区画モデル：単一区画モデル

The single-compartment model represents the human body as a single well-stirred open compartment where drugs distribute instantaneously without barriers. When administered intravenously, a drug rapidly distributes uniformly throughout this compartment and then undergoes biotransformation and elimination. This simplified model assumes uniform drug concentration across the body at any given time.

Drug clearance from the body follows first-order kinetics, meaning the elimination rate is directly proportional to the drug concentration present. As concentration decreases, the elimination rate also decreases proportionally. This results in exponential decline of both plasma and tissue drug concentrations over time, following drug elimination the concept of clearance.

The apparent volume of distribution is a theoretical volume into which an administered drug dose can rapidly and uniformly distribute within the single-compartment model. It represents the relationship between the amount of drug in the body and its plasma concentration. This volume helps predict how extensively a drug distributes throughout the body.

During continuous infusion, drug concentration rises exponentially until reaching a steady-state value where the infusion rate equals the elimination rate. Once steady state is achieved, plasma concentration remains constant. If infusion stops, the concentration falls exponentially following first-order kinetics throughout the body.

When a drug is administered in multiple equal doses, the mean steady-state concentration and time required to reach steady state are similar to those achieved with continuous infusion. Both dosing approaches result in comparable pharmacokinetic profiles, though multiple dosing creates fluctuations around the mean concentration value.

The single-compartment model assumes the body functions as one well-mixed compartment with instantaneous, uniform drug distribution and no physiological barriers. In reality, drugs distribute at different rates into various tissues and organs. This simplification makes calculations manageable for basic pharmacokinetic predictions while acknowledging limitations.

After intravenous bolus administration, the drug distributes instantaneously and uniformly throughout the compartment. Plasma concentration then declines exponentially as the drug undergoes biotransformation and elimination. The tissue drug concentration follows a similar exponential decline pattern proportional to the plasma concentration changes.