= Pharmacodynamics
<Pharmacodynamics> describes how a <drug> changes a biological system as a function of exposure. It connects target binding to a measured response, whereas <pharmacokinetics> describes the time course of exposure. A simple illustrative model is
$$
E(C)=E_0+\frac{E_{max}C}{EC_{50}+C},
$$
where $E_{max}$ is the maximum increment above baseline and $EC_{50}$ is the concentration producing half that increment. Doubling $C$ does not generally double the response. <Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[Chapter 15, Cells Can Adjust Their Sensitivity to a Signal: receptor occupancy and cellular response]{ref}
Potency and maximal effect differ: two <drugs> can have different $EC_{50}$ values but the same $E_{max}$, or the same $EC_{50}$ but different maximal effects. Moreover, $EC_{50}$ need not equal a binding <dissociation constant>, because downstream amplification and measurement time also shape the response.
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