Organization of the Endocrine System

Topics: Signal transduction, Hormone, Endocrine system Pages: 12 (2710 words) Published: March 10, 2014
Chapter 46: Organization of endocrine control
Two major systems evolved to communicate and coordinate body functions Nervous system
Endocrine system
Hormones are carried through the blod to distant target tissue where they are recognized by specific, high-affinity receptors These receptors may be located either on the surface of target tissue, within the cytosol, or in the target cell’s nucleus Once a hormone is recognized by its target tissue or tissues, it can exert its biologic action by a process known as signal transduction PRINCIPLES OF ENDOCRINE FUNCTIION

Chemical signalling can occur through endocrine, paracrine or autocrine pathways In classic endocrine signalling, a hormone carries a signal from a secretory gland across a large distance to a target tissue Hormones secreted into the extracellular space can also resulate nearby cells without ever passing though the systemic circulation This is referred to as paracrine regulation

Chemicals that bind to receptor on or in the cell that is actually secreting the hormone and thus affect the funciotn of the hormone-secreting cell itself, this is referred to as autocrine regulation Endocrine glands

Seven classic endocrine glands: pituitary, thyroid, parathyroid, testes, ovary, adrenal, and pancreas Tissue that are not classically recognized as part of endocrine system produce hormones and play a vital role in endocrine regulation CNS (hypothalamus), GI tract, liver, heart, kidney and others Paracrine factors

Interleukins or lymphokines are example of such paracrine factors, growth factors (platelet-derived growth factor, fibroblast growth factor) These factors are not hormones in the usual senses
They are not secreted by glandular tissue, and their sites of action are usually within the local environment These signalling molecules share many properties of classic peptide and amine hormones in that they bind to surface receptors and regulate one or more of specific intracellular signalling mechanism Hormones may be peptides, metabolites of single amino acids, or metabolites of cholesterol Peptide hormones include a large group of hormones made by a variety of endocrine tissues Insulin, glucagon, and somatostatin are made in the pancreas Pituitary gland makes GH; 2 gonadotropin, LH and FSH; ACTH; TSH and prolactin Parathyroid hormone is made in parathyroid

Calcitonin is made in thyroid gland
Somatostatin and several releasing hormones are made by hypothalamus Secretin, cholecystokinin, are made from GI tract
More restricted number of tissues make catecholamines (from tyrosine) and steroid hormones (from cholesterol) Hormones can circulate either free or bound to carrier proteins Many hormones circulate freely in blood until they reach their target tissue Others form complexes with circulating binding proteins such us thyroid hormones, steroid hormones, insulin-like growth factors I and II, and GH Forming a complex with a circulating binding protein serves several functions First, it provides the blood with a reservoir or pool of hormone minimizing the minute-to minute fluctuations in hormone concentration Second, it extends the half-life of hormone in the circulation (e.g. T4: 7 to 8 days) The presence of plasma binding proteins can affet the total circulating concentration of a hormone without necessarily affecting the concentration of unbound or free hormone in the blood Immunoassays allow measurement of circulating hormones

Hormones can have complementary and antagonistic actions
Regulation of a number of complex physiological functions necessitates the complementary action of several hormones This principle is true both for minute-to-minute homeostasis and for more long-term processes Integration of hormone action can also involve hormones that exert antagonistic actions Endocrine regulation occurs through feedback control

Endocrine regulation can involve hierarchic levels of control Feedback in hierarchic endocrine control system can be quite complex...
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