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Endocrine system
Lesson: Endocrine system
Lesson Developer: Dr. Gayatri Prakash
College/ Department: Daulat Ram College,
University of Delhi
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Endocrine system
Table of Contents

Introduction

Endocrine system




Function of Endocrine system:

Exocrine and Endocrine glands

Glands of the Endocrine System

Location in the Body and Hormone of the Endocrine Glands
Hormones

Characteristics of hormones

Circulation of Hormones

Types of Hormones
Hormonal Regulation

Feedback Mechanisms

Long and Short-Loop Feedback Mechanisms
Endocrine Gland Stimuli

Neural Stimuli

Humoral Stimuli

Hormonal Stimuli

Summary

Practice exercise

Glossary

References/Bibliography/Further reading

Web links
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Endocrine system
Introduction
The vertebrate’s body has two linking systems which co-ordinate functions of all the organs of the
biological systems. Both of these systems are the major means by which the body communicates
information between different cells and tissues, and the transmitted information results in the
regulation of numerous body functions.
These are the endocrine and the nervous systems. The
nervous system coordinates the activities of the body through nerve impulses and neurotransmitters
while the endocrine system does so through the mediator molecules called hormones. Table 1 gives
the characteristics of the endocrine system.
Endocrine system
Endocrine system is a network of glands, which are located throughout the body regulating certain
body functions such as development, growth, body temperature, metabolism and sexual
development and reproduction. These glands together are referred to as endocrine glands. These
glands control the rates of certain chemical reactions, aiding in transporting substances through
membranes, and helping to regulate water and electrolyte balance, blood pressure, reproduction etc
in the body.
Table 1: Characteristics of the Endocrine Systems
System Characteristic
Endocrine System
Mediator molecules
Hormones delivered by blood to tissues throughout the body
Cells affected
Virtually all body cells
Time of onset of action
Varies from seconds to days
Duration of action
Generally longer
The scientific study of the structure and function of endocrine glands is called endocrinology (endo =
with in; crin = to secrete; ology = study of) and Thomas Addison is the father of endocrinology.
Function of Endocrine system:
The endocrine system influences the metabolic activities of cells by means of hormones (Figure 1)
secreted by various endocrine glands. The tissues or organ responses to hormones typically occur
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Endocrine system
after a lag period of seconds or even days but once initiated, these responses tend to be much more
prolonged than those induced by the nervous system.
Figure 1: Types of chemical communication between cells.
Source: ILLL in house
In some tissues, a type of localized chemical communication between cells occurs through the
chemical messenger, which is not transported any distance in blood but acts in a highly
circumscribed area. These secretory cells are not true endocrine cells and are called paracrine cells
(Figure 1). Also, as a result of histochemical studies pioneered in the 1940s, it emerged that certain
nerve cells are neurosecretory, and release chemical agents called neurohormones into the blood.
However, distinction between hormone, neurohormone, and neurotransmitter may not always be
obvious on the basis of chemical identity alone, and the same substance may possess different
functions in different body tissues.
Value Addition: Video
Heading Text: Endocrine system
Click
the
link
and
get
an
interactive
overview
of
of
our
endocrine
system
https://www.youtube.com/watch?v=gjmS4_7kvDM
Source: YouTube
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Endocrine system
Exocrine and Endocrine glands
The glands of the vertebrate body fall into two distinct categories. These arei)
Exocrine glands, and
ii)
Endocrine glands
Figure 2: Exocrine (A) and endocrine (B) glands.
Source: ILLL in house
Glands of the Endocrine System
The endocrine system includes a number of glands and compared to other organs of the body, the
organs of endocrine system are small. In addition, the anatomical continuity typical of most organ
systems of the body does not exist between the endocrine glands.
Instead, they are widely
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Endocrine system
scattered throughout the body (Figure 3). Location of the major endocrine glands of the body is
shown in Figure 3, and the hormones produced by them are given in Table 2.
Glands of endocrine system whose functions are solely endocrine in nature are pituitary
(hypophysis), pineal, thymus, thyroid, parathyroid, and adrenal. In addition, several organs of the
