Acid base balance in the body and it's regulation in detail

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Acid-base balance: definition and normal values

Acid-base balance is the maintenance of hydrogen ion concentration, ([H^+]), within a narrow range so enzymes, ion channels, proteins, and cellular metabolism function normally.

pH = -log₁₀[H⁺]
Normal arterial blood pH: 7.35-7.45 (approximately 7.4)
Acidemia: pH < 7.35
Alkalemia: pH > 7.45
Usual arterial values:
- PaCO₂: 35-45 mmHg
- HCO₃⁻: 22-26 mEq/L

Even small pH changes matter because the pH scale is logarithmic. A fall in pH means a rise in free hydrogen ions. Costanzo notes that normal arterial pH is about 7.37-7.42 and intracellular pH is slightly lower, about 7.2. - Costanzo Physiology, 7th ed., p. 317

The central principle: bicarbonate to carbon dioxide ratio

The main extracellular buffer is the bicarbonate-carbonic acid system:

[ CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^- ]

Carbonic anhydrase, especially abundant in red cells and renal tubular cells, speeds this reaction.

The relationship is expressed by the Henderson-Hasselbalch equation:

[ \text{pH} = 6.1 + \log\left(\frac{[HCO_3^-]}{0.03 \times P_{CO_2}}\right) ]

At normal values:

[ \text{pH} = 6.1 + \log\left(\frac{24}{0.03 \times 40}\right) \approx 7.40 ]

So pH is governed by a ratio:

[ \frac{\text{Kidney-controlled } HCO_3^-}{\text{Lung-controlled } P_{CO_2}} ]

More HCO₃⁻ raises pH.
More CO₂ lowers pH.
The lungs regulate the denominator, PaCO₂.
The kidneys regulate the numerator, plasma HCO₃⁻.
Guyton and Hall Textbook of Medical Physiology, pp. 409-410

Sources of acid in the body

1. Volatile acid: carbon dioxide

Aerobic cellular metabolism continuously produces large quantities of CO₂. CO₂ acts as an acid source because it forms carbonic acid in water. It is called volatile acid because the lungs can excrete it.

[ CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow H^+ + HCO_3^- ]

Approximately 13,000-20,000 mmol of CO₂ may be produced daily. It is eliminated mainly through ventilation. - Costanzo Physiology, 7th ed., p. 317

2. Fixed or nonvolatile acids

These cannot be removed by the lungs and must ultimately be excreted by the kidneys. They include:

Sulfuric acid from metabolism of sulfur-containing amino acids such as methionine and cysteine
Phosphoric acid from phospholipids and dietary phosphate
Lactic acid in hypoxia, shock, seizures, or strenuous exercise
Ketoacids in fasting, starvation, alcoholic ketoacidosis, and diabetic ketoacidosis
Toxic acids, for example formic acid from methanol and oxalic/glycolic acid from ethylene glycol

The routine fixed-acid burden is roughly 50 mmol/day, but rises substantially in disease. - Costanzo Physiology, 7th ed., p. 317

Three lines of defense against pH change

Mechanism	Main action	Onset	Capacity / role
Chemical buffers	Bind or release H⁺	Seconds	Immediate but cannot remove acid
Respiratory regulation	Alters CO₂ excretion	Minutes	Rapid control of volatile acid
Renal regulation	Excretes H⁺, regenerates HCO₃⁻	Hours to days	Slowest but most powerful long-term control

A useful sequence is:

Buffers minimize the initial change. Lungs remove CO₂. Kidneys remove fixed acid and restore bicarbonate.

1. Chemical buffer systems

A buffer is a weak acid plus its conjugate base. It resists a sudden pH change by accepting or donating hydrogen ions.

[ HA \leftrightarrow H^+ + A^- ]

If acid is added: (A^-) binds H⁺ to form (HA)
If base is added: (HA) releases H⁺, reducing the rise in pH

Buffers act immediately but do not eliminate hydrogen ions from the body. They temporarily contain the disturbance until lung and kidney responses occur.

A. Bicarbonate buffer system

[ H^+ + HCO_3^- \leftrightarrow H_2CO_3 \leftrightarrow CO_2 + H_2O ]

This is the most important extracellular buffer because both components are independently controlled:

Lungs remove CO₂.
Kidneys conserve, generate, or excrete HCO₃⁻.

