Medical School · Year 1 · Respiratory · includes a quiz and discussion video

Lecture 6: Acid-Base Physiology

Unit 1.8: Respiratory System

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe normal acid-base homeostasis and buffer systems
2. Apply the Henderson-Hasselbalch equation to understand pH regulation
3. Differentiate respiratory from metabolic acid-base disorders
4. Calculate expected compensation for primary disorders
5. Identify mixed acid-base disorders
6. Apply a systematic approach to ABG interpretation

Lecture Content

I. Acid-Base Fundamentals

Maintenance of blood pH within a narrow range is essential for normal cellular function, enzyme activity, and protein structure. The body possesses multiple mechanisms to regulate hydrogen ion concentration, integrating chemical buffering, respiratory adjustments, and renal handling of acid and base.

Normal arterial blood pH ranges from 7.35 to 7.45, corresponding to a hydrogen ion concentration of 35 to 45 nanomoles per liter. Arterial carbon dioxide tension normally measures 35 to 45 mmHg, while bicarbonate concentration ranges from 22 to 26 milliequivalents per liter. Arterial oxygen tension of 80 to 100 mmHg completes the standard arterial blood gas panel. Base excess, a calculated value representing the amount of strong acid or base required to return blood to normal pH at normal carbon dioxide tension, normally falls between minus 2 and plus 2 milliequivalents per liter.

Precise terminology distinguishes related but distinct concepts. An acid donates protons (hydrogen ions), while a base accepts them. Acidemia denotes a measured blood pH below 7.35, while alkalemia indicates pH above 7.45. Acidosis describes any process tending to lower blood pH, while alkalosis describes a process tending to raise it. A patient may have acidosis without acidemia if a concurrent alkalosis compensates sufficiently, and vice versa. These distinctions become important when analyzing complex acid-base disturbances.

The body produces acid continuously through metabolism. Volatile acid, in the form of carbon dioxide, constitutes the vast majority at approximately 15,000 millimoles daily. Carbon dioxide combines with water to form carbonic acid, which dissociates to release hydrogen ions. The lungs eliminate volatile acid by exhaling carbon dioxide. Fixed acids, including sulfuric acid from sulfur-containing amino acid metabolism, phosphoric acid from phospholipid turnover, and organic acids from intermediary metabolism, total 50 to 100 milliequivalents daily. The kidneys eliminate fixed acids, as these cannot be converted to gaseous form for pulmonary excretion.

<image>Panel A: Volatile acid production showing CO2 from cellular metabolism transported to lungs and exhaled at approximately 15,000 mmol/day. Panel B: Carbonic acid equation CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- with reversibility arrows. Panel C: Fixed acids including sulfuric acid from methionine and cysteine, phosphoric acid from phospholipids, and organic acids totaling 50-100 mEq/day. Panel D: Elimination pathways showing volatile acid via lungs and fixed acids via kidneys with arrows tracing flow from sources to organs.</image>

II. Buffer Systems

Buffers provide the immediate first line of defense against pH changes by absorbing or releasing hydrogen ions. The body employs multiple buffer systems that work in concert to minimize pH fluctuations.

The bicarbonate buffer system functions as the principal extracellular buffer, accounting for approximately 50 percent of buffering capacity. The system operates through the equilibrium between carbon dioxide, carbonic acid, and bicarbonate. Carbon dioxide combines with water to form carbonic acid, which rapidly dissociates to hydrogen ion and bicarbonate. The carbonic anhydrase enzyme catalyzes this reaction in tissues containing the enzyme, particularly red blood cells and renal tubular cells.

What makes the bicarbonate system uniquely effective is the independent regulation of its two components. The lungs control carbon dioxide concentration through ventilation, while the kidneys control bicarbonate concentration through reabsorption and regeneration. This dual regulation allows the bicarbonate system to handle disturbances originating from either respiratory or metabolic sources.

Hemoglobin provides substantial buffering capacity within red blood cells. Histidine residues on the hemoglobin molecule accept and release hydrogen ions depending on pH. Importantly, deoxygenated hemoglobin binds hydrogen ions more avidly than oxygenated hemoglobin, facilitating carbon dioxide transport as described by the Haldane effect.