body contain discrete areas of endocrine tissue and produce hormones and also exocrine products
and therefore, are not exclusively endocrine glands.
These include pancreas and gonads (the
ovaries and testes). These are also major endocrine glands. The hypothalamus also falls into this
category.
Along with its neural functions, hypothalamus produces and releases hormones and is
therefore, it is considered a neuroendocrine organ.
Location in the Body and Hormone of the Endocrine Glands
The hypothalamus lies below the thalamus in the brain and extends from the optic chiasma to the
posterior margin of the mammillary bodies. A stalk of hypothalamic tissue connects the pituitary to
the base of the hypothalamus. The pineal gland is located in the diencephalon and produces the
hormone, melatonin. The Thymus gland lies posterior to the sternum and between the lungs. It
produces a family of hormones called thymosins and thymopoietins that are important for the
normal development of the immune response. The thyroid gland is located in the anterior throat and
its hormones includes thyroxine and triiodothyronine. The parathyroid glands are located on the
dorsal aspect of the thyroid gland and secrete the parathyroid hormone or parathormone. The paired
adrenal (suprarenal) glands sit atop the kidneys and have two functional portions, outer cortex and
inner medulla.
The cortex secretes mineralocorticoids (primarily aldosterone), glucocorticoids
(primarily cortisol) and gonadocorticoids (mainly androgens) while the medulla produces the
catecholamines, epinephrine and norepinephrine. The pancreas located in the abdomen close to the
stomach is both an exocrine and an endocrine gland. The endocrine portion, namely the pancreatic
islets, secretes insulin and glucagon while the exocrine portion is concerned with enzyme secretion.
The male gonads, testes are located in the scrotum and secrete androgens. The female gonads,
ovaries are located in the pelvic cavity and release the hormones, estrogen and progesterone.
Various other tissues and organs in the mammalian body also produce hormones.
For example,
pockets of hormone-producing cells are found in the walls of stomach, small intestine
(duodenum), kidneys, heart, liver, skin and placenta. Placenta is a temporary endocrine
organ formed during pregnancy.
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Endocrine system
Figure 3: Locations of major endocrine glands in Human.
Source: ILLL in house
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Endocrine system
Endocrine glands function smoothly throughout life. However, aging may bring about changes in the
rates of hormone secretion, their breakdown and excretion, or in the sensitivity of target cell
receptors to their respective hormones.
Value Addition: Did you know
Heading Text: Embryonic origin of endocrine glands
Endocrine glands are derived from all three embryonic germ layers. While the mesodermderived endocrine glands produce steroid hormones, all others produce amines or amino
acid and protein hormones.
Source: Author
Table 2: Hormones produced by major endocrine glands and their abbreviations
Gland
Hormone (Abbreviation)
Corticotropin-releasing hormone (CRH)
Gonadotropin- releasing hormone (GnRH )
Growth hormone release-inhibiting hormone (GIH)
Hypothalamus
Growth
hormone-releasing
hormone
(GHRH)
/
Somatostatin (SS)
Prolactin release-inhibiting hormone (PIH)
Prolactin-releasing hormone (PRH)
Thyrotropin-releasing hormone (TRH)
Growth hormone (GH) / Somatotropin (STH)
Follicle-Stimulating hormone (FSH) / Follitropin
Luteinizing hormone (LH) / Interstitial cell stimulating
Anterior
hormone (ICSH) / Lutropin
Corticotropin / Adrenocorticotropic hormone (ACTH)
Pituitary
Thyroid- Stimulating hormone (TSH) / Thyrotropin
Prolactin (PRL)
Intermediate
Melanocyte-stimulating hormone (MSH)
Posterior
Antidiuretic hormone (ADH) / Vasopressin
Oxytocin (OT)
Thyroid
Thyroxin (T4)
Triiodothyronine (T3)
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Endocrine system
Parathyroid
Parathyroid Hormone (PTH) / Parathormone
Mineralocorticoid (Aldosterone)
Cortex
Glucocorticoid (Cortisol)
Adrenal
Gonadocorticoid-androgen
Medulla
Epinephrine (EPI) / Adrenalin
Norepinephrine (NE) / Noradrenalin
Pancreas
Glucagon
Insulin
Somatostatin (SS), pancreatic polypeptide
Testes
Testosterone (T) androgens
Ovaries
Estrogen (E)
Progesterone (P)
Hormone
Hormone (hormon = to excite or get moving) is a chemical substance/messenger secreted by the
glands of the endocrine system into blood stream and is virtually carried to all cells throughout the
body. Hormones have powerful effects even in very low concentration and as a rule, affect only a
few types of cells, called target cells.
Value Addition: Interesting to know
Heading Text: Who coined the term Hormone?
William M. Bayliss and his brother-in-law Ernest H. Starling, both of London University
