Overview
The endocrine system is an extensive network of bodily organs that help control
and maintain the necessary functions of life. It is an important control mechanism
employed by the body, which uses over fifty different hormones to regulate body
functions (1). In contrast to the nervous system, that uses electrical impulses to cause
immediate changes, the endocrine system works chemically over longer periods of time,
causing more lasting effects. The endocrine system can cause minor to major changes
throughout the body or on a specific area to help maintain homeostasis, immune
response, reproductive ability, and ongoing development (1), functions which are all
essential to life.
Important Organs
The endocrine system has a network of nine major organs with vastly varying
functions that are outlined below. A representation of relative size and position of the
different organs are displayed in figure 1. The different functional hormones produced
and secreted by the different organs are summarized below in table 1.
Hypothalamus & Pituitary Gland
These small marble sized organs, often termed the “Task Masters” can be seen as
the control mechanisms of the endocrine system (2). The hypothalamus is a region at the
base of the brain that produces four major hormone groups: oxytocin, antidiuretic,
releasing, and inhibitory. Oxytocin and antidiuretic hormone are sent to the pituitary
gland to be stored after production (1). Releasing and inhibitory hormones activate or
deactivate the pituitary gland from producing its own hormones, as well as releasing
stored ones (1). The hypothalamus also serves as a switch for the adrenal glands (1).
The pituitary gland as can be seen in figure 2 is a pair of small lobes located
below the hypothalamus. Its anterior lobe secretes six pituitary hormones including
gonadotropins, which stimulate the production of sex hormones in the gonads, especially
during puberty (1, 3). The posterior lobes serve as a “storage bay” secreting the two
hypothalamic hormones mentioned earlier (1).
Pineal Body
The pineal body located in the rear portion of the brain secrets melatonin, a
hormone that communicates the conditions of external lighting to other organs (4). This
helps the body consistent circadian rhythm or sleep cycle clock (5).
Thyroid & Parathyroid Glands
The thyroid is two lobe gland located anterior to the trachea and above the voice
box. Thyroid follicles are the functional units of this gland. They secrete two key
hormones; calcitonin which decreases blood-calcium levels and thyroxine which
maintains growth/metabolic rates (1).
The parathyroid glands are four, relatively small glands embedded within the
thyroid tissue. These glands secrete parthormone which like calcitonin is also involved
with regulation of blood-calcium levels (1).
Thymus
Located in the chest, the thymus is a flat mass of lymphoid tissues (1). It grows
until the beginning of puberty, at which point it begins to shrink, merging almost
completely with adjacent tissues in fully developed adults. The thymus secretes
thymosins which convert certain leukocytes to thymocytes (1, 6). These specialized cells
are vital to the immune response against nonnative, damaged, and infected cells (6).
Adrenal Glands
These two glands, which sit atop the kidneys, can be divided into two major parts:
the cortex and the medulla. Both regions are different, and thus play differing roles. The
adrenal cortex secretes a collection of hormones called corticosteroids. Corticosteroids
are part of a variety of functions that can be broken up into two major groups:
glucocorticoids which regulate metabolism and mineralocorticoids which maintain the
water-electrolyte balance (1, 2). Glucocorticoids are often released when the body is
coping with stress (2). Due to the importance of these functions, an adrenal cortex failure
would cause death in less than a week (1).
The adrenal medulla secretes catecholamines such as epinephrine and
norepinephrine (2). Epinephrine initiates the flight-or-fight response when the body is
frightened or under immense stress (2). Norepinephrine, released during situations of
acute stress, causes blood vessels to constrict, thus increasing blood pressure (2).
Pancreas
Located in the lower abdomen, the pancreas is a flat, “pancake” shaped organ that
serves as part both the digestive and endocrine systems (2). Its endocrine function,
generated by specialized units called Islets of Langerhans that are scattered throughout
the organ, involves maintenance of blood-sugar levels, which must be regulated within a
set range to keep optimal health conditions (1, 2). The pancreas produces and releases
two critical hormones. Insulin is introduced to the bloodstream to lower blood sugar
levels by stimulating cells around the body to absorb, metabolize, and store away
ingested glucose. Glucagon is used to produce the reciprocal effect by stimulating cells to
break-down stored glucose and release it back into the bloodstream (7).
