Henderson-Hasselbalch equation

Acid-Base Basics - The pH Party

• Acid: A proton ($H⁺$) donor.
• Base: A proton ($H⁺$) acceptor.
• pH: The measure of acidity, defined as $pH = -log[H⁺]$.
  • Normal arterial blood pH: 7.35-7.45.
• Buffer System: A solution that resists pH change by neutralizing added acid or base. The primary buffer in the ECF is the bicarbonate system ($H₂CO₃/HCO₃⁻$).

⭐ A pH change of 0.3 units corresponds to a 2x change in $[H⁺]$.

The Main Event - H-H Equation

• The Equation: A fundamental formula used to calculate the pH of a buffer solution, linking pH to the ratio of the buffer system's components.

  • General Form: $pH = pKa + log([A⁻]/[HA])$
  • Bicarbonate System: $pH = 6.1 + log([HCO₃⁻]/(0.03 \times PCO₂))$

• Components Defined:

  • pKa: Acid dissociation constant; 6.1 for the bicarbonate system.
  • [A⁻]: Conjugate base (bicarbonate, HCO₃⁻).
  • [HA]: Weak acid (carbonic acid, proportional to PCO₂).

• Primary Use: Crucial for calculating the pH of blood and understanding how the bicarbonate buffer system counteracts changes in acidity.

⭐ High-Yield: The body maintains a [HCO₃⁻] to (0.03 \times PCO₂) ratio of approximately 20:1 to keep blood pH around 7.4. When [A⁻] = [HA], pH = pKa, which represents the point of maximum buffering capacity.

Body's Buffer - Bicarb Brigade

• The most crucial extracellular buffer for maintaining blood pH within the narrow range of 7.35-7.45.

• Henderson-Hasselbalch Equation (Physiological):

  • $pH = 6.1 + log([HCO₃⁻]/(0.03 * PCO₂))$
  • This equation links pH to the ratio of the metabolic base ($[HCO₃⁻]$) and the respiratory acid ($PCO₂$).

• Organ Regulation:

  • Lungs (Fast): Regulate $PCO₂$ via ventilation. Changes occur in minutes.
  • Kidneys (Slow): Regulate $[HCO₃⁻]$ by reabsorbing or excreting it. Takes hours to days.

• 📌 Mnemonic: ROME
  • Respiratory Opposite: pH and $PCO₂$ move in opposite directions.
  • Metabolic Equal: pH and $[HCO₃⁻]$ move in the same direction.

⭐ The pKa of the bicarbonate system is 6.1, far from physiological pH (7.4). Its power lies in the independent and tight regulation of both $PCO₂$ (lungs) and $[HCO₃⁻]$ (kidneys).

Clinical Correlation - pH-inding Problems

• The Henderson-Hasselbalch equation is the key to interpreting arterial blood gas (ABG) results. Normal values are critical:

  • pH: 7.35-7.45
  • $PCO₂$: 35-45 mmHg
  • $[HCO₃⁻]$: 22-26 mEq/L

• Primary acid-base disorders are defined by the initial change in $PCO₂$ (respiratory) or $[HCO₃⁻]$ (metabolic).

• The Henderson-Hasselbalch equation is the key to interpreting arterial blood gas (ABG) results. Normal values are critical:

  • pH: 7.35-7.45
  • $PCO₂$: 35-45 mmHg
  • $[HCO₃⁻]$: 22-26 mEq/L

• Primary acid-base disorders are defined by the initial change in $PCO₂$ (respiratory) or $[HCO₃⁻]$ (metabolic).

| ↓ | ↓ $PCO₂## Clinical Correlation - pH-inding Problems

• The Henderson-Hasselbalch equation is the key to interpreting arterial blood gas (ABG) results. Normal values are critical:

  • pH: 7.35-7.45
  • $PCO₂$: 35-45 mmHg
  • $[HCO₃⁻]$: 22-26 mEq/L

• Primary acid-base disorders are defined by the initial change in $PCO₂$ (respiratory) or $[HCO₃⁻]$ (metabolic).