College in 1903 used the term ‘hormone’ for the first time while referring to a
gastrointestinal tract hormone, secretin. Subsequently, a number of other hormones were
discovered based on the principle that a chemical is responsible for many biological
phenomena. These substances were then called as ‘Chemical messengers’.
Click the link http://joe.endocrinology-journals.org/content/184/1/5.full and read the article
“Ernest Starling and ‘Hormones’: an historical commentary”. Trace the history of
endocrinology up to this important moment.
Source: Author, http://joe.endocrinology-journals.org/content/184/1/5.full
Characteristics of hormones
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Endocrine system
The endocrine glands secrete hormones into the interstitial fluid surrounding the secretory cells.
The secretion then diffuses into capillaries and is carried away by the blood to distant target organ
or cell to exert its action. The concentration of a circulating hormone in blood at any time reflects its
rate of release, and the speed of its inactivation and removal from the body. Enzymes within the
target cells rapidly degrade some hormones, but most are removed from the blood by kidney and
liver enzyme systems, and their breakdown products are excreted from the body in urine or, to a
lesser extent, in feces. As a result, the persistence of a hormone in the blood, referred to as its
half-life, is usually brief-from a fraction of a minute to 30 minutes.
The duration of action of hormone is limited. Depending on the hormone, it ranges from 20 minutes
to several hours. Effects may disappear rapidly as the hormone levels in blood drop, or these may
persist for hours even after reaching very low levels in blood. Because of these variations, hormonal
blood levels are precisely and individually controlled in the body so as to meet the changing needs.
In addition, some hormones such as testosterone secreted by the testes (male gonads), are
secreted in a relatively inactive form termed as prohormone and in order to exert its action, it is
activated in the target cells.
Circulation of Hormones
Hormones circulate in the blood either in free form or bound to a protein.
Free hormones are
generally considered as the active fraction that is directly available to target tissue. Protein-bound
hormones represent the hormone reserve in the plasma.
Protein binding may act to prevent
excretion of this hormone reserve through the kidney glomerulus. Also, in this protein-bound form,
a hormone is able to cross certain plasma membranes to be more available to a specific tissue.
Types of Hormones
Hormones may be classified in a number of ways. This can be on the basis of how far they act from
their site of production, their solubility characteristics, with the kind of molecule a hormone is made
of or its mechanism of action based on the location of receptors.
A.
Site of production and action: Principally hormones are classified into two groups on the
basis of how far from their site of production they act. These are•
Circulating (general) hormones also called endocrine hormones and
•
Local hormones.
Circulating hormones are those that pass into blood and act on distant target cells, for example, the
adrenocorticotropic hormone (ACTH) from the anterior pituitary gland stimulate the adrenal cortex.
These hormones stay in the blood and exert their effects for a few minutes or for a few hours, and
then get inactivated by the liver and excreted by the kidneys.
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Endocrine system
Local hormones are those that act locally without first entering the blood stream and usually are
inactivated quickly. These hormones have specific local effects, for example, secretin released by the
duodenal wall acts directly on the pancreas, or cholecystokinin released from the small intestine
causes the gall bladder to contract, or nitric oxide (NO) released by endothelial cells lining the blood
vessels, causes relaxation of nearby vascular smooth muscle fibers resulting in vasodilation and
increase in blood flow in that region.
The eicosanoids (prostaglandin and leukotriene). are important local hormones and may act as
circulating hormones as well. Figure 8 compares the sites of action of circulating and local
hormones. Among local hormones, those acting on neighboring cells, are called paracrines (para =
beside or near, crin = to secrete), and those acting on the same cell which secretes them, are
termed as autocrines ( auto = self, crin = to secrete).
B.
Chemical Classes of Hormones: Chemically, all hormones can be divided into two broad
classes•
Lipid soluble hormones such as steroids, thyroid hormones and nitric oxide (NO).
•
Water-soluble hormones such as peptides, proteins, glycoproteins, amines and eicosanoids
(prostaglandin and leukotriene).
Water-soluble hormone molecules mostly circulate in a “free” form in blood whereas, lipid-soluble