Example: after addition of HCl,

[ HCl + NaHCO_3 \rightarrow NaCl + H_2CO_3 \rightarrow CO_2 + H_2O ]

The CO₂ can then be exhaled.

B. Protein buffers

Proteins contain negatively charged groups that accept H⁺ and acidic groups that can donate H⁺.

[ Pr^- + H^+ \leftrightarrow HPr ]

Proteins are important intracellular buffers. Hemoglobin is especially important in red blood cells.

In systemic capillaries, deoxygenated hemoglobin binds H⁺ generated from CO₂ hydration. In pulmonary capillaries, oxygenation of hemoglobin promotes H⁺ release, which combines with HCO₃⁻ to form CO₂ for exhalation.

C. Phosphate buffer system

[ HPO_4^{2-} + H^+ \leftrightarrow H_2PO_4^- ]

Phosphate is relatively less important in plasma because its concentration is low. It is much more important:

Intracellularly
In renal tubular fluid, where it buffers secreted H⁺ and permits urinary acid excretion

D. Bone buffering

Bone contains carbonate and phosphate salts that can buffer chronic acid loads. This contributes to buffering in prolonged acidosis, but sustained acid buffering by bone can contribute to mineral loss and bone disease.

A current Merck Manual overview likewise describes immediate intra- and extracellular buffering and notes the role of bone, particularly with acid loads.

2. Respiratory regulation of acid-base balance

The lungs regulate arterial CO₂ through alveolar ventilation.

[ P_{aCO_2} \propto \frac{\dot V_{CO_2}}{\dot V_A} ]

Where:

(\dot V_{CO_2}) = CO₂ production
(\dot V_A) = alveolar ventilation

Thus:

Increased ventilation lowers PaCO₂.
Decreased ventilation raises PaCO₂.

Because CO₂ behaves as an acid:

Hyperventilation causes lower PaCO₂ and tends to raise pH.
Hypoventilation causes higher PaCO₂ and tends to lower pH.

Control of ventilation

A fall in pH, rise in PaCO₂, or both stimulates respiratory drive through chemoreceptors.

Central chemoreceptors

Located in the medulla. They respond mainly to increased CO₂, which diffuses into cerebrospinal fluid and generates H⁺.

Peripheral chemoreceptors

Located in the carotid and aortic bodies. They respond to:

Decreased arterial pH
Increased PaCO₂
Marked hypoxemia

Respiratory compensation

Metabolic acidosis

Primary defect: ↓ HCO₃⁻

The compensatory response is hyperventilation:

[ \downarrow HCO_3^- \rightarrow \downarrow pH \rightarrow \uparrow ventilation \rightarrow \downarrow P_{aCO_2} ]

This is seen clinically as deep, rapid Kussmaul breathing in severe diabetic ketoacidosis.

Expected compensation can be estimated using Winter's formula:

[ Expected\ P_{CO_2} = 1.5 \times [HCO_3^-] + 8 \pm 2 ]

Metabolic alkalosis

Primary defect: ↑ HCO₃⁻

Compensation is hypoventilation, causing CO₂ retention. This is limited because marked hypoventilation would produce dangerous hypoxemia.

Respiratory compensation begins within minutes and is substantial but never fully corrects the primary disorder. A pH that has crossed to the opposite side of normal suggests a mixed acid-base disorder, not simple compensation. The Merck Manual discussion emphasizes that metabolic disorders prompt respiratory compensation and respiratory disorders prompt renal compensation.

3. Renal regulation of acid-base balance

The kidneys provide the final long-term defense. Their three major tasks are:

Reabsorb nearly all filtered bicarbonate
Secrete hydrogen ions
Generate new bicarbonate while excreting net acid

The kidney response begins over hours and reaches maximum effectiveness over days.

A. Reabsorption of filtered bicarbonate

The kidneys filter a large amount of HCO₃⁻ daily. Under normal conditions, nearly all is reclaimed, mainly in the proximal tubule.

