BPS3082 Chap.3 Compartmental Pharmacokinetic Models
Compartmental Pharmacokinetic Models
Compartment models summarise kinetic behaviour rather than anatomical rooms. A one-compartment model treats distribution as effectively instantaneous, while a two-compartment model separates a rapidly distributing central space from a slower peripheral space.
This chapter teaches students to read mono- and bi-exponential concentration decline, connect parameters to movement, account for administration route, and check whether added complexity is supported by residuals and parameter estimates. After an IV bolus, immediate input makes distribution phases easier to see.
A straight semi-log decline supports one exponential phase across the sampled window, while a steep early segment followed by a shallower terminal segment supports two-phase behaviour. The early phase in a two-compartment model represents distribution plus elimination, not simply faster elimination.
Central volume links central amount to measured plasma concentration, clearance removes drug, and intercompartmental exchange connects the spaces. Extravascular absorption can hide early disposition structure, so route must be named before compartments are counted. Sampling also limits what can be identified: early observations inform rapid distribution, while late observations define the terminal phase.
Model choice therefore combines visual signature, candidate fits, residual patterns, parameter precision and purpose rather than rewarding maximum complexity.
What this chapter covers
- 01
Central and peripheral spaces
- 02
IV bolus profile phases
- 03
Mono-exponential decline
- 04
Bi-exponential decline
- 05
Clearance and volumes
- 06
Model identifiability
- 07
Route effects on visible profile phases
- 08
Early sampling for rapid distribution
- 09
Late sampling for terminal behaviour
- 10
Parameter uncertainty in downstream simulation
Worked example · free
Choose a model from an IV profile
- +1Immediate IV input means an absorption phase does not explain the early decline.
- +2Two distinct exponential phases support a two-compartment starting model.
- +2Link the early phase to distribution plus elimination and the late phase to terminal return and elimination.
- +1Confirm with candidate fits, residuals and credible parameter estimates.
Key terms
- Central Compartment
- The rapidly distributing model space that contains the sampled plasma.
- Peripheral Compartment
- A slower model space exchanging drug with the central compartment.
- Clearance
- The model term relating concentration to elimination rate under linear conditions.
- Terminal Phase
- The later portion of a concentration-time profile defining the final exponential decline.
- Identifiability
- The extent to which available data support distinct and precise parameter estimates.
Compartmental Pharmacokinetic Models FAQ
Does a two-compartment model mean two organs?
No. Compartments group tissues with similar kinetic behaviour. The central and peripheral spaces are mathematical representations, and their number is inferred from the concentration-time profile rather than counted from anatomy. A model can represent many tissues with only a few kinetic spaces.
Why is IV bolus useful for seeing compartments?
IV bolus provides immediate input and removes absorption from the early profile. Distinct distribution and terminal phases can therefore be easier to observe on a semi-log concentration-time plot. Adequate early sampling is still required to capture rapid distribution.
Can oral data look simpler than IV data?
Yes. Absorption can blur an early distribution phase, so an extravascular profile may appear one-compartment even when IV data show two-compartment disposition. Route and sampling must be considered together. Do not reinterpret absorption as an extra disposition compartment.
Assessment move
Draw the compartment diagram beside every concentration-time profile. Label dosing, elimination and reversible exchange, then narrate which movement dominates each visible phase. Practise distinguishing a straight semi-log decline from a two-phase curve, but always add a sampling caveat. Create a parameter map linking central volume, peripheral volume, clearance and intercompartmental exchange to a space or arrow.
Finish candidate-model answers with residual behaviour, parameter credibility and a reason the extra compartment is or is not justified. Compare the same hypothetical drug after IV and oral administration, noting which input features can mask the distribution phase. Mark the earliest and latest reliable samples and explain which parameters lose information if either region is missing.
Practise converting verbal descriptions into diagrams and diagrams into short prose. Keep a list of prohibited shortcuts, including treating compartments as organs, calling the early phase only elimination and selecting the most complicated model automatically. End each revision problem with a sentence about what the available sampling cannot resolve.
Build a route-comparison table for IV bolus, infusion and extravascular input, separating what is known about administration from what must be inferred about disposition. For each profile, sketch the expected semi-log view and mark where early or late sampling carries information. Then challenge the preferred model by removing the earliest sample, adding plausible measurement noise or shortening follow-up.
Explain whether the apparent phase count changes and why model identity should remain conditional on observation design. Finish by checking parameter signs, units and correlations before accepting a visually attractive fit.