Intracellular phosphate buffers operate primarily within cells and in urine. The dihydrogen phosphate and monohydrogen phosphate pair buffer effectively at urinary pH, contributing to renal acid excretion. Intracellular proteins provide buffering through their amino acid residues, particularly histidine, with protein buffering predominating in intracellular fluid.

The isohydric principle states that all buffer systems in a solution are in equilibrium with the same hydrogen ion concentration. Consequently, a change affecting any one buffer system affects all others. This interconnection means that measuring one buffer pair, typically bicarbonate and carbon dioxide, provides information about the status of all buffer systems.

<image>Panel A: Central bicarbonate equation CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- with pulmonary control of CO2 and renal control of HCO3- annotated. Panel B: Hemoglobin buffer showing histidine residues binding H+ with oxygenation-dependent affinity change and phosphate buffer HPO4²- + H+ ⇌ H2PO4- operating in urine. Panel C: Protein buffers with amino acid side chains accepting H+ and arrows connecting all systems representing isohydric principle. Panel D: Pie chart showing buffering contributions with bicarbonate 50%, hemoglobin 30%, and proteins plus phosphate 20%.</image>

III. Respiratory Regulation of pH

The respiratory system provides rapid compensation for acid-base disturbances through adjustment of carbon dioxide elimination. Because carbon dioxide hydrates to form carbonic acid, changing the carbon dioxide concentration directly affects blood hydrogen ion concentration and pH.

The relationship between carbon dioxide and pH follows directly from the carbonic acid equilibrium. When arterial carbon dioxide tension rises due to hypoventilation, more carbonic acid forms, more hydrogen ions are released, and pH falls, producing respiratory acidosis. When arterial carbon dioxide tension falls due to hyperventilation, less carbonic acid forms, fewer hydrogen ions are released, and pH rises, producing respiratory alkalosis.

Respiratory compensation operates within minutes to hours, making it the first physiological response to metabolic acid-base disturbances. Chemoreceptors continuously monitor blood gases and pH, adjusting ventilation to maintain homeostasis. Metabolic acidosis stimulates ventilation, lowering carbon dioxide to partially restore the bicarbonate-to-carbon dioxide ratio and raise pH toward normal. Metabolic alkalosis suppresses ventilation, raising carbon dioxide to partially restore the ratio and lower pH toward normal.

Limits constrain respiratory compensation in both directions. Hyperventilation cannot lower carbon dioxide indefinitely; respiratory muscle fatigue, chest wall mechanics, and the work of breathing impose practical limits around 10 to 15 mmHg. More importantly, lowering carbon dioxide requires maintaining adequate oxygenation, which becomes increasingly difficult at very low carbon dioxide levels. Compensation for metabolic alkalosis through hypoventilation is particularly limited because the resulting hypoxemia eventually stimulates ventilation, preventing adequate carbon dioxide retention.

Primary respiratory disturbances originate from abnormal ventilation. Hypoventilation from any cause, including central nervous system depression, neuromuscular weakness, chest wall abnormality, or severe lung disease, produces respiratory acidosis. Hyperventilation from hypoxemia, anxiety, central nervous system disorders, or mechanical ventilation produces respiratory alkalosis.

<image>Panel A: Ventilatory response showing increased ventilation blowing off CO2 in acidosis versus decreased ventilation retaining CO2 in alkalosis with alveolar arrows. Panel B: Feedback loop with chemoreceptors sensing pH changes, signaling respiratory centers, adjusting ventilation, and changing PCO2 to modify pH. Panel C: Compensation limits graph showing minute ventilation versus arterial PCO2 with minimum achievable PCO2 of 10-15 mmHg and hypoventilation limited by hypoxemia. Panel D: Primary respiratory disorders listed with examples for acidosis causes and alkalosis causes.</image>

IV. Renal Regulation of pH

The kidneys provide the definitive long-term correction of acid-base disturbances through bicarbonate handling and acid excretion. Although renal responses develop slowly over hours to days, they have virtually unlimited capacity to restore normal pH.