hormone molecules circulate in the form bound to transport proteins (Table 3). However, 0.1-10 %
of the molecules of a lipid-soluble hormone is not bound to a transport protein and remains free.
This ‘free fraction’ diffuses out of capillaries, binds to receptors, and triggers responses.
As free
hormone molecules leave the blood and bind to their receptors, transport proteins release new ones
to replenish the free fraction.
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Endocrine system
Figure 8: Sites of action of circulating and local hormones.
Source: ILLL in house
Table 3: General features of hormone classes
Character
Group I
Group II
Types
Steroids,
iodothyronine,
calcitriol, gas nitric oxide
Polypeptides,
proteins,
glycoproteins,
amines
(catecholamines), eicosanoids
Solubility
Lipophilic
Hydrophilic
Transport
Proteins
Yes
No
Plasma half-life
Long (hours to days)
Short (minutes)
Receptor
Intracellular
Plasma Membrane
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Endocrine system
Mediator
C.
Receptor-hormone complex
cAMP, cGMP, Ca++, metabolites of
complex phosphoinositols, kinase
cascades
Chemical Structure: Chemically, most hormones are either steroids that are synthesized
from cholesterol or they are amines, peptides, proteins, or glycoproteins that are synthesized from
amino acids.
Vitamin D is a modified steroid and can be converted into a hormone (calcitriol).
Based on their chemical structures, hormones are classified into•
Peptide and protein hormones such as all hypothalamic releasing and inhibiting hormones,
pituitary hormones, insulin, glucagon, parathyroid hormone, etc.
•
Steroid hormones such as sex steroids, corticosteroids, aldosterone, and calcitriol.
•
Thyroid hormones such as triiodothyronine (T3) and thyroxin (T4).
•
Biogenic amines which include the catecholamines such as epinephrine norepinephrine and
dopamine, and the indolamines, which include histamine, melatonin and serotonin.
•
Gases like Nitric oxide (NO).
•
Eicosanoids, which include prostaglandins and leukotrienes.
Table 4. Chemical classes of hormones
Class
Lipid –
Soluble
Compound
Steroids
Amino Acids
Water -
Amines
Sites of Production
Example
Ovary
Estrogen, Progesterone
Testes
Testosterone
Adrenal Cortex
Aldosterone, Cortisol
Kidneys
Calcitriol
Thyroid Gland
Triiodothyronine (I3), Thyroxin (I4)
Endothelial Cells lining
blood vessels
Nitric Oxide (NO)
Adrenal Medulla
Epinephrine and Nor Epinephrine
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Endocrine system
Soluble
Peptides
and Proteins
Glycoproteins
Lipids (20carbon fatty
acids)
Pineal Gland
Melatonin
Mast Cells in Connective
Tissue
Histamine
Platelets in Blood
Serotonin
Hypothalamic
Neurosecretory Cells
All Hypothalamic Releasing and
Inhibiting Hormones
Posterior Pituitary
Hormones
ADH, OT
Anterior Pituitary
Hormones
PTH, GH, PRL, ACTH
Pancreas
Insulin, Glucagon, SS, PP
Parathyroid
Parathyroid Hormone/Parathormone
Anterior Pituitary
Hormones
FSH, LH, TSH
All cells except red blood
cells
Eicosanoids- Prostaglandins and
leukotriene
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Endocrine system
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Endocrine system
Figure 9: Classification of hormone as per chemical structure
Source: http://cnx.org/contents/[email protected]:108/Anatomy_&_Physiology
Hormonal Regulation
Value Addition: Interesting to know
Heading Text: Discovery of regulatory mechanism in body
The concept of regulatory mechanisms initially arose from the work of Claude Bernard, a French
Physician who by 1859 had suggested the importance of maintaining the constancy of the “milieu
interieur” (or internal environment). He pointed out that it is through the milieu interieur that foods,
wastes and gases are exchanged and chemical messengers are distributed.
Source: Author
The development of control system theory in the 1940s provided a framework to describe
physiological and biochemical regulatory mechanisms in a biological system. Two major categories
of control systems exist, and these are ‘open loop’ and ‘closed loop’.
In the open loop control
system input is not affected by the output, while in the closed loop a part of the output is fed back
and participates as an input to the system. Most physiological homeostatic mechanisms are closed
loop systems operating via the principle of negative feedback.
Value Addition: Interesting to know
Heading Text: Who coined the term ‘homeostasis’?
An American Physiologist, Walter B. Cannon coined the term ‘homeostasis’ (homoios = same, stasis
= state) in 1932. Homeostasis is the tendency to maintain uniformity in the internal environment of
the organism and to maintain the normal composition of the body fluids.
Hormones play an
important and decisive role in homeostatic regulation of internal environment in an organism’s body.
Source: Author
Feedback Mechanisms
The synthesis and release of most hormones is regulated by some type of negative and positive
