Acid–Base Balance – Study Notes Normal Blood pH

 Acid–Base Balance – Study Notes

Normal Blood pH

• Normal range: 7.35 – 7.45

• <7.35 = Acidosis

• >7.45 = Alkalosis

• pH measures H⁺ (hydrogen ion) concentration.

Key Chemical Equation

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

• CO₂ = Respiratory component (lungs).

• HCO₃⁻ (bicarbonate) = Metabolic component (kidneys).

• H⁺ = Determines pH.

Seesaw Concept:

• ↑ CO₂ → ↑ H⁺ → ↓ pH (acidosis).

• ↓ CO₂ → ↓ H⁺ → ↑ pH (alkalosis).

Respiratory Disorders

1. Respiratory Acidosis

   • Cause: Hypoventilation → CO₂ retention.

   • Mechanism: ↑ CO₂ → ↑ H⁺ → ↓ pH.

   • Example: COPD, respiratory depression.

2. Respiratory Alkalosis

   • Cause: Hyperventilation → CO₂ loss.

   • Mechanism: ↓ CO₂ → ↓ H⁺ → ↑ pH.

   • Example: Anxiety, early sepsis, high altitude.

Metabolic Disorders

1. Metabolic Acidosis

   • Causes:

     • ↑ H⁺ (e.g., lactic acidosis, ketoacidosis).

     • ↓ HCO₃⁻ (e.g., diarrhea, renal failure).

   • Lab: ↓ HCO₃⁻, ↓ pH.

2. Metabolic Alkalosis

   • Causes:

     • ↑ HCO₃⁻ (e.g., vomiting, excess antacids).

     • ↓ H⁺ (e.g., diuretic use, hypokalemia).

   • Lab: ↑ HCO₃⁻, ↑ pH.

Compensation

• Respiratory system compensates quickly by changing ventilation.

• Metabolic (renal) system compensates slowly by adjusting HCO₃⁻ reabsorption or H⁺ excretion.

• Compensation attempts to return pH toward normal, but the primary disorder is identified by the initial abnormality in CO₂ or HCO₃⁻.

Quick Diagnostic Guide

1. Check pH:

   • <7.35 = Acidosis

   • 7.45 = Alkalosis

2. Check PaCO₂ (respiratory):

   • ↑ CO₂ = Acidosis

   • ↓ CO₂ = Alkalosis

3. Check HCO₃⁻ (metabolic):

   • ↓ HCO₃⁻ = Acidosis

   • ↑ HCO₃⁻ = Alkalosis

4. Determine if compensation is present.

Summary Table

Disorder	pH	CO₂	HCO₃⁻	Common Causes
Respiratory Acidosis	↓	↑	Normal/↑	COPD, hypoventilation, drug OD
Respiratory Alkalosis	↑	↓	Normal/↓	Anxiety, hyperventilation, altitude
Metabolic Acidosis	↓	Normal/↓	↓	DKA, lactic acidosis, renal failure
Metabolic Alkalosis	↑	Normal/↑	↑	Vomiting, diuretics, antacid excess