Three principal mechanisms accomplish renal acid-base regulation. First, the kidneys reabsorb filtered bicarbonate to prevent ongoing bicarbonate loss in urine. Second, the kidneys secrete hydrogen ions into the tubular fluid. Third, the kidneys generate new bicarbonate to replace that consumed in buffering fixed acids.

Bicarbonate reabsorption occurs primarily in the proximal tubule, which reclaims approximately 85 percent of filtered bicarbonate. The mechanism involves hydrogen ion secretion into the tubular lumen, where it combines with filtered bicarbonate to form carbonic acid. Carbonic anhydrase on the luminal membrane catalyzes carbonic acid breakdown to carbon dioxide and water, which enter the tubular cell. Inside the cell, carbonic anhydrase catalyzes the reverse reaction, generating carbonic acid that dissociates to hydrogen ion (secreted again) and bicarbonate (reabsorbed to blood). The thick ascending limb reabsorbs most of the remaining filtered bicarbonate.

Generation of new bicarbonate requires excretion of hydrogen ions in forms that do not simply recombine with bicarbonate. Titratable acid, primarily phosphate, accepts secreted hydrogen ions, forming dihydrogen phosphate that is excreted in urine. Ammonium excretion provides an even more powerful mechanism for acid elimination. Glutamine metabolism in proximal tubule cells generates ammonia and bicarbonate. Ammonia is secreted into the tubular lumen, combines with hydrogen ions to form ammonium, and is excreted while the bicarbonate returns to blood. This process can be markedly upregulated during chronic acidosis.

Renal compensation for respiratory acid-base disorders adjusts bicarbonate concentration. In chronic respiratory acidosis, the kidneys increase bicarbonate reabsorption and generation, raising serum bicarbonate to partially restore the bicarbonate-to-carbon dioxide ratio. In chronic respiratory alkalosis, the kidneys decrease bicarbonate reabsorption and increase bicarbonate excretion, lowering serum bicarbonate to partially restore the ratio.

<image>Panel A: Proximal tubule cell showing bicarbonate reabsorption via H+ secretion through Na+/H+ exchanger with luminal carbonic anhydrase converting H2CO3 to CO2 and H2O. Panel B: Intracellular carbonic anhydrase regenerating H+ and HCO3- with basolateral HCO3- transport to blood. Panel C: New bicarbonate generation via titratable acid pathway HPO4²- + H+ → H2PO4- and ammonium pathway NH3 + H+ → NH4+ with bicarbonate return to blood. Panel D: Timeline comparing respiratory compensation in minutes versus renal compensation over hours to days with greater capacity annotation.</image>

V. Primary Acid-Base Disorders

Four primary acid-base disorders represent disturbances in either the respiratory or metabolic components of acid-base balance. Each produces characteristic changes in pH, carbon dioxide, and bicarbonate that allow diagnosis and guide treatment.

Respiratory acidosis occurs when inadequate alveolar ventilation allows carbon dioxide accumulation. The elevated carbon dioxide shifts the carbonic acid equilibrium toward hydrogen ion production, lowering pH. Causes span the respiratory system from central control (drug overdose, stroke, brainstem injury) through neuromuscular transmission (Guillain-Barré syndrome, myasthenia gravis) to the chest wall (kyphoscoliosis, flail chest), airways (severe asthma, chronic obstructive pulmonary disease), and lung parenchyma (severe pneumonia, acute respiratory distress syndrome). The kidneys compensate by retaining bicarbonate over several days.

Respiratory alkalosis occurs when excessive alveolar ventilation reduces carbon dioxide below normal. The decreased carbon dioxide shifts the carbonic acid equilibrium away from hydrogen ion production, raising pH. Causes include hypoxemia triggering hyperventilation, central nervous system stimulation from anxiety, pain, fever, or salicylate toxicity, normal pregnancy, and iatrogenic over-ventilation. The kidneys compensate by excreting bicarbonate over several days.

