Table 1.

Some of the functions used to describe the unit impulse function.

Parameters	One compartment	Two compartments
V: apparent volume of distribution ke: first order elimination rate constant ka: first order absorption rate constant k12: rate constant for transfer from central to peripheral compartment k21: rate constant for transfer from peripheral to central compartment	·Single compartment = body ·Even distribution of drug throughout body ·Only applicable for rapidly distributed drug	·Central compartment (drug in and out) + peripheral compartment (drug distribution) ·Slow equilibration between compartments
Without absorption ·Very rapidly absorbed drug (almost ideal)	$r\left(t\right)=\frac{1}{V}{e}^{(-k_{e}t)}$	$r\left(t\right)=\frac{k_a}{V(k_a-k_e)}[e^{(-k_{e}t)}-e^{(-k_{a}t)}]$
With absorption ·Commonly applicable ·Severely influenced by physicochemical, biopharmaceutical, and physiological factors	$r\left(t\right)=\frac{k_a}{V}[\frac{k_{21}-k_a}{({\lambda }_1-k_a)({\lambda }_2-k_a)}{e}^{(-k_{a}t)}+\frac{k_{21}-{\lambda }_1}{(k_a-{\lambda }_1)({\lambda }_2-{\lambda }_1)}{e}^{(-{\lambda }_{1}t)}+\frac{k_{21}-{\lambda }_2}{(k_a-{\lambda }_2)({\lambda }_1-{\lambda }_2)}{e}^{(-{\lambda }_{2}t)}]$	$r\left(t\right)=\frac{A}{V}{e}^{(-{\lambda }_{1}t)}+\frac{1-A}{V}{e}^{(-{\lambda }_{2}t)}$

λ₁+λ₂ = k_e+ k₁₂+ k₂₁; λ₁ λ₂ = k_e k₂₁; A = (k₂₁ − λ₁)/(λ₂ − λ₁).