In this model one imagines that a hormone is released in general circulation and can potentially come into contact with all body tissues, but interacts physiologically only with those cells that possess receptors specific for the given hormone. Hormone receptors, often called receptor sites, are mainly large proteins. These proteins confer specificity of hormone recognition upon individual cells within tissues. This is a slightly more complicated area that depends upon a hormone's structure. Here, we will limit our discussion to peptide hormones and a word or two about prostagladins.
Peptides mostly adjust or regulate metabolic events.
An interesting feature of peptides is that they are not able to permeate cell membranes. The hormone itself can not move from extracellular to intracellular compartments. The first issue in the physiology of peptide hormone action is that their specific receptor proteins are located on the outside of target cell membranes. Turning attention to the figure, the transduction process begins when a blood-borne hormone interacts with a specific receptor site. In the example, a hormone interacts with a specific receptor.