Variables, constants and parameters

Another fundamental mathematical concept important in pharmacokinetics is the difference between a variable and a constant. For the purposes of pharmacokinetics, a variable is something that changes over time. Conversely, a constant is time invariant. Box 2.1 presents some examples of variables and constants as well as rules showing whether an expression containing variables and/ or constants will give rise to a variable or a constant. 

Cp varies os a function of time. (There is one exception where o combination of intravenous bolus and intravenous infusion can result in constant Cp over time.) 

The first order elimination rate constant [K] is a constant, as the name implies. 

An expression containing nothing but constants yields a constant  

An expression containing a single variable yields o variable  

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Theories are not used directly, as in the discussion presented in Sect. 3.1, but allow building a mathematical model that describes an experiment in the unambiguous language of mathematics, in terms of variables, constants, and parameters. As an example, when considering the identification of kinetic parameters of chemical reactions from isothermal experiments performed in batch reactors, the relevant equations of mass conservation (presented in Sect. 2.3.1) give a set of ordinary differential equations in the general form... 

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