Acid-Base Foundations

Preparatory Module · Edward Barnes, MD, FACP

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Welcome to Acid-Base.

Let's build the foundation, one concept at a time.

Before you tackle the full Acid-Base curriculum, this preparatory module will get your vocabulary, frameworks, and pattern-recognition in place. Expect short concepts, frequent check-ins, and a few interactive tools to feel out the math.

Take your time. Ideas here will repeat themselves a few times, that's on purpose.

1Distinguish acidemia from acidosis, and alkalemia from alkalosis
2Read the Henderson-Hasselbalch equation and predict pH
3Classify the four primary disorders by direction of change
4Calculate anion gap and delta gap, and apply Winters & Summer's formulas
5Apply Barnes' 6-step framework to a real ABG

What's in this module

Section 1 · Video

Foundations: The Language, pH & Normal Values

Start here. This first video lays the groundwork: the vocabulary of acid-base, what pH actually measures, and the normal values you'll anchor every case to. The sections that follow let you practice each idea.

⬇ Download the slide deck (PowerPoint.pptx)
Video not loading? ▶ Watch on YouTube

While you watch, hold these questions in mind:

  • What's the difference between -emia (the blood) and -osis (the process)?
  • What does pH actually measure, and how do HCO₃⁻ and PCO₂ move it?
  • What are the normal values for pH, HCO₃⁻, and PCO₂, and where's the dividing line for acidemia vs. alkalemia?

The next three sections (The Language, Understanding pH, and Normal Values) let you practice exactly what this video covers.

Section 2

The Language of Acid-Base

Four words sound similar, but they mean very different things. Getting them right is half the battle on rounds.

Definitions
  • Acidemia ← describes the BLOOD
  • Alkalemia ← describes the BLOOD
  • Acidosis → describes a PROCESS →
  • Alkalosis → describes a PROCESS →

Acidemia

The measured pH of arterial blood is < 7.4.
"-emia = in the blood"

Alkalemia

The measured pH of arterial blood is > 7.4.
"-emia = in the blood"

Acidosis

A process that, on its own, would push the pH down. Multiple acidoses can occur at once.
"-osis = a process"

Alkalosis

A process that, on its own, would push the pH up. Can coexist with acidoses.
"-osis = a process"

Why does this distinction matter?

A patient can have a near-normal pH and still have two opposing acid-base disorders happening simultaneously. For example, a patient with both an acidosis and an alkalosis may have a pH close to 7.40 but a very abnormal underlying physiology. If you only know how to talk about "-emia" you'll miss half the picture.

Q1. A patient's arterial pH is 7.50. They are also vomiting heavily from gastric outlet obstruction. Which of the following is the BEST statement?
Section 3

Understanding pH

pH is the language we use to describe how acidic or basic the blood is. Two equations do most of the heavy lifting, let's see them in action.

First equation: what pH actually is

pH = −log10 [H+]

Because of the negative log, a small drop in pH means a big rise in [H⁺]. A pH of 7.0 has ten times the hydrogen ion concentration of a pH of 8.0. The scale feels gentle but the chemistry is steep.

Second equation: where the H⁺ comes from

H2O + CO2 ⇌ H2CO3 ⇌ H+ + HCO3−

This single reversible reaction is the engine of acid-base physiology. Water plus carbon dioxide makes carbonic acid, which dissociates into hydrogen and bicarbonate. Push the equation either way and pH follows.

Interactive: shift the equilibrium
CO2 ⇌ H2CO3 ⇌ H+ + HCO3−
Pick a perturbation. Watch which side the reaction shifts to, and what happens to H⁺.

Third equation: pH from bicarb and CO₂

pH = 6.1 + log10 [HCO3−] PCO₂ × 0.03

This is the Henderson-Hasselbalch equation applied to blood. Don't memorize the constants, memorize the relationship: pH tracks the ratio of HCO₃⁻ to PCO₂.

The relationship that matters
pH ∝ HCO3− PCO2
↑ HCO3− → pH up   •   ↑ PCO2 → pH down
Q2. A patient hyperventilates and their PCO₂ falls from 40 to 25 mmHg. With no other changes, what happens to the pH?
Section 4

Normal Values & the pH Number Line

Every acid-base problem starts here: is the pH normal, low, or high? Internalize these numbers, they are your anchor.

pH: 7.40

The dividing line. Below 7.40 = acidemia. Above 7.40 = alkalemia. The lab reference range (7.35 – 7.45) is just measurement uncertainty around this midpoint.

