The endocrine system refers to organs including endocrine and exocrine glands which produce and secrete hormones, the chemicals necessary for the regulation of body processes. Hormones, enzymes, and metabolites are secreted by the endocrine system to prompt changes in specific tissues. Types of endocrine system organs include the pituitary gland, the hypothalamus, and glands involved in reproduction such as the ovaries and testes.
Exocrine glands, like the sweat glands and salivary glands, differ from endocrine glands in that they secrete into ducts. Endocrine glands do not have any ducts and hormones are secreted directly into the blood or interstitial fluid and dispersed into the capillaries, very tiny blood vessels connecting arteries to veins important for transporting nutrients and oxygen. In addition, cells with specialized functions are present in several other tissues and organs such as the heart and gastrointestinal system, which also secrete hormones.
A chemically-varied group, hormones may fall into one of three categories including the steroids, monoamines, and peptides. Stimulated by a negative feedback mechanism, hormone secretion occurs in response to a particular change, which then initiates a counteracting or opposite action. Homeostasis is a balanced internal state, which is the primary purpose of hormone function.
Frequently referred to as the "master gland," the pituitary gland is the principle controller and regulator of other endocrine system organs. Located at the base portion of the brain and inferior to the hypothalamus, the pituitary gland functions as either a producer of hormones or it manufactures hormone messengers which are necessary for prompting target glands to produce hormones for release into the blood. An example of a hormone messenger is the thyroid-stimulating hormone (TSH), essential for triggering the thyroid gland into producing thyroid hormone which is necessary for metabolism and nerve cell function. Prolactin, the hormone responsible for lactation in humans and other mammals, is also produced by the pituitary gland.
Another important structure among endocrine system organs is the hypothalamus, which is located in the brain beneath the thalamus. Playing a major role in controlling homeostatic balance, the hypothalamus is essential in motivation behaviors and emotion. Also responsible for influencing the activity of the pituitary gland, the hypothalamus regulates many functions of the endocrine system organs, particularly through the secretion of hormones which signal the release of other hormones like the gonadotropin-releasing hormone (GnRH) and the thyrotropin-releasing hormone (TRH). This organ is the primary reason for the interconnection between the endocrine and nervous systems.