Proximal-tubule mechanism

Tubular cells secrete H⁺ into the lumen, mainly through the Na⁺-H⁺ exchanger.
Secreted H⁺ combines with filtered HCO₃⁻:

[ H^+ + HCO_3^- \rightarrow H_2CO_3 ]

Luminal carbonic anhydrase converts carbonic acid to CO₂ and water:

[ H_2CO_3 \rightarrow CO_2 + H_2O ]

CO₂ enters the tubular cell.
Intracellular carbonic anhydrase reforms H⁺ and HCO₃⁻.
H⁺ is secreted again; HCO₃⁻ returns to blood.

This process reclaims filtered bicarbonate. It does not produce net acid excretion or new bicarbonate.

B. Hydrogen-ion secretion

H⁺ is secreted in:

Proximal tubule: predominantly through Na⁺-H⁺ exchange
Thick ascending limb
Distal tubule and collecting duct: mainly through H⁺-ATPase and H⁺-K⁺-ATPase in alpha-intercalated cells

The collecting duct can generate a very low urine pH, usually to a minimum around 4.5. Free H⁺ excretion alone is small because urine cannot contain enough free H⁺ to dispose of the daily acid burden. Hence urinary buffers are essential.

C. Excretion as titratable acid

Secreted H⁺ binds filtered buffers, mainly phosphate:

[ HPO_4^{2-} + H^+ \rightarrow H_2PO_4^- ]

The hydrogen ion is then excreted as (H_2PO_4^-). This is called titratable acid because it can be measured by titrating urine back to plasma pH.

Each H⁺ excreted in this form adds one new HCO₃⁻ to blood.

Mechanism for excretion of hydrogen ion as titratable acid

Renal tubular secretion of H⁺ and generation of bicarbonate during titratable acid excretion. - Costanzo Physiology, 7th ed., p. 325

D. Excretion as ammonium: the major adaptive mechanism in acidosis

Ammonium excretion is particularly important when acid production is high.

In proximal tubular cells:

[ Glutamine \rightarrow NH_4^+ + HCO_3^- ]

More precisely, metabolism of glutamine produces ammonium and new bicarbonate. The bicarbonate moves into the bloodstream; ammonium is secreted into tubular fluid.

In the collecting duct:

[ NH_3 + H^+ \rightarrow NH_4^+ ]

NH₃ diffuses into the lumen, binds secreted H⁺, and becomes NH₄⁺. Because NH₄⁺ crosses membranes poorly, it becomes trapped in tubular fluid and is excreted. This is called diffusion trapping.

In acidosis

↑ Glutamine metabolism
↑ NH₃/NH₄⁺ production
↑ H⁺ secretion
↑ NH₄⁺ excretion
↑ New HCO₃⁻ added to blood

In alkalosis

↓ Ammonium production and excretion
↓ H⁺ secretion
More bicarbonate may be excreted

Net acid excretion is:

[ \text{Net acid excretion} = NH_4^+ + \text{titratable acid} - urinary\ HCO_3^- ]

The Merck Manual renal-regulation summary describes ammonia as especially important because renal tubular cells can increase ammonia production in response to an acid load.

Renal cellular responses

Alpha-intercalated cells

These cells are active during acidosis.

Secrete H⁺ into tubular lumen via H⁺-ATPase and H⁺-K⁺-ATPase
Move HCO₃⁻ into blood
Produce acidic urine
Help restore plasma bicarbonate

Beta-intercalated cells

These are more active during alkalosis.

Secrete HCO₃⁻ into urine via the apical Cl⁻-HCO₃⁻ exchanger, pendrin
Return H⁺ to blood
Help eliminate excess base

Factors affecting renal acid excretion

Potassium

Potassium and hydrogen balance are closely linked.

Hypokalemia promotes H⁺ secretion, HCO₃⁻ reabsorption, ammoniagenesis, and metabolic alkalosis.
Hyperkalemia suppresses ammonium production and can contribute to metabolic acidosis, particularly type 4 renal tubular acidosis.

Aldosterone

Aldosterone increases distal Na⁺ reabsorption and supports H⁺ secretion by alpha-intercalated cells.

Excess aldosterone can promote metabolic alkalosis.
Aldosterone deficiency or resistance can impair H⁺ excretion and promote metabolic acidosis.