Metabolic acidosis occurs when acid accumulation or bicarbonate loss reduces serum bicarbonate. The reduced bicarbonate lowers the bicarbonate-to-carbon dioxide ratio, reducing pH. Classification by the anion gap helps identify causes. Elevated anion gap acidosis results from addition of unmeasured acids: lactic acidosis, ketoacidosis, renal failure, and ingestions of methanol, ethylene glycol, or salicylates follow the mnemonic MUDPILES. Normal anion gap (hyperchloremic) acidosis results from bicarbonate loss or impaired acid excretion: diarrhea, renal tubular acidosis, early chronic kidney disease, and certain medications. The lungs compensate by hyperventilating to reduce carbon dioxide.

Metabolic alkalosis occurs when acid loss or base gain increases serum bicarbonate. The elevated bicarbonate raises the bicarbonate-to-carbon dioxide ratio, increasing pH. Volume-responsive causes include vomiting, nasogastric suction, and diuretic use; these respond to saline administration because volume depletion and chloride depletion perpetuate the alkalosis. Volume-resistant causes include primary aldosteronism and Cushing syndrome, where mineralocorticoid excess drives ongoing hydrogen ion loss. The lungs compensate weakly by hypoventilating, limited by the hypoxemic stimulus to breathe.

<image>Panel A: Respiratory acidosis quadrant showing decreased pH, increased PCO2 as primary change, increased HCO3- as compensation with causes listed. Panel B: Respiratory alkalosis quadrant showing increased pH, decreased PCO2 as primary, decreased HCO3- as compensation with causes listed. Panel C: Metabolic acidosis quadrant showing decreased pH, decreased HCO3- as primary, decreased PCO2 as compensation with causes divided by anion gap. Panel D: Metabolic alkalosis quadrant showing increased pH, increased HCO3- as primary, increased PCO2 as compensation with causes divided by volume responsiveness and directional arrows.</image>

VI. Compensation

The body responds to primary acid-base disorders by activating compensatory mechanisms that partially restore pH toward normal. Understanding expected compensation allows recognition of simple versus mixed disorders.

Several principles govern compensation. The body compensates but never overcompensates: if the primary disorder causes acidemia, compensation may bring pH toward 7.40 but will not produce alkalemia, and vice versa. Respiratory compensation for metabolic disorders develops rapidly over minutes to hours. Renal compensation for respiratory disorders develops slowly over three to five days. Compensation moves pH toward normal but rarely achieves complete normalization; persistently abnormal pH suggests either inadequate time for full compensation or a concurrent additional disorder.

For metabolic acidosis, the expected respiratory compensation follows Winter's formula: expected arterial carbon dioxide equals 1.5 times the serum bicarbonate plus 8, with a range of plus or minus 2. For example, if bicarbonate has fallen to 12 milliequivalents per liter, expected carbon dioxide equals 1.5 times 12 plus 8, which equals 26, with an acceptable range of 24 to 28 mmHg.

For metabolic alkalosis, expected carbon dioxide increases by approximately 0.7 mmHg for each milliequivalent per liter increase in bicarbonate above normal. Because of the hypoxemic limitation, carbon dioxide rarely exceeds 55 mmHg in compensation for metabolic alkalosis.

Compensation for respiratory disorders differs between acute and chronic phases. In acute respiratory acidosis, bicarbonate increases only 1 milliequivalent per liter for each 10 mmHg increase in carbon dioxide, reflecting only chemical buffering. In chronic respiratory acidosis with full renal compensation, bicarbonate increases 3.5 milliequivalents per liter for each 10 mmHg increase in carbon dioxide. In acute respiratory alkalosis, bicarbonate decreases 2 milliequivalents per liter for each 10 mmHg decrease in carbon dioxide. In chronic respiratory alkalosis, bicarbonate decreases 5 milliequivalents per liter for each 10 mmHg decrease in carbon dioxide.

