Each hormone has a unique chemical structure that also gives it a unique pattern of electrical charges. The theory of hormone action is that on a target cell’s surface are receptor sites, in the form of a molecule-sized pit or slot, into which one hormone fits like a key in a lock. Once inserted, the result is a change in the cell’s function – it takes in more glucose, or releases some of its contents, or makes more of a certain product. The theory explains specificity of: for example, nearly all body possess receptors for thyroxine (a thyroid hormone) but only kidney have receptors for ADH.
A number of chemical substances are secreted by some cells of the body that have an effect only on cells in their close vicinity. These substances are sometimes known as local or tissue hormones. Examples of local hormones are gastrin, histamine, serotonin and prostaglandins.
Gastrin is secreted by certain cells in the lower part of the stomach wall in response to food intake. It stimulates glands in the stomach to secrete hydrochloric acid, which facilitates food digestion. Histamine and serotonin are released by specialized cells – present in nearly all theof the body – in response to local tissue damage such as a burn or an infection. These chemical substances cause the small vessels to widen and leak fluid and proteins into the surrounding, damaged tissue, to speed healing. These changes account for the inflammation and swelling seen after tissue damage. Histamine release is also responsible for some allergic reactions, including hay fever and urticaria or nettle . Prostaglandins are also known to be widespread in the body. They are released into nearby tissue fluids and have a variety of local effects, including widening (or even, sometimes, narrowing) of vessels, slowing down blood clotting, induction of uterine contractions in labour, physiological changes in the ovary, and relaxation of the bronchial muscles in the lungs. They are released in inflamed and may mimic the action of other hormones. Their precise role in the body is still under investigation.