How can a hormone carry a message through the whole body?

A hormone is a chemical messenger released by cells into body fluids, often into the bloodstream. Blood can distribute it widely. That does not mean every cell responds in the same way: a cell needs the appropriate receptor and working signaling machinery.

A receptor is a protein that recognizes a particular chemical signal. In the illustrated example, a water-soluble hormone binds to a receptor at the cell surface. Binding changes the receptor’s activity and starts events inside the cell. These can alter enzyme activity, transport or the production of other substances. The hormone does not have to enter the cell to pass on its message.

Other hormones, including steroid hormones, can act through receptors inside cells. Receptors are not literal keyholes; recognition depends on molecular shape and chemical interactions. A matching receptor also does not guarantee that all cell types give the same response, because their internal machinery differs.

Hormone amounts and receptor activity are regulated, often through feedback. This allows organs far apart to coordinate processes such as growth and energy use. The simple matching-symbol drawing explains selective communication, not a prediction of the strength or speed of a hormonal response.