Comparing measured compensation to expected compensation reveals additional disorders. If measured carbon dioxide or bicarbonate falls short of the expected value, a concurrent acidosis of the other type exists. If measured values exceed expected, a concurrent alkalosis exists.

<image>Panel A: Central pH graph showing relationship between respiratory and metabolic components with lines for each disorder and expected compensation. Panel B: Compensation formulas including Winter's formula for metabolic acidosis and 0.7 multiplier for metabolic alkalosis with example calculation. Panel C: Acute versus chronic respiratory compensation rules showing 1 and 3.5 for acidosis and 2 and 5 for alkalosis. Panel D: Decision tree interpreting measured versus expected compensation with less than expected indicating additional acidosis, more than expected indicating additional alkalosis.</image>

VII. Anion Gap

The anion gap represents a calculated value that helps classify metabolic acidosis and detect certain mixed disorders. It quantifies unmeasured anions and provides crucial diagnostic information.

The anion gap equals serum sodium minus the sum of chloride and bicarbonate. Normal values range from 8 to 12 milliequivalents per liter when potassium is not included in the calculation, or 10 to 14 when potassium is included. This calculation reflects the principle of electroneutrality: total cations must equal total anions. The measured cation (sodium) exceeds measured anions (chloride plus bicarbonate), and the difference represents unmeasured anions.

Albumin constitutes the major unmeasured anion under normal conditions. Because hypoalbuminemia reduces the anion gap, the expected gap should be adjusted in patients with low albumin: corrected anion gap equals measured anion gap plus 2.5 times (4 minus the serum albumin in grams per deciliter).

Elevated anion gap metabolic acidosis occurs when an unmeasured acid accumulates in blood. The acid dissociates, releasing hydrogen ions that consume bicarbonate while leaving behind the acid's anion. The anion gap increases by the same amount that bicarbonate decreases. The mnemonic MUDPILES lists major causes: Methanol, Uremia, Diabetic ketoacidosis (and other ketoacidosis), Propylene glycol, Isoniazid or Iron toxicity, Lactic acidosis, Ethylene glycol, and Salicylates.

Normal anion gap metabolic acidosis, also called hyperchloremic acidosis, occurs when bicarbonate is lost or acid is added without an accompanying unmeasured anion. Chloride increases to maintain electroneutrality, keeping the anion gap normal. Diarrhea causes direct bicarbonate loss in stool. Renal tubular acidosis impairs either distal hydrogen ion secretion (type 1), proximal bicarbonate reabsorption (type 2), or aldosterone effect (type 4). Early chronic kidney disease reduces ammonium excretion. Large-volume normal saline administration dilutes bicarbonate while adding chloride.

<image>Panel A: Bar chart showing cations (sodium, potassium) versus anions (chloride, bicarbonate, unmeasured) with anion gap as difference and calculation formula. Panel B: Elevated anion gap acidosis bar chart showing decreased bicarbonate, increased unmeasured anion like lactate or ketones, and widened gap. Panel C: Normal anion gap acidosis bar chart showing decreased bicarbonate, proportionally increased chloride, and unchanged gap. Panel D: Cause lists with MUDPILES mnemonic for elevated gap and diarrhea, RTA, and saline dilution for normal gap acidosis.</image>

VIII. Delta-Delta Ratio

The delta-delta ratio refines the analysis of anion gap metabolic acidosis by detecting concurrent metabolic disorders that might otherwise escape notice. This calculation compares the increase in anion gap to the decrease in bicarbonate.

The delta-delta calculation divides the change in anion gap from normal (typically 12) by the change in bicarbonate from normal (typically 24). In pure anion gap acidosis without other metabolic disturbances, each milliequivalent of acid added produces one milliequivalent increase in anion gap and one milliequivalent decrease in bicarbonate, yielding a delta-delta ratio between 1 and 2.

A ratio less than 1 suggests that bicarbonate has fallen more than the anion gap has risen. This pattern indicates that a normal anion gap acidosis coexists with the anion gap acidosis. For example, a patient with diabetic ketoacidosis who also has diarrhea will have the expected bicarbonate reduction from ketoacid accumulation plus additional bicarbonate loss in stool, producing a ratio less than 1.