Extracellular-fluid volume and chloride

Volume contraction activates angiotensin II and enhances proximal Na⁺-H⁺ exchange, increasing bicarbonate reabsorption. This helps explain contraction alkalosis, such as after vomiting or loop/thiazide diuretic use.

Chloride depletion also limits bicarbonate excretion and helps maintain metabolic alkalosis. - Costanzo Physiology, 7th ed., pp. 324-325

Primary acid-base disorders and compensation

Primary disorder	Primary change	Direction of pH	Expected compensation
Metabolic acidosis	↓ HCO₃⁻	↓	Hyperventilation, ↓ PaCO₂
Metabolic alkalosis	↑ HCO₃⁻	↑	Hypoventilation, ↑ PaCO₂
Respiratory acidosis	↑ PaCO₂	↓	Renal H⁺ excretion and ↑ HCO₃⁻
Respiratory alkalosis	↓ PaCO₂	↑	Renal HCO₃⁻ excretion and ↓ HCO₃⁻

Important principle

Compensation reduces the pH disturbance but does not correct the underlying cause. Complete normalization usually requires correction of the primary disease process.

Summary flowchart

Acid or base disturbance
        ↓
Immediate: chemical buffering
        ↓
Minutes: respiratory response alters PaCO₂
        ↓
Hours to days: renal response alters HCO₃⁻ and net acid excretion
        ↓
pH moves toward normal

Acidosis:
↑ H⁺ → buffers bind H⁺ → lungs increase ventilation → kidneys retain/generate HCO₃⁻
     → kidneys excrete H⁺ as phosphate and NH₄⁺

Alkalosis:
↓ H⁺ → buffers release H⁺ → lungs reduce ventilation within hypoxemia limits
     → kidneys excrete HCO₃⁻ and reduce H⁺/NH₄⁺ excretion

Key takeaways

Body pH is controlled chiefly by the HCO₃⁻ to PaCO₂ ratio.
Buffers act in seconds, but only temporarily.
Lungs control volatile acid, CO₂, in minutes.
Kidneys control bicarbonate and fixed-acid removal over hours to days.
Renal ammonium excretion is the principal adaptive pathway for sustained acid loading.
Compensation does not overcorrect a simple disorder. An inappropriate compensation suggests an additional mixed acid-base disorder.

Each acid base disorder in detail

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Acid-base disorders

There are four primary acid-base disorders:

Metabolic acidosis
Metabolic alkalosis
Respiratory acidosis
Respiratory alkalosis

They are classified by the primary abnormality:

Metabolic disorders: primary change in serum bicarbonate, (HCO_3^-)
Respiratory disorders: primary change in arterial carbon dioxide, (PaCO_2)

[ \text{pH} = 6.1 + \log\left(\frac{HCO_3^-}{0.03 \times PaCO_2}\right) ]

Thus, pH falls if bicarbonate falls or CO₂ rises; pH rises if bicarbonate rises or CO₂ falls.

Normal approximate values:

Variable	Normal value
Arterial pH	7.35-7.45
(PaCO_2)	35-45 mmHg
(HCO_3^-)	22-26 mEq/L

A simple disorder has one primary abnormality and expected compensation. If compensation is outside the expected range, suspect a mixed acid-base disorder. - Costanzo Physiology, 7th ed., pp. 330-336

1. Metabolic acidosis

Definition

Metabolic acidosis is a primary reduction in plasma bicarbonate concentration, causing a fall in pH.

[ \downarrow HCO_3^- \rightarrow \downarrow pH ]

Typical arterial blood gas pattern

Variable	Finding
pH	Low
(HCO_3^-)	Low, primary change
(PaCO_2)	Low, due to respiratory compensation

Mechanisms

Metabolic acidosis occurs due to one of four mechanisms:

Increased endogenous acid production
Ingestion of acid or toxin
Loss of bicarbonate
Failure of renal acid excretion

Respiratory compensation

A low pH stimulates peripheral chemoreceptors, especially carotid bodies, producing hyperventilation. This lowers PaCO₂.

[ \downarrow HCO_3^- \rightarrow \downarrow pH \rightarrow \uparrow ventilation \rightarrow \downarrow PaCO_2 ]

Severe acidosis may cause Kussmaul respiration: deep, labored, rapid breathing, classically seen in diabetic ketoacidosis.

