Endocrinology at a Glance
Location of different Endocrine glands.
Source: Tortora Gerald, Principles of Anatomy and Physiology 12th edition
Nearly a hundred years ago, Starling coined the term hormone to describe
secretin, which is a substance that stimulates secretion of pancreatic juice
and bile that is secreted by the small intestine via the blood stream. In his
Croonian Lectures, Starling considered the endocrine and nervous systems as the
main organ systems that works in coordination with each other in order to
regulate and control bodily organ functions. Thus, endocrinology found its earliest domicile in the mammalian
physiology discipline.
Over the next several decades, many works by
physiologists, biochemists, and clinical investigators led to the categorization
of an array of hormones secreted into the blood stream from discrete glands and/or
other organs. These investigators were the first to show that diseases such as
hypothyroidism and diabetes could be successfully treated by replacing specific
hormones. These initial triumphs fashioned the foundation of the clinical
specialty of endocrinology. Distinct advances in molecular biology, cell
biology, and genetics over the consequent years began to help elucidate the
mechanisms of hormone secretion and action and nature of endocrine diseases.
Both circulating and local hormones of
the endocrine system contribute to homeostasis by the regulation of the growth
and activity of the different target cells in the body. Hormones also
participate in the regulation of body metabolism.
As boys and girls come into puberty,
they start to develop remarkable differences in their behavior and physical
appearances. There may be no other period in life so significantly shows the
endocrine system’s impact in regulating body functions and directing
development. In girls, estrogen promotes the buildup of adipose tissue in the
breasts and hips, molding a feminine shape. Meanwhile, boys begin to build
muscle mass and enlarge the vocal cords, producing a lower pitched voice; due
to increasing production of testosterone in their body. These bodily changes
are just a few examples of the powerful and dynamic influence of endocrine
secretions. Perhaps, multitudes of hormones help maintain and regulate homeostasis
in the body on a daily basis. They control the activity of cardiac muscle,
smooth muscle, and some glands; modify body metabolism; prompt growth and
development; manipulate reproductive processes; and take part in the circadian
(daily) rhythms recognized by the suprachiasmatic nucleus of the hypothalamus.
Moreover, both the nervous
and endocrine systems work hand in hand to bring together functions of all body
systems. Bear in mind that the nervous system’s actions are made through nerve
impulses (action potentials) that are conducted along axons of neurons. At
synapses, nerve impulses elicit the release of substances called neurotransmitters
which serves as mediator or messenger molecules. Also, the endocrine system
controls the body’s activities by releasing mediators, called hormones.
However, the means of control of the two systems are very different.
Responses of the endocrine system often
are slower compare with those of the responses of the nervous system; even
though some hormones act only within seconds, to be able to cause a response, most
get several minutes or more. The effects of the activation of the nervous
system are generally briefer than those of the endocrine system. The nervous
system actually acts on specific muscles and glands. The endocrine system’s
influence is much broader; it helps control almost all types of cells in the
body.
Endocrine glands exude their products (hormones) not into ducts
of the body, but into the interstitial fluid adjacent to the secretory cells. Then
from the interstitial fluid, hormones disseminate into the blood capillaries
and blood serves as carrier that will deliver them to target cells throughout
the body. Since most hormones are required only in very small amounts, most of
the time the circulating levels are typically low.
The endocrine glands
include the pituitary, parathyroid, adrenal, pineal glands thyroid. In
addition, several organs and tissues are not solely classified as endocrine
glands but contain cells that liberate hormones. These include the
hypothalamus, thymus, stomach, liver, small intestine, pancreas, ovaries,
testes, kidneys, skin, heart, adipose tissue, and placenta. When taken
together, all endocrine glands and cells that secrete hormones constitute the endocrine
system. The vast body of knowledge that deals with the structure and
function of the endocrine glands and
the diagnosis and treatment of disorders of the endocrine system is endocrinology.
References:
Larsen, Reed et. al, William’s Textbook of
Endocrinology 10th edition, 2003
Tortora Gerald,
Principles of Anatomy and Physiology 12th edition, 2009
