Evolution of plasma concentrations of a drug after a single administration: One compartment model, Intravenous administration:

After administration of a drug by intravenous injection of short duration, as a bolus, its plasma concentration is immediately maximal. It then decreases according to time. When one has a simple exponential decay, i.e. linear in semi-logarithmic scale, the elimination and the inactivation are simply dependant on the concentration. The variation of the plasma concentration is represented by the following equation:

C = C0.e -KelT

in which C represents the concentration at a given time , C0, initial concentration, - Kel, the constant of apparent elimination and T, time.

Evolution of the plasma concentrations of a drug injected by intravenous route (1 compartment)

C = Concentration of the drug at the time considered C = C0.e -KelT	1 = Absorption (injection)
C0 = Initial concentration	2 = Biotransformation
Kel = Elimination constant	3 = Elimination

One compartment model, oral administration

After a single administration of a drug by oral route, also called “per os”, its plasma concentration according to time increases, reaches a maximum (Cmax), then decreases exponentially. While the concentration increases, the quantity of drug arriving into the blood is higher than that is eliminated and metabolized. At the equilibrium, i.e. at Cmax, they are equal and thereafter the elimination and the biotransformations predominate.

Evolution of the plasma concentrations of a drug taken by oral route (1 compartment)

Increasing curve, 0 to Cmax, 1> 2 + 3	1 = Absorption (injection)
Cmax, 1 = 2 + 3	2 = Biotransformations
Decreasing curve, 1 < 2 + 3	3 = Elimination

Two compartment model, Intravenous administration

After intravenous administration of a drug by an injection of short duration, in the two compartment model, the plasma concentration (measured in compartment 1, i.e. blood or plasma) decreases initially

quickly (phase alpha) and then more slowly (phase beta). The fast decrease (phase alpha) corresponds to the elimination of the drug and its transfer from compartment 1 into compartment 2. When the equilibrium between compartments 1 and 2 is reached, elimination alone is responsible for the decrease, but this decrease is slower because there is a greater volume to clean. Slope A corresponds to the constant of elimination to which the diffusion into the second compartment is added. The slope B corresponds to elimination alone.

Increasing curve, 0 to Cmax, 1> 2 + 3 + 4	1 = Absorption (injection)
Cmax, 1 = 2 + 3 + 4	2 = Biotransformations
Decreasing curve, 1 < 2 + 3 + 4	3 = Elimination
Slope alpha, 2 + 3 + 4	4 = Transfer from C1 to C2
Slope beta, 2 + 3	5 = Transfer from C2 to C1