HCO3−: 22 – 26 mEq/L

The metabolic / kidney side. Normal ~24.

PCO2: 35 – 45 mmHg

The respiratory / lung side. Normal ~40.

The number line

Drag the red pin along the line. Anything below 7.40 is acidemic; anything above 7.40 is alkalemic. There is no "normal" zone, even small shifts from 7.40 indicate a process at work.

Current pH
7.40
, exactly 7.40 ,
Q3. A patient has a pH of 7.32. Independent of any other lab value, what can you say with confidence?
Section 5 · Video

Disorders, Compensation & the Gaps

Now that the vocabulary and normal values are in place, this video builds the working machinery: the four primary disorders, how the body compensates, and how the anion gap and delta gap expose what's really going on. The next four sections give you the hands-on practice.

Video not loading? ▶ Watch on YouTube

While you watch, hold these questions in mind:

  • How do pH and PCO₂ direction sort a disorder into metabolic vs. respiratory?
  • How do you check whether compensation is appropriate, Winters for acidosis, Summer's for alkalosis?
  • How is the anion gap calculated, and when do you correct it for albumin?
  • What does the delta gap reveal about a hidden, coexisting disorder?

The next four sections (Four Primary Disorders, Compensation, Anion Gap, and Delta Gap) let you practice exactly what this video covers.

Section 6

The Four Primary Disorders

Every primary acid-base disturbance falls into one of four boxes. The classification is purely mechanical: look at the direction of pH, then ask which side, HCO₃⁻ or PCO₂, is driving it.

Acidosis (pH ↓)
Alkalosis (pH ↑)
Metabolic
HCO3− drives
Metabolic Acidosis
↓ pH   ↓ HCO3−
e.g., DKA, lactic acidosis, diarrhea
Metabolic Alkalosis
↑ pH   ↑ HCO3−
e.g., vomiting, diuretics
Respiratory
PCO2 drives
Respiratory Acidosis
↓ pH   ↑ PCO2
e.g., COPD exacerbation, opioid OD
Respiratory Alkalosis
↑ pH   ↓ PCO2
e.g., anxiety, PE, sepsis (early)

Pattern recognition: which disorder is this?

Click a clinical scenario. The widget will show you which primary disorder it produces and which way pH, HCO₃⁻, and PCO₂ move.
pick a scenario
pH
HCO3−
PCO2
Q4. An ABG shows pH 7.28, PCO₂ 60, HCO₃⁻ 28. What is the primary disorder?
Section 7

Compensation

The body hates an abnormal pH. When one side (HCO₃⁻ or PCO₂) drives an acidosis or alkalosis, the other side pushes back. That counter-move is compensation.

The compensation idea, in one line
If the kidneys cause a problem, the lungs try to fix it.
If the lungs cause a problem, the kidneys try to fix it.

Direction of compensation

Compensation moves in the same direction as the primary disturbance, never opposite.

Winters formula for metabolic acidosis

Winters tells you the PCO₂ the lungs should compensate to. If the measured PCO₂ is much different, there's a second disorder hiding.

Expected PCO2 = 1.5 × [HCO3−] + 8   (± 2)
Winters Formula Calculator
Enter the measured HCO₃⁻. The widget computes the expected PCO₂ range.
Expected PCO₂ Range
Enter HCO₃⁻ to compute.

Summer's Formula for metabolic alkalosis

The mirror image of Winters. When HCO₃⁻ rises (metabolic alkalosis), the lungs hypoventilate to retain CO₂ and pull the pH back down. Same idea: if measured PCO₂ doesn't fit, there's a second disorder.

A joking name from Dr. Barnes (Winters for the acidosis, Summer's for the alkalosis). Opposite seasons, opposite disorders.

Expected PCO2 = 0.7 × [HCO3−] + 20   (± 5)
Summer's Formula Calculator
Enter the measured HCO₃⁻ in a patient with metabolic alkalosis. The widget computes the expected PCO₂ range.
Expected PCO₂ Range
Enter HCO₃⁻ to compute.

Other compensation formulas (for reference)

Acute Respiratory Acidosis
Δ HCO₃⁻ = 1 per 10 mmHg ↑ PCO₂
Chronic Respiratory Acidosis
Δ HCO₃⁻ = 4 per 10 mmHg ↑ PCO₂
Acute Respiratory Alkalosis
Δ HCO₃⁻ = 2 per 10 mmHg ↓ PCO₂
Chronic Respiratory Alkalosis
Δ HCO₃⁻ = 5 per 10 mmHg ↓ PCO₂

The two to memorize: Winters (for metabolic acidosis) and Summer's (for metabolic alkalosis).