Gonads
The testes in males and the ovaries in females are classified broadly as gonads.
Among other functions for the reproductive system, the gonads release sex hormones
which are responsible for the development of secondary sex characteristics during
puberty (2). Estrogen and progesterone are the primary hormones secreted by the ovaries
and initiate the development characteristics such as: growth of breasts, collection of fat
around the thighs, and widening of the pelvis (2). Progesterone also helps prepare the
uterus for pregnancy (2). Similarly testosterone released by the testicles, causes growth of
the genitalia, deepening of the voice and development of facial hair (2). As females also
have low levels of testosterone and males low levels of estrogen, certain characteristics
developed during puberty such as the growth of pubic hair are common to both genders
(2).
Significant Hormones
table 1 – Compiled from sources (1, 2, 3, 4, 5, 6, 7)
Summary of various hormones and their functions in order of appearance in above text
Hormone
Produced by:
Secreted by:
Effect/Function
Oxytocin
Hypothalamus
Posterior
Stimulates pre and post birth
Pituitary
processes
Antidiuretic
Hypothalamus
Posterior
Stimulates kidney to prevent
Hormone
Pituitary
dehydration
Releasing
Hypothalamus Hypothalamus
Stimulate release of pituitary
Hormone
hormones
Inhibitory
Hypothalamus Hypothalamus
Inhibit production of pituitary
Hormone
hormones
Gonadotropins
Pituitary
Pituitary
Stimulate gonads to produce sex
hormones
Melatonin
Pineal Body
Pineal Body
Maintenance of circadian
rhythm or body clock
Calcitonin
Thyroid
Thyroid
Reduction of blood-calcium
levels
Thyroxin
Thyroid
Thyroid
Maintence of metabolism, and
growth rates in children
Parthormone
Parathyroid
Parathyroid
Regulation of blood-calcium
Glands
Thymus
Glands
Thymus
Adrenal Cortex
Insulin
Glucagon
Estrogen
Adrenal
Cortex
Adrenal
Cortex
Adrenal
Medulla
Adrenal
Medulla
Pancreas
Pancreas
Ovaries
Progesterone
Ovaries
Ovaries
Testosterone
Testes
Testes
Thymosins
Glucocorticoids
Mineralocorticoids
Epinephrine
Norepinephrine
Corticosteroids
Adrenal Cortex
Adrenal
Medulla
Adrenal
Medulla
Pancreas
Pancreas
Ovaries
levels
Coversion of leukocytes to
thymocytes
Regulation of metabolism and
blood-glucose levels
Regulation of electrolyte or ion
levels
Trigger flight-or-fight response
Trigger constriction of blood
vessels, increase blood pressure
Decrease blood-glucose levels
Increase blood-glucose levels
Initiation and development of
female secondary sex
characteristics
Preparation of uterus for
pregnancy
Initiation and development of
male secondary sex
characteristics.
Major Hormone Groups:
Catecholamines
Pancreatic Hormones
Sex Hormones
Works Cited
1) Nagel, Rob. (2000) Body By Design Vol. 1., U•X•L - Imprint of the Gale Group,
Detroit, MI
2) Siegfried, Donna Rae. (2002) Anatomy & Physiology for Dummies., Wiley
Publishing Inc, New York, NY
3) Lewis, Thomas, et Al. (1994) The Incredible Machine., The National Geographic
Society, Washington D. C.
4) Carey, Joseph, et Al. (2006) Brain Facts: A Primer on the Brain and Nervous
System., Society for Neuroscience, Washington D. C.
5) Bowen, R. "The Pineal Gland and Melatonin". 17 March 2003. Colorado State
University. 6 December 2009.
http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/otherendo/pineal.html
6) "Dictionary of Cancer Terms". National Cancer Institute. 7 December 2009.
http://www.cancer.gov/dictionary/?CdrID=467345
7) Lysaght, Dr. Michael, et Al. "Insulin/Glucagon". 28 March 2002. Brown
University. 7 December 2009.
http://biomed.brown.edu/Courses/BI108/BI108_2002_Groups/pancstems/stemcell
/insulin_glucagon.htm