Hormones 2016

An overview of the endocrine system
The endocrine system is a group of ductless glands that regulate
the body processes by their secretion of chemical substances called
hormones, which are carried to specific target organs and tissues by the
bloodstream. Hormones are necessary for normal growth and development,
for reproduction, and for homoeostasis 32 . They stimulate or inhibit
various biochemical processes by combining with specific receptors on
the membranes of target organs. Thus, although a hormone circulates
throughout the body in the bloodstream, it does not affect every cell with
which it comes into contact but only those cells that contain a specific
receptor site. The major endocrine glands in the human body are the
hypothalamus, the pituitary, the thyroid, the Islets of Langerhans in the
pancreas (these produce insulin), the adrenals, the parathyroids, the ovaries
and the testes. Almost every organ or tissue of the body (including the
intestinal tract, the stomach, and the heart) has been found to be involved
in endocrine secretions (unknown, 2014d). It has also been found that the
ovaries produce small amounts of testosterone, and the testicles produce
small amounts of oestrogen (unknown, 2014g).
Hormonal secretions are generally regulated by negative-feedback loops.
In the simple loops, the concentration of another hormone or a metabolite
(i.e. calcium) influences sensitive regulators in an endocrine gland to inhibit
or stimulate hormonal secretions in the target organ. The complex loops
involve a mechanism called the hypothalamo-pituitary-target-organ axis,
in which the hypothalamus secretes releasing hormones that stimulate the
pituitary to secrete a target hormone, which then enters the circulation
and binds with the receptors of the target organ (unknown, 2014d). The
hypothalamus produces gonadotropin-releasing hormone GnRH 33 , which
stimulates the anterior pituitary gland to synthesize and release luteinizing
hormone luteinizing hormone (LH) 34 . To a lesser degree, GnRH also
triggers the synthesis and release of follicle stimulating hormone follicle
stimulating hormone (FSH) 35 . Subsequently, LH and FSH stimulate the
gonads (ovaries in females, testes in males) to synthesize and release
hormones that cause differentiation of the body tissue into female or
male form: oestrogen, progesterone, and testosterone. A small quantity
of testosterone is also produced by the adrenal gland. Proportionally,
females have more oestrogen and progesterone than males; males have
more testosterone (J. V. Turner, Agatonovic-Kustrin, and Glass, 2007).
Two body systems control all physiologic processes in the human body via
a process of messaging — the nervous system with its electrical point-topoint
control via nerves, and our system of interest — the endocrine system.
32 Automatic self-regulation to maintain the normal or standard state of the body under
variations in the environment i.e. the body producing sweat to help cool it down on a hot
day
33 gonadotropin-releasing hormone
34 luteinizing hormone
35 follicle stimulating hormone
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