Winter's formula

Use this to determine whether respiratory compensation is appropriate:

[ Expected\ PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2 ]

Example: if (HCO_3^- = 12) mEq/L:

[ Expected\ PaCO_2 = 1.5(12) + 8 = 26 \pm 2 ]

Actual PaCO₂ higher than predicted: additional respiratory acidosis
Actual PaCO₂ lower than predicted: additional respiratory alkalosis

Anion gap

The anion gap identifies whether unmeasured acids are present.

[ AG = Na^+ - (Cl^- + HCO_3^-) ]

A common reference range is about 8-12 mEq/L when potassium is omitted, but interpret according to the laboratory reference interval.

Because albumin is a major unmeasured anion, a low albumin can conceal a raised anion gap:

[ Corrected\ AG = Measured\ AG + 2.5 \times (4 - albumin\ in\ g/dL) ]

A. High-anion-gap metabolic acidosis

Here, bicarbonate is consumed buffering an added acid, while the acid's unmeasured anion remains in plasma.

[ \downarrow HCO_3^- + \uparrow unmeasured\ anions = \uparrow AG ]

Common causes: GOLD MARK

Cause	Examples
Glycols	Ethylene glycol, propylene glycol
Oxoproline	Chronic acetaminophen use, usually in susceptible patients
L-lactate	Shock, hypoxemia, sepsis, seizures, severe liver dysfunction
D-lactate	Short-bowel syndrome
Methanol	Formic acid accumulation
Aspirin	Salicylate poisoning
Renal failure	Retention of sulfate, phosphate, and other acids
Ketoacidosis	Diabetic, alcoholic, starvation ketoacidosis

Major clinical examples

Diabetic ketoacidosis

Insulin deficiency causes lipolysis and hepatic ketone production, especially beta-hydroxybutyrate and acetoacetate.

Typical findings:

Hyperglycemia
Ketones
High anion gap acidosis
Dehydration
Kussmaul breathing
Total-body potassium deficit, even when initial serum potassium is normal or high

Lactic acidosis

Occurs when lactate production exceeds clearance.

Common settings:

Sepsis or septic shock
Hypovolemia or hemorrhage
Cardiogenic shock
Severe hypoxemia
Seizures
Mesenteric ischemia
Severe liver dysfunction
Some drugs and toxins

Renal failure

Reduced nephron mass impairs ammonium production, titratable-acid excretion, and regeneration of bicarbonate. Fixed acids accumulate. - Costanzo Physiology, 7th ed., pp. 328-331

B. Normal-anion-gap metabolic acidosis

Also called hyperchloremic metabolic acidosis.

Here, bicarbonate is lost and replaced electrically by chloride:

[ \downarrow HCO_3^- \quad \text{with} \quad \uparrow Cl^- ]

The anion gap stays normal.

Causes

Mechanism	Examples
Gastrointestinal bicarbonate loss	Diarrhea, ileostomy, pancreatic or biliary drainage, intestinal fistula
Renal bicarbonate loss	Proximal renal tubular acidosis, carbonic anhydrase inhibitors
Reduced distal H⁺ secretion	Distal renal tubular acidosis
Low aldosterone effect	Type 4 renal tubular acidosis, adrenal insufficiency
Chloride administration	Large-volume 0.9% saline administration
Urinary diversion	Ureterosigmoidostomy

Urine anion gap

In normal renal response to metabolic acidosis, the kidney excretes ammonium chloride, so urinary chloride rises.

[ Urine\ anion\ gap = (Urine\ Na^+ + Urine\ K^+) - Urine\ Cl^- ]

Negative urine anion gap: high urinary NH₄⁺ excretion, suggesting an extrarenal cause such as diarrhea
Positive urine anion gap: low urinary NH₄⁺ excretion, suggesting impaired renal acid excretion, such as renal tubular acidosis

Symptoms and signs

Symptoms depend on severity and cause:

Deep rapid breathing
Fatigue, weakness
Nausea and vomiting
Confusion, drowsiness, coma in severe cases
Reduced cardiac contractility and hypotension in severe acidemia
Hyperkalemia may occur, especially with mineral acidosis or renal failure

Management principles

Treatment is directed at the cause:

Restore perfusion and oxygenation in shock or lactic acidosis
Insulin, fluids, electrolyte monitoring, and potassium management in DKA
Treat toxins urgently, with antidotes and dialysis where indicated
Stop bicarbonate-losing medications or treat diarrhea
Address renal failure or renal tubular acidosis

Bicarbonate is not routine for all acidoses. It is considered selectively in severe acidemia or specific bicarbonate-loss states, with careful monitoring.

