Acid-Base
Disturbances
Master the four primary disorders of acid-base balance, learn the Henderson-Hasselbalch equation, and understand how the body compensates to restore homeostasis.
Anatomy & Physiology
What Is pH?
The pH scale measures the concentration of hydrogen ions (\( \text{H}^+ \)) in a solution. It is defined by the equation:
A lower \( [\text{H}^+] \) means a higher pH (more alkaline), while a higher \( [\text{H}^+] \) means a lower pH (more acidic). Normal blood pH is maintained in a remarkably tight range.
The pH Scale and Blood
Key Players in Acid-Base Balance
Lungs
Regulate \( P_{\text{CO}_2} \) by adjusting respiratory rate. Fast response (minutes).
Kidneys
Excrete or reabsorb \( \text{H}^+ \) and \( \text{HCO}_3^- \). Slow response (hours to days).
Buffer System
The bicarbonate buffer: \( \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^- \). Instant response.
The Henderson-Hasselbalch Equation
\( [\text{HCO}_3^-] \)
Metabolic component. Normal: 22 – 26 mEq/L. Regulated by kidneys.
\( P_{\text{CO}_2} \)
Respiratory component. Normal: 35 – 45 mmHg. Regulated by lungs.
The Four Primary Disturbances
Respiratory Disturbances
Respiratory Acidosis
Cause: \( P_{\text{CO}_2} \uparrow \) from hypoventilation
Examples: COPD, respiratory depression, airway obstruction
Compensation: Kidneys retain \( \text{HCO}_3^- \)
Examples: COPD, respiratory depression, airway obstruction
Compensation: Kidneys retain \( \text{HCO}_3^- \)
Respiratory Alkalosis
Cause: \( P_{\text{CO}_2} \downarrow \) from hyperventilation
Examples: Anxiety, pain, high altitude, fever
Compensation: Kidneys excrete \( \text{HCO}_3^- \)
Examples: Anxiety, pain, high altitude, fever
Compensation: Kidneys excrete \( \text{HCO}_3^- \)
Metabolic Disturbances
Metabolic Acidosis
Cause: \( \text{HCO}_3^- \downarrow \) from acid gain or base loss
Examples: DKA, renal failure, severe diarrhea, lactic acidosis
Compensation: Lungs increase rate (\( P_{\text{CO}_2} \downarrow \))
Examples: DKA, renal failure, severe diarrhea, lactic acidosis
Compensation: Lungs increase rate (\( P_{\text{CO}_2} \downarrow \))
Metabolic Alkalosis
Cause: \( \text{HCO}_3^- \uparrow \) from acid loss or base gain
Examples: Prolonged vomiting, antacid overuse, diuretics
Compensation: Lungs decrease rate (\( P_{\text{CO}_2} \uparrow \))
Examples: Prolonged vomiting, antacid overuse, diuretics
Compensation: Lungs decrease rate (\( P_{\text{CO}_2} \uparrow \))
How Compensation Works
The body never overcompensates. Compensation moves pH toward normal but does not fully correct it unless the primary problem resolves.
Respiratory vs Metabolic
| Respiratory | Metabolic | |
|---|---|---|
| Primary Change | \( P_{\text{CO}_2} \) | \( \text{HCO}_3^- \) |
| Organ Responsible | Lungs | Kidneys / Metabolism |
| Compensating Organ | Kidneys | Lungs |
| Speed of Compensation | Hours to days | Minutes to hours |
| Acidosis Trigger | \( P_{\text{CO}_2} \uparrow \) | \( \text{HCO}_3^- \downarrow \) |
| Alkalosis Trigger | \( P_{\text{CO}_2} \downarrow \) | \( \text{HCO}_3^- \uparrow \) |
How to Interpret an ABG
1
Look at the pH. Is it < 7.35 (acidosis) or > 7.45 (alkalosis)?
2
Check \( P_{\text{CO}_2} \). Normal = 35 – 45 mmHg. If abnormal and explains the pH, the disturbance is respiratory.
3
Check \( \text{HCO}_3^- \). Normal = 22 – 26 mEq/L. If abnormal and explains the pH, the disturbance is metabolic.
4
Assess compensation. Is the other value also abnormal? If yes, the body is compensating. If both are abnormal in conflicting directions, suspect a mixed disorder.
5
Determine degree. Full compensation: pH normal. Partial compensation: pH still abnormal. Uncompensated: only one value abnormal.
Worked Example 1
1
pH = 7.30: Below 7.35, so this is acidosis.
