Overview of acid-base balance disorders, compensation mechanisms

Maintenance of acid–base homeostasis is essential for normal cellular function, enzymatic activity, and metabolic processes. The normal arterial blood pH is tightly regulated between 7.35 and 7.45. Acid–base balance is achieved through the integrated function of buffer systems, lungs, and kidneys. Disorders arise when there is an imbalance between acid production and acid elimination or between base loss and base gain.

Physiological Regulation of Acid–Base Balance

Buffer Systems

Buffer systems provide immediate defense against changes in pH.

• Bicarbonate–carbonic acid buffer system (most important)
• Phosphate buffer system
• Protein buffers (hemoglobin and plasma proteins)

The bicarbonate buffer system is governed by the Henderson–Hasselbalch equation and depends on the ratio of bicarbonate (HCO₃⁻) to carbon dioxide (CO₂).

Role of the Lungs

The lungs regulate acid–base balance by controlling the elimination of carbon dioxide, a volatile acid.

• Increased ventilation → ↓ PaCO₂ → ↑ pH
• Decreased ventilation → ↑ PaCO₂ → ↓ pH

Role of the Kidneys

The kidneys maintain long-term acid–base balance by:

• Reabsorbing filtered bicarbonate
• Generating new bicarbonate
• Excreting hydrogen ions as titratable acid and ammonium

Classification of Acid–Base Disorders

Acid–base disorders are broadly classified into metabolic and respiratory disturbances.

Disorder	Primary Change	pH
Metabolic acidosis	↓ HCO₃⁻	↓
Metabolic alkalosis	↑ HCO₃⁻	↑
Respiratory acidosis	↑ PaCO₂	↓
Respiratory alkalosis	↓ PaCO₂	↑

Metabolic Acidosis

A primary decrease in plasma bicarbonate resulting in decreased pH.

Common Causes:

• Increased acid production (lactic acidosis, ketoacidosis)
• Loss of bicarbonate (diarrhea)
• Reduced renal acid excretion (renal failure)

Compensation Mechanism:

• Respiratory compensation via hyperventilation (Kussmaul respiration)
• Decreases PaCO₂ to raise pH

Metabolic Alkalosis

A primary increase in plasma bicarbonate resulting in elevated pH.

Common Causes:

• Loss of hydrogen ions (vomiting, nasogastric suction)
• Excess bicarbonate intake
• Diuretic use

Compensation Mechanism:

• Respiratory compensation via hypoventilation
• Retains CO₂ to lower pH

Limitation:

Compensation is limited by hypoxia, which prevents marked hypoventilation.

Respiratory Acidosis

Definition:

A primary increase in PaCO₂ due to hypoventilation.

Common Causes:

• Chronic obstructive pulmonary disease (COPD)
• Airway obstruction
• Central respiratory depression
• Neuromuscular disorders

Compensation Mechanism:

• Renal compensation by increased bicarbonate reabsorption and hydrogen ion excretion
• More effective in chronic respiratory acidosis

Respiratory Alkalosis

Definition:

A primary decrease in PaCO₂ due to hyperventilation.

Common Causes:

• Anxiety or panic attacks
• High altitude
• Fever
• Pregnancy
• Sepsis (early)

Compensation Mechanism:

• Renal compensation by increased bicarbonate excretion
• Decreased hydrogen ion excretion

Mixed Acid–Base Disorders

Mixed disorders occur when two or more primary acid–base abnormalities coexist.

• Example: Metabolic acidosis with respiratory alkalosis in sepsis
• Compensation mechanisms are insufficient or inappropriate

Recognition requires arterial blood gas analysis and clinical correlation.

Summary of Compensation Mechanisms

Primary Disorder	Compensatory System	Mechanism
Metabolic acidosis	Respiratory	Hyperventilation ↓ PaCO₂
Metabolic alkalosis	Respiratory	Hypoventilation ↑ PaCO₂
Respiratory acidosis	Renal	↑ HCO₃⁻ reabsorption
Respiratory alkalosis	Renal	↓ HCO₃⁻ reabsorption

Key Principles of Compensation

• Compensation never fully normalizes pH
• The body compensates in the opposite direction of the primary disorder
• Respiratory compensation is rapid (minutes–hours)
• Renal compensation is slow (hours–days)