Questions
2 questions per B.Pharm exam
Difficulty
Medium
Importance
High yield for B.Pharm University exams
Overview
Compartment models are mathematical frameworks used in pharmacokinetics to represent the body as a system of interconnected compartments where drugs distribute and eliminate. Mastering these models is essential for predicting drug concentration time profiles and dosage regimens in clinical practice. Students must grasp the distinction between kinetic homogeneity and actual physiological reality.
Principles of Compartmental Modeling
Compartmental modeling simplifies the complex physiological body into distinct, kinetic units where drug distribution occurs at different rates. These models use differential equations to describe the rate of change of drug concentration over time.
- Compartments represent kinetic pools, not necessarily anatomical regions
- Open systems permit drug input and exit
- Kinetics often follow first-order processes
- Rate constants denote fractional drug transfer per unit time
- Assumes instantaneous distribution within a compartment
One-Compartment Model
The one-compartment open model assumes the entire body acts as a single, uniform compartment where drug distribution is instantaneous. It is the simplest model, effectively describing the kinetics of many drugs that distribute rapidly into the plasma and extracellular fluids.
- Cp = C0 * e^(-k*t)
- Apparent volume of distribution (Vd) is constant
- Elimination follows first-order kinetics
- Plasma concentration declines mono-exponentially
- Applicable for rapid equilibrium drugs
Two-Compartment Model
The two-compartment model acknowledges that drugs distribute at different rates into different tissues, characterized by a central compartment (blood/highly perfused organs) and a peripheral compartment (less perfused tissues). This model is necessary for drugs that show a biphasic decline in plasma concentration.
- Cp = A * e^(-alpha*t) + B * e^(-beta*t)
- Alpha phase represents rapid distribution
- Beta phase represents slower elimination
- Includes micro-rate constants k12, k21, and k10
- Central and peripheral equilibrium is not instantaneous
Formula Sheet
One-compartment: Cp = C0 * e^(-k*t)
Two-compartment: Cp = A * e^(-alpha*t) + B * e^(-beta*t)
Vd = Dose / C0
Exam Tip
Always state whether your calculation assumes a central or peripheral compartment first, as the volume of distribution and rate constant definitions vary significantly between them.
Common Mistakes
- Confusing the physiological reality of organs with mathematical kinetic compartments
- Assuming zero-order kinetics when most drugs in these models follow first-order processes
- Failing to distinguish between the distribution phase (alpha) and elimination phase (beta) in two-compartment calculations
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