A ratio greater than 2 suggests that bicarbonate remains higher than expected given the increase in anion gap. This pattern indicates a concurrent metabolic alkalosis. For example, a patient with lactic acidosis who has also been vomiting will have bicarbonate consumption from lactic acid buffering partially offset by hydrogen ion loss from vomiting, producing a ratio greater than 2.

This analysis proves valuable in complex patients with multiple pathological processes. Critical illness often involves simultaneous acid-generating conditions (shock producing lactic acidosis) and acid-losing conditions (nasogastric suction causing metabolic alkalosis). The delta-delta ratio helps disentangle these overlapping disturbances.

<image>Panel A: Delta-delta calculation formula (measured AG - 12) / (24 - measured HCO3-) with normal values and interpretation ranges 1-2. Panel B: Scenario 1 ratio 1-2 showing pure anion gap acidosis with proportional AG increase and HCO3- decrease in bar chart. Panel C: Scenario 2 ratio less than 1 showing combined anion gap plus normal gap acidosis with HCO3- fallen more than AG risen as in DKA plus diarrhea. Panel D: Scenario 3 ratio greater than 2 showing anion gap acidosis plus metabolic alkalosis with HCO3- maintained higher than expected as in lactic acidosis plus vomiting.</image>

IX. Systematic ABG Interpretation

A structured approach to arterial blood gas interpretation prevents errors and ensures detection of mixed disorders. Following a stepwise algorithm allows systematic analysis of even complex acid-base disturbances.

Step 1 assesses the pH to determine whether acidemia (pH below 7.35) or alkalemia (pH above 7.45) exists. Normal pH does not exclude acid-base disturbance, as opposing processes may produce normal pH.

Step 2 identifies the primary disorder by determining whether the carbon dioxide or bicarbonate change explains the pH abnormality. If acidemia exists and carbon dioxide is elevated, respiratory acidosis is present. If acidemia exists and bicarbonate is reduced, metabolic acidosis is present. Similar logic applies to alkalemia with reduced carbon dioxide (respiratory alkalosis) or elevated bicarbonate (metabolic alkalosis).

Step 3 calculates the expected compensation using the formulas for each primary disorder. This calculation establishes the anticipated range for the compensating variable if only a simple disorder exists.

Step 4 compares the measured compensating variable to the expected value. If the measurement falls within the expected range, the disorder is appropriately compensated. If the measurement differs significantly from expected, a mixed disorder exists.

Step 5 calculates the anion gap if metabolic acidosis is present. An elevated gap indicates addition of an unmeasured acid. A normal gap indicates bicarbonate loss or impaired acid excretion.

Step 6 calculates the delta-delta ratio if an elevated anion gap exists. This step detects concurrent normal gap acidosis (ratio below 1) or metabolic alkalosis (ratio above 2).

Step 7 assesses oxygenation. While not directly related to acid-base status, hypoxemia may influence clinical decisions and may contribute to lactic acidosis.

<image>Panel A: ABG interpretation flowchart beginning with pH assessment branching to acidemia, normal, or alkalemia with PCO2 and HCO3- evaluation to identify primary disorder. Panel B: Compensation calculation steps with decision points for appropriate versus inappropriate compensation and anion gap calculation for metabolic acidosis. Panel C: Delta-delta calculation step when gap is elevated with terminal diagnostic boxes. Panel D: Worked example showing pH 7.32 acidemia, HCO3- 12, expected PCO2 26±2, measured PCO2 24, AG 28 elevated, delta-delta 1.3 concluding compensated anion gap metabolic acidosis.</image>

X. Clinical Applications

Several clinical scenarios illustrate the application of acid-base principles to patient care, demonstrating how systematic analysis guides diagnosis and treatment.

