The drugs that enter an individual tend to stimulate certain receptors, ion channels, act on enzymes or transport proteins.
Drugs interact with receptors by bonding at specific binding sites.
These interactions are very basic, just like that of other chemical bondings: Mainly occur through attractions between opposite charges; for example, between protonated amino (on salbutamol) or quaternary ammonium (e.g. acetylcholine), and the dissociated carboxylic acid group.
These so-called hydrogen bonds only act over short distances, and are dependent on the correct alignment between functional groups.
Potency is a measure of how much a drug is required in order to produce a particular effect.
Therefore, only a small dosage of a high potency drug is required to induce a large response.
The affinity is a measure of how tightly a drug binds to the receptor.
But the value of KD is also affected by the conformation, bonding and size of the drug and the receptor.