 Hormonal control systems are found in all animals, the complexity has increased throughout evolution.  These are often the response section in a negative feedback control mechanism.  Hormones modify the activity of specific cells due to a interaction with highly specific receptors.  Hormones are involved in controlling many metabolic functions, including rates of rxns, transport of substances across plasma membranes, secretion and growth of cells.

 Hormone does not refer to a particular type of chemical compound, but a diverse group of compounds that act as intercellular messengers and regulate cell function.

 In animals hormones are produced by cells in one part of the organism and usually pass throughout the organism in internal transport systems (blood).  They transmit their signal to target cells by altering specific biochemical reactions in those cells. Target cells are the cells that have the specific receptor for that hormone.  Hormones are effective in low concentrations.

 A particular stimulus will only affect a specific group of hormone-secreting cells.  Only those cells that possess the specific receptor are capable of responding to the hormone.  While a few hormones affect most cells in the body, most hormones only affect a specific organ and even in that organ only a small number of cells.

 Hormones are generally slower than nervous responses, longer in duration and often affect cells widely distributed throughout the body.  This is because hormones are typically transported throughout the system in the blood to reach their target cell.

 There are two main groups those synthesized from fatty acids and those made from amino acids.  Hormones from fatty acids are able to pass into the cell through the cell membrane as they are small lipid-soluble molecules.  Hormones from amino acids act on specific receptors in the plasma membrane which triggers a specific receptor.

 Animals usually have cells whose main function it is to produce hormones. In more complex animals (vertebrates, insects, crustaceans, and some molluscs) they are usually clustered together into specific organs called endocrine glands.

 The pituitary gland has a central roll in overall endocrine regulation. More than half the hormones in mammals are either released from the pituitary gland or used to regulate the release of hormones from the pituitary gland.  The pituitary gland hormones are responsible for regulation of growth, lactation, reproductive state, skin pigmentation, fat tissue, kidney function, and the activity of the thyroid and adrenal glands.

 The pituitary gland lies just below the hypothalamus, which is the collecting centre for information relating to the body’s state of well-being. (satiety, smell, pain, and emotions).  This information is used to regulate secretion from the pituitary gland.  The hypothalamus produces releasing hormones which control secretion of some hormones in the pituitary gland.