Q5. A patient has HCO₃⁻ of 10 and measured PCO₂ of 35. Is the respiratory compensation appropriate?
Q5b. A patient with chronic vomiting has HCO₃⁻ of 40 and PCO₂ of 52. Using Summer's formula, is the respiratory compensation appropriate?
Section 8

The Anion Gap

When you find a metabolic acidosis, the next question is what kind: is bicarbonate being consumed by a new acid in the blood, or is it being lost? The anion gap answers this.

Electroneutrality, why the gap exists

Blood must be electrically neutral. Cations (+) must equal anions (−).

Na+  =  Cl−  +  HCO3−  +  [other anions we don't measure]

We measure Na, Cl, HCO₃⁻. We don't directly measure albumin, lactate, ketoacids, sulfates. The "gap" is the cushion of unmeasured anions:

Anion Gap = Na+ − (Cl− + HCO3−)

Normal anion gap is 12 ± 2 (assuming a normal serum albumin of ~4 g/dL, more on that on the next page). When a new acid like lactate or β-hydroxybutyrate enters the blood, it consumes HCO₃⁻ but is not measured, so the gap goes up.

Two flavors of metabolic acidosis

High Anion Gap (HAGMA)

Bicarbonate is consumed by a new acid
MUDPILES: Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates

Normal Anion Gap (NAGMA)

Bicarbonate is lost; chloride takes its place
HARDUPS / USED CARP: GI losses (diarrhea), renal tubular acidosis, ureteral diversions, post-hypocapnia, NH₄Cl, Acetazolamide
Anion Gap Calculator
Enter Na, Cl, and HCO₃⁻. The widget computes AG and flags the type.
Anion Gap
Enter all three values.
Q6. Labs: Na 140, Cl 105, HCO₃⁻ 12. What does the anion gap suggest?
Section 9

Delta Gap: Hidden Disorders

A high anion gap acidosis usually has a partner. The delta gap reveals whether a second metabolic disorder is hiding underneath the obvious one.

First: what's the expected anion gap?

Before we calculate a delta gap, we need to know what "normal" is for this patient. The textbook normal is 12, but that assumes a normal serum albumin (~4 g/dL). Albumin is a negatively charged protein, it's the largest single unmeasured anion in the blood. When albumin drops (common in ICU, cirrhosis, nephrotic syndrome, malnutrition), the expected anion gap drops with it.

Correction for serum albumin

For every 1 g/dL fall in albumin below 4, the expected AG falls by ~2.5.

Expected AG = 12 − 2.5 × (4 − albumin)

Equivalently, the corrected AG for comparison purposes is:

Corrected AG = measured AG + 2.5 × (4 − albumin)

Example: albumin 2 g/dL → expected AG ≈ 7. A measured AG of 14 in that patient is really a 7-point elevation, a hidden HAGMA that would have been called "normal" without the correction.

Now the delta gap

In a "pure" high anion gap acidosis, every 1-point rise in AG above normal should correspond to a 1-point fall in HCO₃⁻ below normal, because each new acid molecule consumed one bicarbonate. If those numbers don't match, there's something else going on.

Calculating delta gap
  1. Calculate the AG = Na − (Cl + HCO₃⁻). Correct it for albumin if low.
  2. ΔAG = (corrected) AG − 12
  3. Add ΔAG to the measured HCO₃⁻, this is the "corrected" bicarb

Then check the corrected HCO₃⁻ against normal (24):

  • ≈ 24 → pure HAGMA
  • < 22 → coexisting NAGMA also present
  • > 26 → coexisting metabolic alkalosis also present
Delta Gap Calculator
Enter Na, Cl, HCO₃⁻, and (optionally) albumin. The widget corrects the AG for hypoalbuminemia, then computes ΔAG and the corrected HCO₃⁻.
Measured AG
Corrected AG
Δ Gap
Corrected HCO₃⁻
Interpretation
Enter Na, Cl, and HCO₃⁻ to compute. Albumin is optional (assumes 4.0 if blank).
Q7. Labs: Na 140, Cl 95, HCO₃⁻ 14, albumin 4.0. AG = 31. ΔAG = 19. Corrected HCO₃⁻ = 14 + 19 = 33. What's the interpretation?
Section 10 · Video

Putting It Together: Barnes' 6-Step Approach

You've built every piece. This final video shows how they snap together into one repeatable framework, the 6 steps Dr. Barnes runs on every blood gas, and walks through worked cases. After this, you'll apply the steps yourself and finish with the synthesis check.

