# Linear 2-compartments

Linear 2-compartments, 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]
$$Q$$ Inter-compartmental 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 2-compartments, 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]
$$Q$$ Inter-compartmental 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 2-compartments, 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]
$$Q$$ Inter-compartmental 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*}