2. Metabolic alkalosis

Definition

Metabolic alkalosis is a primary elevation in plasma bicarbonate, causing increased pH.

[ \uparrow HCO_3^- \rightarrow \uparrow pH ]

Typical blood gas pattern

Variable	Finding
pH	High
(HCO_3^-)	High, primary change
(PaCO_2)	High, compensatory

Mechanisms

Metabolic alkalosis results from:

Loss of hydrogen ions
Gain of bicarbonate
Renal retention of bicarbonate

For metabolic alkalosis to persist, the kidney must be unable or signaled not to excrete excess bicarbonate. Common maintaining factors are:

Volume depletion
Chloride depletion
Hypokalemia
Reduced kidney function
Mineralocorticoid excess

Respiratory compensation

The compensatory response is hypoventilation, causing CO₂ retention:

[ \uparrow HCO_3^- \rightarrow \uparrow pH \rightarrow \downarrow ventilation \rightarrow \uparrow PaCO_2 ]

Expected response:

[ Expected\ PaCO_2 \approx 40 + 0.6\text{ to }0.75 \times (HCO_3^- - 24) ]

Respiratory compensation is limited because excessive hypoventilation causes hypoxemia. A compensatory PaCO₂ generally should not rise above about 55 mmHg. - Merck diagnostic guidance

Causes

A. Chloride-responsive metabolic alkalosis

Usually caused by volume depletion and chloride loss. Urine chloride is typically low:

[ Urine\ Cl^- < 20\ mEq/L ]

Common causes:

Vomiting
Nasogastric suction
Loop diuretics
Thiazide diuretics
Post-hypercapnic alkalosis
Chloride-losing diarrhea, rarely
Cystic fibrosis with marked sweat chloride loss

Why vomiting causes alkalosis

Loss of gastric hydrochloric acid causes direct H⁺ and Cl⁻ loss:

[ Loss\ of\ H^+ \rightarrow \uparrow HCO_3^- ]

Volume and chloride depletion activate renin-angiotensin-aldosterone mechanisms, enhancing proximal sodium-bicarbonate reabsorption and maintaining alkalosis.

B. Chloride-unresponsive metabolic alkalosis

Urine chloride is usually high:

[ Urine\ Cl^- > 20\ mEq/L ]

Main causes:

Primary hyperaldosteronism
Cushing syndrome or ectopic mineralocorticoid effect
Severe hypokalemia
Bartter syndrome
Gitelman syndrome
Excess alkali intake in reduced renal function
Licorice ingestion, causing apparent mineralocorticoid excess

Potassium relationship

Hypokalemia promotes:

H⁺ movement into cells
Increased renal H⁺ secretion
Increased bicarbonate reabsorption
Increased ammonium production

Therefore, hypokalemia both accompanies and perpetuates metabolic alkalosis.

Symptoms and signs

Mild cases may be asymptomatic. Severe alkalemia can cause:

Weakness
Headache
Confusion or lethargy
Muscle cramps
Paresthesia
Tetany
Seizures
Cardiac arrhythmias

Alkalemia increases protein binding of calcium and lowers ionized calcium, which contributes to tetany and carpopedal spasm. - Costanzo Physiology, 7th ed., p. 336

Management principles

Treat vomiting, gastric suction, or diuretic excess
Correct extracellular-volume depletion with chloride-containing fluid when appropriate
Correct potassium and magnesium deficits
Treat mineralocorticoid excess when present
Consider acetazolamide in selected patients, especially when volume expansion is undesirable
Severe, resistant alkalemia requires urgent specialist management

A urine chloride below 20 mEq/L generally suggests a chloride-responsive cause, whereas a higher value supports chloride-unresponsive alkalosis, assuming kidney function is adequate. - Merck metabolic alkalosis review

3. Respiratory acidosis

Definition

Respiratory acidosis is a primary increase in PaCO₂ due to inadequate alveolar ventilation.