feedback system (Figure 4) and as a result, blood levels of many hormones vary only within a
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Endocrine system
narrow “desirable” range (Figure 5). Release of a hormone from the parent gland, is controlled by
its levels in the blood.
In general, each gland has a tendency to over-secrete, but once its
physiological effect is achieved, information is transmitted back to the gland to check further
secretions.
This is called negative feedback.
Once the secretion of a hormone is reduced below
optimum levels, positive feedback system operates to stimulate the involved gland to increase its
secretion.
In such a feedback system, an endocrine gland or the system controlling it is sensitive to
the concentration of a substance the gland secretes or to the concentration of a product from a
process it controls. For example, increase in the levels of a target-gland hormone in blood signals
the hypothalamus-pituitary axis to decrease output of the stimulating hormone (a negative effect),
which in turn decreases the secretory activity of the target-gland (Figure 6). Hypothalamic control
of the peripheral endocrine glands may utilize as many as three types of hormones, with multiple
negative feedback controls as indicated by negative signs in Figure 6.
A positive effect on the
secretory activity of the gland occurs with a decrease in the levels of its hormone in blood.
For
example, the function of the thyrotropic hormone from the pituitary is to stimulate the thyroid gland
to produce more thyroxin.
When the thyroid produces a sufficient amount, the blood level of
thyroxin rises and as the blood circulates back to the hypothalamus and pituitary, the pituitary
reduces its output of the stimulating hormone-thyrotropin. This in turn results in decreased
production of thyroxin. Vice versa occurs with low levels of thyroxin in blood. Such negative and
positive feedback systems, work to keep the hormones concentrations relatively stable.
Figure 4: Hormone regulation through negative (-) and positive (+)
feedback systems, hormone A stimulates secretion of gland B, hormone B
has action on target cells and also inhibits secretion of gland A.
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Endocrine system
Source: ILLL in house
Figure 5: Fluctuations in hormone concentration.
Source: ILLL in house
Value Addition: Interesting to know
Heading text: Negative feedback loop
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Endocrine system
Source:
https://upload.wikimedia.org/wikipedia/commons/5/52/1813_A_Classic_Negative_Feedback_Loop.jpg
Long and Short-Loop Feedback Mechanisms
Three mechanisms of feedback control of hypophysiotropin and hypophyseal hormone secretion are
recognized. Hormones from peripheral target tissue may feed back through a long-loop system to
act at the level of the pituitary gland, at the level of the hypothalamus, or even at the higher brain
centers. In contrast, a short-loop system involves circulation of hypophysial hormones by retrograde
portal system blood flow to affect secretion of hypothalamic hypophysiotropic hormone. Also, the
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Endocrine system
secreted hypophysial hormones may feedback on their cells of origin to inhibit their own secretion.
This is referred as autofeedback inhibition or autoinhibition (Figure 7).
Figure 6: Hypothalamic control mechanism of peripheral endocrine glands.
Source: Author
Thus, a hormone secreted by a target gland can have both a direct feedback on the
adenohypophyseal hormone and an indirect feedback on the release of the hypothalamic hormone.
This involvement of the hypothalamus is particularly important in the control of hormones secreted
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Endocrine system
by the adrenal cortex and gonads (Figure 7). There is some evidence for the existence of a short
feedback loop in the control of pituitary function whereby a pituitary hormone has a negative
feedback action on the secretion of a hypothalamic releasing or release-inhibiting factor. Prolactin is
one hormone, which is thought to regulate its own secretion by a short feedback mechanism.
However, the general circulation could just as well mediate a short-loop feedback.
Figure 7: Feedback mechanisms in hormones secretion-autoregulation,
short-loop and long-loop.
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Endocrine system
Source: ILLL in house
Although some of the pituitary hormones such as prolactin, melanotropin, and neurohypophyseal
hormones stimulate peripheral target tissues, these target tissues themselves do not secrete
hormones and therefore, their secretion is not controlled by long-loop hormonal feedback
mechanisms.
The reflex mechanisms and other sensory stimuli reaching the hypothalamus from
sensory receptors control the secretion of these hormones. For example, suckling of breast by the
