Fig. 1. Schematic representation of the model used to calculate morphine-6-glucuronide (M6G) formation after an intravenous bolus dose of morphine (morphine infusion). V^1–6  represent the three compartments for morphine and M6G; V^m  represents a metabolism compartment; Cl^1–6  represent the clearance from the respective compartments 1–6; and F^m  represents the fraction of morphine clearance responsible for M6G formation. MOR =μ-opioid receptor.