Learning Objective: By the end of this lesson, the learner will be able to identify major extracellular solutes, interpret their normal ranges in a basic metabolic panel (BMP), and explain the concept and clinical relevance of the osmolar gap.
Extracellular Solutes
A Basic Metabolic Panel (BMP) provides key laboratory values for major extracellular solutes. These solutes maintain osmotic balance, nerve and muscle function, acid–base status, and metabolic homeostasis. Although memorization is not required for exams (values are provided), knowing ranges improves test speed and clinical reasoning.
Normal Ranges of Major Extracellular Solutes
| Solute | Normal Range | Notes |
|---|---|---|
| Na+ | 136–145 mEq/L | Major determinant of ECF osmolality |
| K+ | 3.5–5.0 mEq/L | Small changes → large cardiac effects |
| Cl– | 100–106 mEq/L | Varies by lab |
| HCO₃– | 22–26 mEq/L | Key metabolic component of acid–base balance |
| BUN | 8–25 mg/dL | Reflects renal perfusion & protein breakdown |
| Creatinine | 0.8–1.2 mg/dL | Indicator of GFR |
| Glucose | 70–100 mg/dL | Fasting value |
Activity
Osmolar Gap
The osmolar gap is the difference between: Measured Osmolality – Calculated Osmolality
Calculated osmolality ≈ 2 × Na⁺ + (Glucose / 18) + (BUN / 2.8)
A normal osmolar gap is ≤ 10 mOsm/kg. Increased osmolar gap occurs in:
- Ethylene glycol poisoning
- Methanol poisoning
- Propylene glycol
- Mannitol therapy
- Ketoacidosis (mild ↑)
When the osmolar gap is elevated, think of toxic alcohol ingestion.