infant leads to secretion of oxytocin from the posterior pituitary gland.
The hormones from the
pancreas and parathyroid are not under the control of the hypothalamus-pituitary axis, but it is the
blood level of certain substances like glucose and calcium that regulate the secretion of these
hormones. These substances are the stimulating factor for decreasing or increasing the hormonal
output. For example, insulin release from the pancreas depends on levels of glucose in blood, and
release of parathormone (PTH) from the parathyroid depends on the levels of calcium in the blood.
There is at least one example in the endocrine system of a positive feedback mechanism. These are
extremely rare in biological systems because basically they are unstable and lead to an ‘explosive’
situation when no other factors are involved. The principle of positive feedback is that an increase in
a hormone concentration leads to an enhancement of its effect by stimulating further release. This
situation occurs prior to ovulation when estrogens are secreted in increasing amounts by the
developing ovarian follicle (s). Normally these steroids have a negative feedback action on pituitary
gonadotropin secretion but when they reach a critical plasma concentration, they stimulate the
release of gonadotropins from the pituitary. This results in a surge of luteinizing hormone (LH) and
follicle stimulating hormone (FSH) secretion. This surge acts on the ovary to cause follicular rupture
and release of the ovum. The reason why this positive feedback does not become uncontrollable is
that by some unknown mechanism the feedback effect of estrogen reverts to a negative feedback
action thereby stabilizing the system.
The endocrine system is not dependent solely on circulatory communication, since the nervous
system has a stimulating effect on the release of hormones from the posterior pituitary, adrenal
medulla, and the pineal. As such, the posterior pituitary and the adrenal medulla are of neural
origin, and thus a part of the autonomic nervous system, and if these glands are destroyed, their
function is taken over to some extent by the nervous system.
Endocrine Gland Stimuli
The endocrine glands are stimulated to manufacture and release their hormones by three major
types of stimuli. These are i) humoral, ii) neural, and iii) hormonal (Figure 9).
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Endocrine system
Figure 9: Three major types of endocrine gland stimuli.
Source: http://4.bp.blogspot.com/43iCIiQEozw/U2fB1_SuFjI/AAAAAAAABT0/8LXaMh2SfP8/s1600/endocrine-gland-stimuli-humoralneural-and-hormonal-stimulus.jpg
Neural Stimuli
In a few cases, nerve fibers stimulate hormone release.
system
stimulation
of
the
adrenal
medulla
norepinephrine) during periods of stress.
to
The example is sympathetic nervous
release
catecholamines
(epinephrine
and
Also, oxytocin and anti-diuretic hormone are released
from the posterior pituitary in response to nerve impulses from hypothalamic neurons.
Humoral Stimuli
Some endocrine glands secrete their hormones in direct response to changing concentration of
certain ions and nutrients in blood.
These stimuli are called humoral stimuli to distinguish them
from hormonal stimuli, which are also blood borne chemicals. This is the simplest of the endocrine
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control systems. For example, cells of the parathyroid glands directly monitor the concentration of
calcium ions in blood and when they detect a decline from normal Ca++ values, they secrete
parathyroid hormone (PTH). Since PTH acts by several routes to reverse that decline, blood Ca++
levels soon rise, ending the initiative for PTH release.
Other hormones released in response to
humoral stimuli include insulin, produced by the pancreas, and aldosterone, one of the adrenal
cortex hormones.
Value Addition: Interesting to know
Heading Text: Origin of the term ‘homeostasis’?
The term humoral dates back to the ancient use of the word humor, as it refers to various body
fluids (blood, bile, and others).
Source: Author
Hormonal Stimuli
Many endocrine glands release their hormones in response to hormones produced by other
endocrine organs. For example, releasing and inhibiting hormones produced by the hypothalamus
regulates release of most of the anterior pituitary hormones. And further, many anterior pituitary
hormones in turn stimulate other endocrine glands to release their hormones into the blood. As the
hormones produced by the final target gland increase in the blood, they inhibit the release of
anterior pituitary hormones and thus their own release (feedback inhibition). The hormonal
mechanism promotes rhythmic hormone release, with the rising and falling levels of hormone in
blood in a specific pattern.
While these three stimuli mechanisms typify most systems that control hormone release, yet they