2
\( P_{\text{CO}_2} \) = 50: Elevated (normal 35 – 45). High \( P_{\text{CO}_2} \) causes acidosis. This matches.
3
\( \text{HCO}_3^- \) = 24: Normal (22 – 26). No metabolic compensation yet.
Worked Example 2
1
pH = 7.32: Below 7.35 = acidosis.
2
\( P_{\text{CO}_2} \) = 30: Low (would cause alkalosis, not acidosis). This does not match the pH.
3
\( \text{HCO}_3^- \) = 15: Low (below 22). Low bicarb causes acidosis. This matches.
4
Compensation: \( P_{\text{CO}_2} \) is low = lungs are hyperventilating to blow off \( \text{CO}_2 \). Partial compensation (pH still abnormal).
Worked Example 3: Fully Compensated
1
pH = 7.37: Within normal range (7.35 – 7.45), but on the acidic side of 7.40.
2
\( P_{\text{CO}_2} \) = 55: High. Would cause acidosis. This aligns with the slightly acidic pH.
3
\( \text{HCO}_3^- \) = 32: High. Kidneys have retained bicarb to buffer the excess acid.
Common Mistakes
✕ Confusing Cause and Compensation
Always start with pH to identify the primary disturbance. The compensating value moves in the same direction as the primary change.
✕ Assuming Normal pH = No Problem
A normal pH with two abnormal values means full compensation or a mixed disorder. Always check \( P_{\text{CO}_2} \) and \( \text{HCO}_3^- \).
✕ Expecting Overcompensation
The body never overcompensates. If the pH swings past normal to the opposite side, suspect a second primary disorder.
Master Summary Table
| Disorder | pH | \( P_{\text{CO}_2} \) | \( \text{HCO}_3^- \) | Compensation |
|---|---|---|---|---|
| Resp. Acidosis | ↓ | ↑ | Normal → ↑ | Kidneys retain \( \text{HCO}_3^- \) |
| Resp. Alkalosis | ↑ | ↓ | Normal → ↓ | Kidneys excrete \( \text{HCO}_3^- \) |
| Met. Acidosis | ↓ | Normal → ↓ | ↓ | Lungs hyperventilate |
| Met. Alkalosis | ↑ | Normal → ↑ | ↑ | Lungs hypoventilate |
Question 1 of 5
A patient presents with pH = 7.28, \( P_{\text{CO}_2} \) = 60 mmHg, and \( \text{HCO}_3^- \) = 25 mEq/L. What is the most likely diagnosis?
The pH is low (acidosis), and \( P_{\text{CO}_2} \) is elevated, which causes acidosis. The \( \text{HCO}_3^- \) is normal, meaning the kidneys have not yet compensated. This is uncompensated respiratory acidosis.
Question 2 of 5
In metabolic acidosis, how does the body compensate?
In metabolic acidosis, \( \text{HCO}_3^- \) is low. The lungs compensate by hyperventilating to blow off \( \text{CO}_2 \), raising pH back toward normal. This is called Kussmaul breathing in severe cases.
Question 3 of 5
A patient has pH = 7.50, \( P_{\text{CO}_2} \) = 28 mmHg, and \( \text{HCO}_3^- \) = 23 mEq/L. What is the diagnosis?
The pH is high (alkalosis). \( P_{\text{CO}_2} \) is low, which would cause alkalosis: this matches. \( \text{HCO}_3^- \) is normal (no renal compensation yet). This is uncompensated respiratory alkalosis, often caused by hyperventilation from anxiety or pain.
Question 4 of 5
A patient's ABG shows pH = 7.38, \( P_{\text{CO}_2} \) = 55 mmHg, \( \text{HCO}_3^- \) = 33 mEq/L. Which statement is most accurate?
Although pH is normal, both \( P_{\text{CO}_2} \) and \( \text{HCO}_3^- \) are elevated. The pH sits on the acidic side of 7.40 (7.38), so the original disturbance was acidosis. High \( P_{\text{CO}_2} \) is the cause (respiratory). The kidneys have fully compensated by retaining \( \text{HCO}_3^- \).
Question 5 of 5
Which of the following is not a common cause of metabolic acidosis?
Prolonged vomiting causes metabolic alkalosis, not acidosis, because the body loses \( \text{HCl} \) (hydrochloric acid) from the stomach, which raises pH. DKA, renal failure, and lactic acidosis all cause metabolic acidosis by adding acid or losing bicarbonate.