[ \uparrow PaCO_2 \rightarrow \uparrow H_2CO_3 \rightarrow \uparrow H^+ \rightarrow \downarrow pH ]

Typical blood gas pattern

Variable	Finding
pH	Low
(PaCO_2)	High, primary change
(HCO_3^-)	High if compensation occurs

Main mechanism: hypoventilation

[ \downarrow alveolar\ ventilation \rightarrow \uparrow PaCO_2 ]

Any condition that reduces effective ventilation can cause respiratory acidosis.

Causes

A. Decreased respiratory drive

Opioid intoxication
Sedatives, anesthetics, barbiturates
Brainstem stroke, tumor, trauma, or infection
Central sleep apnea
Obesity hypoventilation syndrome

B. Neuromuscular failure

Guillain-Barré syndrome
Myasthenia gravis
Motor neuron disease
Spinal cord injury
Severe hypokalemia or hypophosphatemia
Neuromuscular blocking drugs

C. Airway obstruction

Severe COPD exacerbation
Severe asthma with fatigue or impending respiratory failure
Upper airway obstruction
Obstructive sleep apnea

D. Impaired gas exchange or increased work of breathing

Severe pneumonia
Pulmonary edema
Acute respiratory distress syndrome
Severe obesity or chest-wall restriction
Costanzo Physiology, 7th ed., pp. 333-334

Acute versus chronic respiratory acidosis

The distinction matters because the kidneys require time to retain bicarbonate and excrete acid.

Acute respiratory acidosis

Occurs over minutes to hours, for example opioid overdose or sudden airway obstruction.

Intracellular buffering is the main initial response.
Bicarbonate rises only slightly.

[ \text{For every 10 mmHg rise in } PaCO_2,\ HCO_3^- \text{ rises by about 1-2 mEq/L} ]

Example: PaCO₂ rises from 40 to 60 mmHg.

Expected acute bicarbonate:

[ HCO_3^- \approx 24 + 2\text{ to }4 = 26\text{ to }28 ]

Chronic respiratory acidosis

Occurs over days, for example stable COPD with chronic CO₂ retention.

Kidneys increase H⁺ secretion.
Ammonium and titratable-acid excretion increase.
New bicarbonate is generated and retained.

[ \text{For every 10 mmHg rise in } PaCO_2,\ HCO_3^- \text{ rises by about 3-4 mEq/L} ]

Example: PaCO₂ rises from 40 to 60 mmHg.

Expected chronic bicarbonate:

[ HCO_3^- \approx 24 + 6\text{ to }8 = 30\text{ to }32 ]

The pH is lower in acute respiratory acidosis than in chronic respiratory acidosis at the same PaCO₂ because chronic renal compensation is more effective.

Symptoms and signs

Severity depends on how rapidly CO₂ rises.

Headache
Drowsiness
Confusion
Asterixis
Warm, flushed skin
Dyspnea
Reduced level of consciousness
CO₂ narcosis and coma in severe cases

Chronic hypercapnia may be tolerated better than an abrupt CO₂ rise.

Management principles

Support airway, breathing, and oxygenation
Reverse or stop the underlying cause where possible
Naloxone for opioid-induced hypoventilation, when appropriate
Treat COPD or asthma exacerbation
Manage pneumonia, pulmonary edema, or neuromuscular weakness
Use noninvasive ventilation or invasive ventilation when indicated

In a chronic CO₂ retainer, abrupt overventilation can rapidly lower PaCO₂ while bicarbonate remains elevated, producing post-hypercapnic metabolic alkalosis. Correction must be monitored clinically and with serial blood gases.

4. Respiratory alkalosis

Definition

Respiratory alkalosis is a primary reduction in PaCO₂ caused by excessive ventilation.

