Linear 2compartments, bolus  

Parameters  
\(CL\)  Clearance. Ratio of the drug’s elimination rate from the body over its circulating concentration, [L/h]  
\(V_1\)  Central Volume of distribution. Volume into which the drug distributes initially without delay after its delivery into the circulation, [L]  
\(V_2\)  Peripheral volume of distribution, second compartment. Volume into which a drug is considered to distribute secondly with retardation, from and back to the central compartment, [L]  
\(Q\)  Intercompartmental clearance. Ratio of the drug’s distribution rate between the central compartment and the peripheral compartments over its circulating concentration , [L/h]  
\(k_e\)  \(=\frac{CL}{V_1}\)  
\(k_{12}\)  \(=\frac{Q}{V_1}\)  
\(k_{21}\)  \(=\frac{Q}{V_2}\)  
Equations  
Equation:  \(\begin{align*} \frac{dC_1}{dt} &= k_{21} C_2 – k_{12} C_1 – k_e C_1 \\ \frac{dC_2}{dt} &= k_{12} C_1 k_{21} C_2 \end{align*}\) 

Initial conditions:  \(\begin{align*} C_1(t_0) &= C_{1residual} + \frac{D}{V_1} \\ C_2(t_0) &= C_{2{residual}} \end{align*}\) 
Linear 2compartments, infusion  

Parameters  
\(CL\)  Clearance. Ratio of the drug’s elimination rate from the body over its circulating concentration, [L/h] 
\(V_1\)  Central Volume of distribution. Volume into which the drug distributes initially without delay after its delivery into the circulation, [L] 
\(V_2\)  Peripheral volume of distribution, second compartment. Volume into which a drug is considered to distribute secondly with retardation, from and back to the central compartment, [L] 
\(Q\)  Intercompartmental clearance. Ratio of the drug’s distribution rate between the central compartment and the peripheral compartments over its circulating concentration , [L/h] 
\(T_{inf}\)  Infusion time, [h] 
\(k_e\)  \(=\frac{CL}{V_1}\) 
\(k_{12}\)  \(=\frac{Q}{V_1}\) 
\(k_{21}\)  \(=\frac{Q}{V_2}\) 
\(k_0\)  \(=\frac{D}{T_{inf}}\) 
Equations  
Equation:  \(\begin{align*} \frac{dC_1}{dt} &= \begin{cases} k_{21} C_2 – k_{12} C_1 – k_e C_1 + \frac{k_0}{V_1}, &\text{for $t\leq t_0 + T_{inf}$}\\ k_{21} C_2 – k_{12} C_1 – k_e C_1, &\text{for $t> t_0 + T_{inf}$} \end{cases} \\ \frac{dC_2}{dt} &= k_{12} C_1 k_{21} C_2 \end{align*} \) 
Initial conditions:  \(\begin{align*} C_1(t_0) &= C_{1residual} \\ C_2(t_0) &= C_{2{residual}} \end{align*}\) 
Linear 2compartments, extravascular  

Parameters  
\(CL\)  Clearance. Ratio of the drug’s elimination rate from the body over its circulating concentration, [L/h] 
\(V_1\)  Central Volume of distribution. Volume into which the drug distributes initially without delay after its delivery into the circulation, [L] 
\(V_2\)  Peripheral volume of distribution, second compartment. Volume into which a drug is considered to distribute secondly with retardation, from and back to the central compartment, [L] 
\(Q\)  Intercompartmental clearance. Ratio of the drug’s distribution rate between the central compartment and the peripheral compartments over its circulating concentration , [L/h] 
\(T_{inf}\)  Infusion time, [h] 
\(k_a\)  Absorption rate constant. Relative rate constant of the drug’s absorption into the body, [h⁻¹] 
\(F\)  Bioavailability. Fraction of the drug’s administered dose that reaches unchanged the systemic circulation, [%] 
\(k_e\)  \(=\frac{CL}{V}\) 
\(k_{12}\)  \(=\frac{Q}{V_1}\) 
\(k_{21}\)  \(=\frac{Q}{V_2}\) 
Equations  
Equation:  \(\begin{align*} \frac{dC_1}{dt} &= k_{21} C_2 – k_{12} C_1 + k_a C_3 – k_e C_1 \\ \frac{dC_2}{dt} &= k_{12} C_1 k_{21} C_2 \\ \frac{dC_3}{dt} &= k_{a} C_3 \end{align*} \) 
Initial conditions:  \( \begin{align*} C_1(t_0) &= C_{1residual} \\ C_2(t_0) &= C_{2residual} \\ C_3(t_0) &= C_{3residual} + \frac{F D}{V_1} \end{align*}\) 