Endocrine System

Most endocrine hormones are maintained at specific concentrations in the plasma, the non-cellular, liquid portion of the blood. Receptors at set locations monitor plasma hormonal levels and inform the gland responsible for producing that hormone if levels are too high or too low for a particular time of day, month, or other life period. When excess hormone is present, a negative feedback loop is initiated such that further hormone production is inhibited. Most hormones have this type of regulatory control. However, a few hormones operate on a positive feedback cycle such that high levels of the particular hormone will activate release of another hormone. With this type of feedback loop, the end result is usually that the second hormone released will eventually decrease the initial hormone's secretion. An example of positive feedback regulation occurs in the female menstrual cycle, where high levels of estrogen stimulate release of the pituitary hormone, luteinizing hormone (LH).

All hormones are influenced by numerous factors. The hypothalamus can release inhibitory or stimulatory hormones the determine pituitary function. And every physiological component that enters the circulation can effect some endocrine function. Overall, this system uses multiple bits of chemical information to hormonally maintain a biochemically balanced organism.

Endocrine hormones do not fall into any one chemical class, but most are either a protein (polypeptides, peptides, and glycoproteins are also included in this category) or steroids. Protein hormones bind cell-surface receptors and activate intracellular events that carry out the hormone's response. Steroid hormones, on the other hand, usually travel directly into the cell and bind a receptor in the cell's cytoplasm or nucleus. From there, steroid hormones (bound to their receptors) interact directly with genes in the DNA to elicit a hormonal response.