| | Metabolic Alkalosis | ↑ $[HCO₃⁻]## Clinical Correlation - pH-inding Problems

• The Henderson-Hasselbalch equation is the key to interpreting arterial blood gas (ABG) results. Normal values are critical:

  • pH: 7.35-7.45
  • $PCO₂$: 35-45 mmHg
  • $[HCO₃⁻]$: 22-26 mEq/L

• Primary acid-base disorders are defined by the initial change in $PCO₂$ (respiratory) or $[HCO₃⁻]$ (metabolic).

| ↑ | ↑ $PCO₂## Clinical Correlation - pH-inding Problems

• The Henderson-Hasselbalch equation is the key to interpreting arterial blood gas (ABG) results. Normal values are critical:

  • pH: 7.35-7.45
  • $PCO₂$: 35-45 mmHg
  • $[HCO₃⁻]$: 22-26 mEq/L

• Primary acid-base disorders are defined by the initial change in $PCO₂$ (respiratory) or $[HCO₃⁻]$ (metabolic).

| | Respiratory Acidosis | ↑ $PCO₂## Clinical Correlation - pH-inding Problems

• The Henderson-Hasselbalch equation is the key to interpreting arterial blood gas (ABG) results. Normal values are critical:

  • pH: 7.35-7.45
  • $PCO₂$: 35-45 mmHg
  • $[HCO₃⁻]$: 22-26 mEq/L

• Primary acid-base disorders are defined by the initial change in $PCO₂$ (respiratory) or $[HCO₃⁻]$ (metabolic).

| ↓ | ↑ $[HCO₃⁻]## Clinical Correlation - pH-inding Problems

• The Henderson-Hasselbalch equation is the key to interpreting arterial blood gas (ABG) results. Normal values are critical:

  • pH: 7.35-7.45
  • $PCO₂$: 35-45 mmHg
  • $[HCO₃⁻]$: 22-26 mEq/L

• Primary acid-base disorders are defined by the initial change in $PCO₂$ (respiratory) or $[HCO₃⁻]$ (metabolic).

| | Respiratory Alkalosis | ↓ $PCO₂## Clinical Correlation - pH-inding Problems

• The Henderson-Hasselbalch equation is the key to interpreting arterial blood gas (ABG) results. Normal values are critical:

  • pH: 7.35-7.45
  • $PCO₂$: 35-45 mmHg
  • $[HCO₃⁻]$: 22-26 mEq/L

• Primary acid-base disorders are defined by the initial change in $PCO₂$ (respiratory) or $[HCO₃⁻]$ (metabolic).

| ↑ | ↓ $[HCO₃⁻]## Clinical Correlation - pH-inding Problems

• The Henderson-Hasselbalch equation is the key to interpreting arterial blood gas (ABG) results. Normal values are critical:

  • pH: 7.35-7.45
  • $PCO₂$: 35-45 mmHg
  • $[HCO₃⁻]$: 22-26 mEq/L

• Primary acid-base disorders are defined by the initial change in $PCO₂$ (respiratory) or $[HCO₃⁻]$ (metabolic).

|

⭐ In metabolic acidosis, always calculate the anion gap ($Na⁺ - (Cl⁻ + HCO₃⁻)$). A high anion gap (HAGMA) indicates the addition of an unmeasured acid (e.g., MUDPILES).

High‑Yield Points - ⚡ Biggest Takeaways

• The Henderson-Hasselbalch equation links pH, pKa, and the bicarbonate buffer system.
• It calculates blood pH using the ratio of bicarbonate [HCO₃⁻] (metabolic component) to dissolved CO₂ (respiratory component).
• The key is the ratio of base to acid, not their absolute values.
• A 20:1 ratio of [HCO₃⁻] to dissolved PCO₂ maintains a normal blood pH of 7.4.
• Metabolic acidosis/alkalosis primarily alters [HCO₃⁻].
• Respiratory acidosis/alkalosis primarily alters PCO₂.