Endocrine System Introduction

The Endocrine System is an interwoven set of biologic agents that activate and inhibit and thereby regulate the bodies systems.   If you have to much sugar (glucose) in your blood, insulin will reduce it, if you have too little sugar, glucagon will increase it.  The endocrine systems is tightly integrated with the nervous system.    It regulates the flight or fight response via the adrenal glands by raising the heart rate and constricting blood vessels.  To help understand the system, first a little anatomy.  The endocrine system is comprised of very specific organs that secrete into the fluid around them or into the blood stream directly. 

The endocrine system is made up of glands that produce and secrete hormones. These hormones regulate the body's growth, metabolism (the physical and chemical processes of the body), and sexual development and function. The hormones are released into the bloodstream and may affect one or several organs throughout the body.

Hormones are chemical messengers created by the body. They transfer information from one set of cells to another to coordinate the functions of different parts of the body.

The major glands of the endocrine system are the hypothalamus, pituitary, thyroid, parathyroids, adrenals, pineal body, and the reproductive organs (ovaries and testes). The pancreas is also a part of this system; it has a role in hormone production as well as in digestion.

The endocrine system is regulated by feedback in much the same way that a thermostat regulates the temperature in a room. For the hormones that are regulated by the pituitary gland, a signal is sent from the hypothalamus to the pituitary gland in the form of a "releasing hormone," which stimulates the pituitary to secrete a "stimulating hormone" into the circulation. The stimulating hormone then signals the target gland to secrete its hormone. As the level of this hormone rises in the circulation, the hypothalamus and the pituitary gland shut down secretion of the releasing hormone and the stimulating hormone, which in turn slows the secretion by the target gland. This system results in stable blood concentrations of the hormones that are regulated by the pituitary gland.

The organs and effects are:

Organ	Agent	Regulates
Pineal	Melatonin, Growth Hormone, Adrenocorticotropic hormone, prolactin, Luentizing Hormone	Circadian rhythms, bone and soft tissue growth, stimulates adrenal cortex, milk production and ovulation/sperm production
Hypothalmus	Trophic hormones	Controls the anterior pituitary
Pituitary	Oxytocin, Antiduretic, endorphins	Uterine contractions, decrease urine output
Thyroid	Thyroxin, Triiodothyrone	Increases metabolism
Parathyroid	Parathyroid Hormone	Release calcuim from bone, reabsorpotion of calcium from intestine and kidney, Vitaman D synthesis
Skin	Vitamin D	Bone growth
Thymus	Thymosin, Thymopoitin	T Lympocytes  - a type of white blood cell
Adipose	Leptin, adipopectin	Food intake and metabolism
Adrenal gland  - cortex	Glucocorticoids, mineralcorticoids	Raise blood glucose level, reabsorb sodium, secrete potassium
Adrenal gland – medulla	Epinephrine, norepinephrine	Increase cardiac output, blood vessel regulation, glycogen metabolism
Pancrease	Insulin, glucagon	Regulates glucose levels in the blood steam
Kidney	Erythropoitin	Red blood cell production
Ovary/Testis	Estrogen/Androgen	Sex drive and sexual features