Anatomy and Physiology 2e · Fluid, Electrolyte, and Acid-Base Balance
Disorders of Acid-Base Balance
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In 30 seconds
When the acid-base defenses are overwhelmed, one of four primary disturbances develops: Respiratory acidosis Acid-base disorder from CO₂ retention (hypoventilation) Full entry →, Respiratory alkalosis Acid-base disorder from excess CO₂ loss (hyperventilation) Full entry →, Metabolic acidosis Disorder from bicarbonate loss or fixed-acid accumulation Full entry →, or Metabolic alkalosis Disorder from bicarbonate excess or hydrogen ion loss Full entry →. The names tell the story: respiratory disorders come from changes in ventilation that alter CO₂ (and thus carbonic acid), while metabolic disorders come from changes in bicarbonate or in the production/loss of nonvolatile acids. The body then mounts Compensation The opposite system pulling pH back toward normal Full entry → — the opposite system pulls pH back toward normal: lungs compensate metabolic disturbances by changing ventilation; kidneys compensate respiratory disturbances by changing bicarbonate handling.
Why this matters
Acid-base disorders accompany some of the most common and serious conditions in healthcare: diabetic ketoacidosis, sepsis, kidney failure, chronic lung disease, severe vomiting, and drug overdoses. Reading a blood gas tells the clinician what is wrong and how long it has been developing. For students, the compensation logic is a favorite exam question: given a pH, PCO₂, and HCO₃⁻, name the disorder and whether it is compensated.
The college version
Core Concepts
The four primary disorders at a glance
| Disorder | Primary change | Common causes (educational examples) |
|---|---|---|
| Respiratory acidosis | PCO₂ ↑ (hypoventilation) | COPD, airway obstruction, sedative overdose, chest-wall injury |
| Respiratory alkalosis | PCO₂ ↓ (hyperventilation) | Anxiety/panic, high altitude, fever, early aspirin toxicity, mechanical overventilation |
| Metabolic acidosis | HCO₃⁻ ↓ or fixed acid ↑ | Diabetic ketoacidosis, lactic acidosis, severe diarrhea (bicarbonate loss), kidney failure |
| Metabolic alkalosis | HCO₃⁻ ↑ or H⁺ loss | Prolonged vomiting (loss of stomach acid), diuretic use, excess bicarbonate administration |
These are educational examples of mechanism, not treatment guidance; the actual cause in any patient must be established by a clinician.
Respiratory acidosis: too much CO₂
Respiratory acidosis develops when ventilation fails to keep pace with CO₂ production, so PCO₂ rises and pH falls. Because the disturbance is in the lungs, compensation must come from the kidneys: over hours to days they secrete more H⁺ and return more bicarbonate to the blood. That is why chronic respiratory acidosis shows a high bicarbonate: it is the kidney's compensation, not a separate metabolic problem.
Respiratory alkalosis: too little CO₂
Respiratory alkalosis is the mirror image: ventilation exceeds metabolic need, CO₂ is blown off, PCO₂ falls, the buffer equation shifts left, H⁺ is consumed, and pH rises. The kidneys compensate by excreting more bicarbonate in the urine. Because hyperventilation can be triggered by stress and pain, respiratory alkalosis is often brief and self-correcting.
Metabolic acidosis: losing bicarbonate or gaining fixed acid
Metabolic acidosis means bicarbonate is falling relative to the acid load — either bicarbonate is lost (as in severe diarrhea) or fixed acids pile up faster than the kidneys can excrete them (as in diabetic ketoacidosis, where ketone bodies accumulate, or kidney failure). The lungs compensate rapidly and dramatically: deeper, faster breathing (Kussmaul respirations Deep, rapid breathing in metabolic acidosis (classic teaching) Full entry → in classic teaching) blows off CO₂, pulling pH up. The kidneys eventually excrete the excess acid — if they can — and bicarbonate must be regenerated. In diabetic ketoacidosis, the deep breathing is the body's attempt to exhale the acid load, but ketones are fixed acids with no gas phase, so exhalation can only blunt the problem.
Metabolic alkalosis: too much bicarbonate or too little H⁺
Metabolic alkalosis develops when bicarbonate rises or hydrogen ions are lost without equivalent replacement. The classic teaching example is prolonged vomiting: stomach acid (HCl) is lost, leaving relatively more base. Diuretics can also cause it by increasing loss of H⁺ and chloride in the urine. The lungs compensate by slowing ventilation to retain CO₂ — blood gases may show a raised PCO₂ — but ventilation cannot slow too far without driving down oxygen levels. The kidneys are supposed to excrete the excess bicarbonate, but they hold onto it if volume depletion signals "save salt and water," which is why the disorder can persist until the cause is corrected.
Compensation versus correction
Compensation must not be confused with correction. Compensation is the opposite system keeping pH near normal while the primary problem persists — it changes the numbers but does not fix the cause. Correction resolves the disturbance. Exam logic: uncompensated = pH abnormal, other component normal; partially compensated = pH abnormal, other component moving; fully compensated = pH in range, both components abnormal in opposite directions.
Reading a blood gas: a four-step approach
- Look at pH: < 7.35 → acidemia; > 7.45 → alkalemia (commonly taught reference; verify against current texts).
- Identify the primary mover: abnormal PCO₂ explaining the pH → respiratory; abnormal HCO₃⁻ explaining the pH → metabolic.
- Check for compensation: is the other value moving opposite to the primary change?
- Name it: e.g., "metabolic acidosis with respiratory compensation."
This is a reasoning tool for study and exam use; actual interpretation requires clinical judgment and full laboratory context.