Video not loading? ▶ Watch on YouTube

While you watch, hold these questions in mind:

  • What are the six steps, and in what order do you run them on every ABG?
  • How do the steps combine to catch a mixed disorder you'd otherwise miss?
  • How does the framework hold up across the worked cases, and what's the differential at the end?

The next three sections (Barnes' Steps, Sample Cases, and the Final Check) let you apply exactly what this video covers.

Section 11

Barnes' Steps to Acid-Base Cases

Every concept so far comes together as a six-step framework. Run every ABG through these steps and you won't miss a mixed disorder.

  1. Determine if the blood is acidemic or alkalemic. Look at pH alone. pH < 7.4 = acidemic; pH > 7.4 = alkalemic.
  2. Determine the direction of change in pH and PCO₂. Primary metabolic if pH and PCO₂ move together. Primary respiratory if they move opposite.
  3. Calculate the anion gap. Na − (Cl + HCO₃⁻). Normal ≈ 12. Correct for albumin if low.
  4. Check compensation. Winters for metabolic acidosis: Expected PCO₂ = 1.5 × HCO₃⁻ + 8 ± 2. Summer's for metabolic alkalosis: Expected PCO₂ = 0.7 × HCO₃⁻ + 20 ± 5. If measured PCO₂ doesn't fit, there's a second disorder.
  5. Calculate the delta gap if AG is elevated. Add ΔAG to HCO₃⁻ to look for hidden disorders.
  6. Map out a differential diagnosis for each disorder. Then treat the underlying cause, unless pH is in a dangerous range (acute management).
Worked example

Given: pH 7.20, PCO₂ 22, HCO₃⁻ 8, Na 140, Cl 110.

Step 1   pH 7.20 → acidemic.

Step 2   pH ↓ and PCO₂ ↓ → they moved together → primary metabolic acidosis.

Step 3   AG = 140 − (110 + 8) = 22 → high anion gap.

Step 4   Winters: expected PCO₂ = 1.5(8) + 8 = 20 (± 2) → measured 22 → appropriate compensation.

Step 5   ΔAG = 22 − 12 = 10 → corrected HCO₃⁻ = 8 + 10 = 18 → below 22 → coexisting NAGMA.

Step 6   Differential: HAGMA + NAGMA → e.g., DKA (high gap) with concurrent diarrhea (non-gap), or DKA being resuscitated with normal saline (hyperchloremic NAGMA).

Section 12

Sample Cases

Five ABGs from the original module. For each, click Walk through it to see Barnes' steps applied. Try to work it out in your head first.

Barnes' Steps, apply to each case below
1Acidemic or alkalemic? pH < 7.4 vs > 7.4.
2Direction of pH & PCO₂. Same way → metabolic. Opposite → respiratory.
3Anion gap = Na − (Cl + HCO₃⁻). Normal ≈ 12. Correct for albumin.
4Compensation. Winters (acidosis) or Summer's (alkalosis).
5Delta gap if AG ↑. Corrected HCO₃⁻ = HCO₃⁻ + ΔAG vs 24.
6Differential dx for each disorder. Treat the cause.
Winters:1.5·HCO₃⁻ + 8 ± 2 Summer's:0.7·HCO₃⁻ + 20 ± 5 Alb correction:+2.5 × (4 − alb)
Final Check

Putting It All Together

Five questions covering the whole module. Take your time.

F1. A patient on long-term diuretics presents with pH 7.52, HCO₃⁻ 36, PCO₂ 46. Which is the primary disorder and is it compensated?
F2. Labs: Na 138, Cl 105, HCO₃⁻ 24. What is the anion gap?
F3. Which disorder moves pH and PCO₂ in the SAME direction (both up or both down)?
F4. A patient with DKA has Na 135, Cl 95, HCO₃⁻ 10, AG = 30. ΔAG = 18. Corrected HCO₃⁻ = 28. What does that suggest?
F5. Which statement is TRUE about compensation?
🎉

Module Complete

You're ready for the full course material.
FINAL CHECK SCORE
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You now know how to read the language of acid-base, identify the primary disorder, check compensation, calculate gaps, and detect hidden disorders. The full course will build on every one of these foundations.