are by no means all-inclusive or mutually exclusive, and some endocrine glands respond to multiple
stimuli.
Both “turn on” factors (humoral, hormonal and neural stimuli) and “turn off” factors
(feedback inhibition) may be modified or modulated by the nervous system. The nervous system
can, in certain cases, override normal endocrine controls as needed to maintain homeostasis. For
example, blood sugar levels are normally kept within a range of 90-110 mg glucose per 100 ml of
blood by the action of insulin and several other hormones. However, when the body is under severe
stress, blood sugar levels rise much higher because the hypothalamus and sympathetic nervous
system centers are strongly activated. This ensures that body cells will have sufficient fuel for the
more vigorous activity required of them during such periods.
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Endocrine system
Value Addition: Interesting to know
Heading Text: Endocrine Disruptors
Endocrine disruptors are chemicals that may interfere with the body’s endocrine system and produce
adverse developmental, reproductive, neurological, and immune effects in both humans and wildlife.
A wide range of substances, both natural and man-made, are thought to cause endocrine disruption,
including pharmaceuticals, dioxin and dioxin-like compounds, polychlorinated biphenyls, DDT and
other pesticides, and plasticizers such as bisphenol A. Endocrine disruptors may be found in many
everyday products– including plastic bottles, metal food cans, detergents, flame retardants, food,
toys, cosmetics, and pesticides. The NIEHS supports studies to determine whether exposure to
endocrine disruptors may result in human health effects including lowered fertility and an increased
incidence of endometriosis and some cancers. Research shows that endocrine disruptors may pose
the greatest risk during prenatal and early postnatal development when organ and neural systems
are forming.
Source: http://www.niehs.nih.gov/health/topics/agents/endocrine/
Value Addition: Interesting to know
Heading Text: Bisphenol A and Endocrine Disruption
There have been may reports about the effects of a chemical called bisphenol A (BPA) in various
types of food packaging. BPA is used in the manufacturing of hard plastics and epoxy resins.
Common food-related items that may contain BPA include the lining of aluminum cans, plastic foodstorage containers, drinking cups, as well as baby bottles and “sippy” cups. Other uses of BPA
include medical equipment, dental fillings, and the lining of water pipes.
BPA is an endocrine disruptor, meaning that it negatively interferes with the endocrine system,
particularly during the prenatal and postnatal development period. In particular, BPA mimics the
hormonal effects of estrogens and has the opposite effect—that of androgens. The U.S. Food and
Drug Administration (FDA) notes in their statement about BPA safety that although traditional
toxicology studies have supported the safety of low levels of exposure to BPA, recent studies using
novel approaches to test for subtle effects have led to some concern about the potential effects of
BPA on the brain, behavior, and prostate gland in fetuses, infants, and young children. The FDA is
currently facilitating decreased use of BPA in food-related materials.
The potential harmful effects of BPA have been studied in both animal models and humans and
include a large variety of health effects, such as developmental delay and disease. For example,
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Endocrine system
prenatal exposure to BPA during the first trimester of human pregnancy may be associated with
wheezing and aggressive behavior during childhood. Adults exposed to high levels of BPA may
experience altered thyroid signaling and male sexual dysfunction. BPA exposure during the prenatal
or postnatal period of development in animal models has been observed to cause neurological
delays, changes in brain structure and function, sexual dysfunction, asthma, and increased risk for
multiple cancers. In vitro studies have also shown that BPA exposure causes molecular changes that
initiate the development of cancers of the breast, prostate, and brain. Although these studies have
implicated BPA in numerous ill health effects, some experts caution that some of these studies may
be flawed and that more research needs to be done. In the meantime, the FDA recommends that
consumers take precautions to limit their exposure to BPA. In addition to purchasing foods in
packaging free of BPA, consumers should avoid carrying or storing foods or liquids in bottles with the
recycling code 3 or 7. Foods and liquids should not be microwave-heated in any form of plastic: use
paper, glass, or ceramics instead.
Source: http://cnx.org/contents/[email protected]:108/Anatomy_&_Physiology
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Endocrine system
Summary