How It Works / Step-by-Step Process
Interpret this scenario with the four-step read. A patient with uncontrolled diabetes is producing ketone bodies faster than the kidneys can clear them.
- pH: falls below the reference range → acidemia.
- Which component moved? HCO₃⁻ is low (consumed buffering ketone acids) → metabolic acidosis.
- Compensation? Deep, rapid breathing (Kussmaul respirations) blows off CO₂; PCO₂ drops below normal → respiratory compensation underway.
- Name it: metabolic acidosis with respiratory compensation.
The numbers tell you what; the clinical picture tells you why.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Acidosis | Acidemia | Acidosis is the process (e.g., CO₂ retention); acidemia is the pH state. Compensation can keep pH normal despite ongoing acidosis |
| Respiratory acidosis | Metabolic acidosis | One is caused by high CO₂ (lungs); the other by low bicarbonate or high fixed acid (metabolism/kidneys) — cause and compensation differ |
| Compensation | Correction | Compensation is the opposite system masking the pH change; correction resolves the underlying cause. A compensated patient is not "cured" |
| High bicarbonate in respiratory acidosis | A separate metabolic alkalosis | In chronic respiratory acidosis, high HCO₃⁻ is usually the kidney's compensation, not an independent disorder |
| Hyperventilation | Hypoventilation | Hyperventilation blows off CO₂ → alkalosis; hypoventilation retains CO₂ → acidosis. Breathing direction predicts pH direction |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of your blood as lemonade that must stay just right — not too sour, not too sweet. Breathe too slowly and too much "fizzy gas" builds up: sour (respiratory acidosis). Breathe too fast and you blow the fizz away: too sweet (respiratory alkalosis). Too much sour stuff, or losing the sweet stuff through diarrhea: sour in a different way (metabolic acidosis). Throw up stomach acid: too sweet (metabolic alkalosis). When one part goes wrong, the other part patches the taste — the lungs breathe faster or slower, the kidneys add or remove the sweet stuff — until the real problem is fixed.
Worked example
Two patients present with the same blood pH of 7.32 (acidemia, using a commonly taught reference range). Patient A has chronic lung disease: PCO₂ high, bicarbonate high, pH only mildly low — respiratory acidosis with renal compensation building for a long time. Patient B has severe diarrhea: bicarbonate low, PCO₂ low (lungs compensating), pH low — acute metabolic acidosis with respiratory compensation. Same pH, completely different stories: the PCO₂ and bicarbonate values — not the pH alone — reveal which system failed. Exam questions always give all three numbers: the pH names the direction, the other two values tell the mechanism.
Key takeaways
- Four primary disorders: respiratory acidosis (PCO₂ ↑), respiratory alkalosis (PCO₂ ↓), metabolic acidosis (HCO₃⁻ ↓ or fixed acid ↑), metabolic alkalosis (HCO₃⁻ ↑ or H⁺ loss).
- Name logic: "respiratory" = the CO₂/ventilation side changed; "metabolic" = the bicarbonate/fixed-acid side changed.
- Compensation rule: lungs compensate metabolic disorders; kidneys compensate respiratory disorders.
- Compensation ≠ correction: compensation keeps pH near normal while the cause persists; the numbers move, the problem stays.
- Fully compensated: pH normal, both PCO₂ and HCO₃⁻ abnormal in opposite directions.
- Kidney failure → metabolic acidosis because fixed acids are not excreted and bicarbonate is not regenerated.
- Classic cause pairs (educational): COPD → respiratory acidosis; hyperventilation → respiratory alkalosis; diabetic ketoacidosis and diarrhea → metabolic acidosis; prolonged vomiting → metabolic alkalosis.
- Kussmaul respirations (deep, rapid breathing) are the classic lung compensation in metabolic acidosis.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What is the primary disturbance in respiratory acidosis, and which organ system usually compensates?
Show answer
PCO₂ is elevated (CO₂ retention); the kidneys compensate over hours to days by secreting more H⁺ and returning more bicarbonate to the blood.
Why does prolonged vomiting tend to cause metabolic alkalosis?
Show answer
Vomiting removes hydrochloric acid from the stomach, so the body loses H⁺ and is left with relatively more base — pH rises.
How do the kidneys compensate for respiratory acidosis?
Show answer
The kidneys secrete more H⁺ and generate/reabsorb more bicarbonate, which is why bicarbonate rises in chronic respiratory acidosis.
What does "fully compensated" mean on a blood gas?
Show answer
pH is back in the normal range while both PCO₂ and HCO₃⁻ are abnormal in opposite directions — the opposite system is fully compensating.
Why does kidney failure cause metabolic acidosis?
Show answer
The kidneys are the only route for excreting fixed acids and regenerating bicarbonate; in kidney failure, acids accumulate.
In metabolic acidosis, what does the lungs' compensation look like, and why is it only a partial fix?
Show answer
Deep, rapid breathing (Kussmaul respirations) blows off CO₂, shifting the buffer equation to consume H⁺ and raise pH; it is only partial because ketones cannot be exhaled — they still need renal excretion.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Respiratory acidosis
- Acid-base disorder from CO₂ retention (hypoventilation)
- Respiratory alkalosis
- Acid-base disorder from excess CO₂ loss (hyperventilation)
- Metabolic acidosis
- Disorder from bicarbonate loss or fixed-acid accumulation
- Metabolic alkalosis
- Disorder from bicarbonate excess or hydrogen ion loss
- Compensation
- The opposite system pulling pH back toward normal
- Kussmaul respirations
- Deep, rapid breathing in metabolic acidosis (classic teaching)
- Arterial blood gas (ABG)
- Blood test measuring pH, PCO₂, and bicarbonate
Sources & references
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