[ \downarrow PaCO_2 \rightarrow \downarrow H^+ \rightarrow \uparrow pH ]

Typical blood gas pattern

Variable	Finding
pH	High
(PaCO_2)	Low, primary change
(HCO_3^-)	Low if compensation occurs

Causes

A. Hypoxemia-driven hyperventilation

Pulmonary embolism
Pneumonia
Pulmonary edema
Asthma exacerbation
High altitude
Severe anemia, rarely
Right-to-left shunt

B. Increased respiratory drive

Pain
Anxiety or panic
Fever
Sepsis
Pregnancy, due partly to progesterone
Liver disease
CNS disorders: stroke, trauma, tumor, meningitis, encephalitis
Salicylate toxicity, especially early

C. Iatrogenic causes

Excessive mechanical ventilation

Never assume hyperventilation is due to anxiety until potentially serious causes, especially hypoxemia, sepsis, pulmonary embolism, and salicylate poisoning, have been considered. - Merck respiratory alkalosis review

Acute versus chronic respiratory alkalosis

Acute respiratory alkalosis

The kidneys have not yet adapted. Bicarbonate falls only slightly due to buffering.

[ \text{For every 10 mmHg fall in } PaCO_2,\ HCO_3^- \text{ decreases by about 1-2 mEq/L} ]

Example: PaCO₂ falls from 40 to 20 mmHg.

[ HCO_3^- \approx 24 - 2\text{ to }4 = 20\text{ to }22 ]

Chronic respiratory alkalosis

The kidney adapts by excreting more bicarbonate and reducing H⁺ secretion.

[ \text{For every 10 mmHg fall in } PaCO_2,\ HCO_3^- \text{ decreases by about 4-5 mEq/L} ]

Example: PaCO₂ falls from 40 to 20 mmHg.

[ HCO_3^- \approx 24 - 8\text{ to }10 = 14\text{ to }16 ]

Symptoms and signs

Acute hypocapnia can reduce cerebral blood flow and lower ionized calcium.

Features include:

Light-headedness
Dizziness
Perioral numbness
Tingling of hands and feet
Muscle cramps
Carpopedal spasm
Chest discomfort
Confusion
Syncope in severe cases

The fall in ionized calcium occurs because alkalemia increases calcium binding to albumin.

Management principles

Treat the cause:

Give oxygen and treat hypoxemia
Investigate pulmonary embolism, sepsis, lung disease, or salicylate toxicity when relevant
Adjust excessive mechanical ventilation
Treat pain, fever, or anxiety only after excluding important organic causes

Rebreathing into a paper bag is not recommended because it can worsen unrecognized hypoxemia or delay diagnosis of a dangerous cause. - Merck guidance

Quick comparison table

Disorder	Primary change	pH	Compensation	Typical causes
Metabolic acidosis	↓ HCO₃⁻	↓	↓ PaCO₂ by hyperventilation	DKA, lactic acidosis, diarrhea, renal failure, RTA
Metabolic alkalosis	↑ HCO₃⁻	↑	↑ PaCO₂ by hypoventilation	Vomiting, diuretics, hypokalemia, mineralocorticoid excess
Respiratory acidosis	↑ PaCO₂	↓	↑ HCO₃⁻ by kidneys	COPD, CNS depression, neuromuscular failure, airway obstruction
Respiratory alkalosis	↓ PaCO₂	↑	↓ HCO₃⁻ by kidneys	Hypoxemia, PE, sepsis, pregnancy, pain, anxiety, liver disease

Practical ABG interpretation sequence

Check pH
- < 7.35: acidemia
- 7.45: alkalemia
Identify the primary process
- In acidemia: high PaCO₂ suggests respiratory acidosis; low HCO₃⁻ suggests metabolic acidosis.
- In alkalemia: low PaCO₂ suggests respiratory alkalosis; high HCO₃⁻ suggests metabolic alkalosis.
Check expected compensation
- Use Winter's formula in metabolic acidosis.
- Use acute/chronic compensation rules for respiratory disorders.
Calculate anion gap if metabolic acidosis is present.
Look for a mixed disorder if compensation is inappropriate or pH, PaCO₂, and bicarbonate do not fit one simple pattern.
Find and treat the underlying cause, not merely the blood-gas abnormality.

Severe acidemia, severe alkalemia, altered consciousness, shock, significant hyperkalemia, suspected toxic ingestion, severe dyspnea, or respiratory fatigue requires urgent in-person assessment.

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