raising “new HCO3−” generation. Hyperkalemia suppresses ammoniagenesis—one reason type 4 RTA maintains mild acidosis.

Distal α-intercalated cells secrete H+ via H+-ATPase (and H+/K+-ATPase), lowering urine pH to ~4.5–5.0 under maximal stimulation when the distal nephron is intact.

Renal Tubular Acidosis Types

Type 2 RTA mechanism detail: When plasma HCO3− is normal, a large fraction of filtered load escapes the PCT and overwhelms distal capacity → bicarbonaturia, high urine pH, Na+ delivery with K+ wasting. As plasma HCO3− falls to the lowered threshold, filtered load matches residual proximal capacity, bicarbonaturia stops, and urine can acidify—so steady-state acidosis is often moderate.

Type 1 RTA: Even with systemic acidosis, distal H+ secretion fails → urine pH stays high, net acid excretion collapses, bone buffering and hypercalciuria promote nephrocalcinosis/stones.

Type 4 RTA: Hyperkalemia is the signature. Low aldosterone (or resistance) reduces principal-cell Na+ reabsorption and K+ secretion; hyperkalemia further suppresses NH4+ production → mild hyperchloremic acidosis. Contrast with types 1 and 2, which are hypokalemic.

Worked Mental Algorithm (Use on Every Item)

1. Look at pH: acidemia (<7.35) or alkalemia (>7.45).
2. Look at PaCO2 and HCO3−: which change explains the pH?
3. If metabolic acidosis → compute AG (correct for albumin).
4. Apply Winter’s formula (or chronic respiratory rules) → pure vs mixed.
5. Match cause: gap list vs non-gap list; if non-gap + renal, branch to RTA type by K+ and urine pH.
6. Tie back to transporters: CA/NHE3 (proximal), H+-ATPase (distal), ENaC/aldosterone (type 4 and K+).

Mastery of this algorithm converts acid–base from memorized mnemonics into predictable physiology—exactly how CBSE and Step-style items are written.