The nervous and endocrine systems are the major controlling systems of the body.

The endocrine system consists of endocrine glands and several organs containing endocrine
tissue.

The endocrine glands are small and widely separated in the body.

Endocrine glands are ductless, and well vascularised.
They release their products called
hormones directly into the blood or lymph.

Major endocrine glands are hypothalamus, pituitary, pineal, thymus, thyroid, parathyroid,
adrenal, pancreas and gonads (testes and ovaries).

The hypothalamus lies below the thalamus.

The pituitary gland lies in the cavity of the third ventricle called sella turcica.

The pineal gland is located in the diencephalon.

The thymus gland lies posterior to the sternum and between the lungs.

The thyroid gland is located in the anterior throat.

The parathyroid glands are located on the dorsal aspect of the thyroid gland.

The paired adrenal (suprarenal) glands sit atop the kidneys.

The pancreas located in the abdomen close to the stomach is a compound gland consisting of
an exocrine and an endocrine gland.

The male gonads, testes are located in the scrotum.

The female gonads, ovaries are located in the pelvic cavity.

Endocrine glands operate smoothly throughout life until old age.

Endocrine glands via hormones regulate many processes in the body such as reproduction,
growth
and
development,
maintenance
of
electrolyte,
water
and
nutrient
balance,
mobilization of body defenses to stressors, and regulation of cellular metabolism.

Endocrine glands are derived from all three germ layers.
Those derived from mesoderm
produce steroidal hormones, while others produce the amino acid-based hormones.

Hormone is a chemical substance secreted by the glands of the endocrine system into blood
stream. It is carried to all cells throughout the body.

Hormones affect only a few types of cells, called target cells.
They exert powerful effects
even in very low concentrations. They have a brief half-life.

Some type of negative feedback system regulates the synthesis and release of the hormones.

Hormones can be of circulating and local type depending on how far from their site of
production they act. Local hormones can be paracrine or autocrine in nature.

Hormones circulate in the blood in a free form and also in a form that is bound to proteins.

Hormones may be water or lipid soluble.
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Endocrine system

Chemically, most hormones are either steroids that are synthesized from cholesterol or they
are amines, peptides, proteins, or glycoproteins that are synthesized from amino acids.

Steroid hormones include sex hormones, such as estrogen, progesterone, and testosterone,
and the ones secreted by the adrenal cortex include aldosterone and cortisol.

Some neurons and adrenal medulla produce amines such as epinephrine and norepinephrine, derived from amino acids.

The peptide hormones are short chains of amino acids, produced in the hypothalamus.

Protein hormones are long chains of amino acids secreted by parathyroid gland and anterior
pituitary gland.

Feedback mechanisms control the secretion of the hormones.

Endocrine glands are activated to release their hormones by humoral, neural, or hormonal
stimuli.
Value Addition: Video
Heading Text: Hormones and the Endocrine System
Click the link and recap the concepts you have learnt in the chapter in interactive way
https://www.youtube.com/watch?v=WMVEGAVdEoc
Source: YouTube
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Endocrine system
Exercise
1.
Give the locations of the following:
(a)
Hypothalamus
(b)
Thyroid
(c)
Thymus
(d)
Adrenal gland
(e)
Pituitary
2.
Differentiate between:
(a)
Endocrine and exocrine
(b)
Local hormone and circulating hormone
(c)
Group I and II hormone classes
(d)
Apocrine and paracrine
3.
Answer the following questions in short:
(a)
Name the hormones secreted by a) Parathyroid b) Adrenal cortex c) Ovary d) Testes c)
Hypothalamus
4.
Answer the following questions in details:
1.
What is an endocrine system?
2.
Enumerate the characteristics of the endocrine system.
3.
Contrast the control of homeostasis by the nervous and endocrine systems.
4.
Define hormone and target cell.
5.
Distinguish between exocrine and endocrine glands.
6.
List the major endocrine organs and give their location in the body.
7.
Besides the major endocrine organs, what are the other hormone-producing cells clusters?
8.
Distinguish between circulating and local hormones.
9.
Compare the two chemical classes of hormones based on their solubility.
10.
Identify the chemical classes of hormones. Give an example of each.
11.
How are hormones transported in the blood?
12.
Describe the three types of signals that can control hormone secretion.
13.
What is a feedback system and why is it important?
14.
Distinguish between down and up-regulation of a hormone.
15.
How do negative and positive feedback systems differ from one another?
16.
Explain how hormone release is regulated.
17.
How endocrine glands are stimulated to release their hormones?
18.
List the hormones secreted by the endocrine glands and give their functions.
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Endocrine system
References
REFERENCES, BIBLIOGRAPHY AND FURTHER READING:
1. Tortora, G.J. & Grabowski, S. Principles of Anatomy & Physiology. 13th Edition, p.642.
2. Moyes, C. D. and Schulte, P. M. (2006). Principles of Animal Physiology, p. 248.
3. Hill, R. W., Wyse, G. A. and Anderson, M. (2006). Animal Physiology. p.355.
4. Randall, D., Burggren W. and French, Kathleen (2001). Eckert Animal Physiology.
5. Widmaier, E.P., Raff, H. and Strang, K.T. (2008). Vander’s Human Physiology, XI Edition,
McGraw Hill.
6. Guyton, A.C. and Hall, J.E. (2011). Textbook of Medical Physiology, XII Edition,
Harcourt Asia Pvt. Ltd./W.B. Saunders Company.
7. Ganong, William F. Review of Medical Physiology. XXI Edition. Mc Graw Hill
USEFUL WEB LINKS:
http://cnx.org/contents/[email protected]:108/Anatomy_&_Physiology
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