Diabetic ketoacidosis produces severe anion gap metabolic acidosis from ketone body accumulation. The arterial blood gas typically shows markedly reduced pH, very low bicarbonate, elevated anion gap from beta-hydroxybutyrate and acetoacetate, and low carbon dioxide from respiratory compensation (Kussmaul breathing). Concurrent lactic acidosis from volume depletion may contribute. Vomiting, common in diabetic ketoacidosis, may add a metabolic alkalosis component, and the delta-delta ratio helps detect this overlap.

Chronic obstructive pulmonary disease with acute exacerbation produces an instructive pattern. At baseline, these patients often have compensated chronic respiratory acidosis with elevated carbon dioxide and proportionally elevated bicarbonate yielding near-normal pH. During exacerbation, carbon dioxide rises acutely but bicarbonate cannot increase quickly enough, producing acute-on-chronic respiratory acidosis with significantly reduced pH. Distinguishing acute from chronic respiratory acidosis using the compensation rules guides treatment intensity.

Salicylate toxicity produces a characteristic mixed disorder that evolves over time. Early in poisoning, salicylate directly stimulates the respiratory center, causing primary respiratory alkalosis. As toxicity progresses, salicylate inhibits oxidative phosphorylation, producing lactic acidosis that adds a metabolic acidosis component. The late picture shows mixed respiratory alkalosis and metabolic acidosis, sometimes with near-normal pH but distinctly abnormal carbon dioxide and bicarbonate.

Renal tubular acidosis represents a group of disorders characterized by normal anion gap metabolic acidosis with distinct features. Type 1 (distal) renal tubular acidosis involves impaired hydrogen ion secretion in the collecting duct, producing urine pH persistently above 5.5 despite systemic acidosis, with low serum potassium. Type 2 (proximal) renal tubular acidosis involves impaired proximal bicarbonate reabsorption; urine pH initially exceeds 5.5 but falls below 5.5 once serum bicarbonate drops below the reduced reabsorptive threshold, also with low serum potassium. Type 4 renal tubular acidosis involves aldosterone deficiency or resistance, producing high serum potassium and urine pH below 5.5.

<image>Panel A: Diabetic ketoacidosis showing very low pH, low HCO3-, high AG, low PCO2 interpreted as anion gap metabolic acidosis with appropriate respiratory compensation. Panel B: COPD exacerbation comparing baseline compensated chronic respiratory acidosis versus acute-on-chronic decompensated pH with compensation calculations. Panel C: Salicylate toxicity timeline showing early respiratory alkalosis evolving to mixed disorder with ABG values at each phase. Panel D: Renal tubular acidosis table comparing types 1, 2, and 4 by defect location, urine pH, serum potassium, and distinguishing features.</image>

Summary

Acid-base homeostasis maintains arterial pH between 7.35 and 7.45 through coordinated buffering, respiratory, and renal mechanisms. The bicarbonate buffer system predominates, with its components independently regulated by the lungs (carbon dioxide) and kidneys (bicarbonate). The Henderson-Hasselbalch equation relates pH to the ratio of bicarbonate to carbon dioxide, explaining how changes in either component affect pH.

Respiratory compensation operates rapidly through ventilation adjustments that change carbon dioxide concentration. Renal compensation develops slowly but has greater capacity, adjusting bicarbonate through reabsorption and regeneration mechanisms.

Four primary disorders exist: respiratory acidosis (elevated carbon dioxide), respiratory alkalosis (reduced carbon dioxide), metabolic acidosis (reduced bicarbonate), and metabolic alkalosis (elevated bicarbonate). Each triggers compensatory responses from the opposite system. Expected compensation can be calculated using established formulas, and comparison of measured to expected values identifies mixed disorders.

The anion gap classifies metabolic acidosis as elevated (MUDPILES: unmeasured acid accumulation) or normal (hyperchloremic: bicarbonate loss or impaired acid excretion). The delta-delta ratio detects concurrent metabolic disorders in the presence of anion gap acidosis.

Systematic ABG interpretation follows a stepwise approach: assess pH, identify primary disorder, calculate expected compensation, compare to measured, calculate anion gap if indicated, and calculate delta-delta if the gap is elevated. This approach reliably identifies simple and mixed disorders.

Key Terms

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