You have already learnt that the neural system provides a
point-to-point rapid coordination among organs. The neural
coordination is fast but short-lived. As the nerve fibres do not innervate
all cells of the body and the cellular functions need to be continuously
regulated; a special kind of coordination and integration has to be
provided. This function is carried out by hormones. The neural system
and the endocrine system jointly coordinate and regulate the
physiological functions in the body.
Endocrine glands lack ducts and are hence, called ductless glands. Their
secretions are called hormones. The classical definition of hormone as a
chemical produced by endocrine glands and released into the blood and
transported to a distantly located target organ has current scientific
definition as follows: Hormones are non-nutrient chemicals which
act as intercellular messengers and are produced in trace amounts.
The new definition covers a number of new molecules in addition to the
hormones secreted by the organised endocrine glands. Invertebrates
possess very simple endocrine systems with few hormones whereas a large
number of chemicals act as hormones and provide coordination in the
vertebrates. The human endocrine system is described here.
22.1 Endocrine
Glands and
22.2 Human
22.3 Hormones of
Heart, Kidney
22.4 Mechanism of
Hormone Action
The endocrine glands and hormone
producing diffused tissues/cells located
in different parts of our body constitute
the endocrine system. Pituitary, pineal,
thyroid, adrenal, pancreas, parathyroid,
thymus and gonads (testis in males and
ovary in females) are the organised
endocrine bodies in our body
(Figure 22.1). In addition to these, some
other organs, e.g., gastrointestinal tract,
liver, kidney, heart also produce
hormones. A brief account of the
structure and functions of all major
endocrine glands and hypothalamus
of the human body is given in the
following sections.
22.2.1 The Hypothalamus
As you know, the hypothalamus is the
basal part of diencephalon, forebrain
(Figure 22.1) and it regulates a wide
spectrum of body functions. It contains
several groups of neurosecretory cells
called nuclei which produce hormones.
These hormones regulate the synthesis and secretion of pituitary
hormones. However, the hormones produced by hypothalamus ar
e of
two types, the releasing hormones (which stimulate secretion of pituitary
hormones) and the inhibiting hormones (which inhibit secretions of
pituitary hormones). For example a hypothalamic hormone called
Gonadotrophin releasing hormone (GnRH) stimulates the pituitary
synthesis and release of gonadotrophins. On the other hand, somatostatin
from the hypothalamus inhibits the release of growth hormone from the
pituitary. These hormones originating in the hypothalamic neurons, pass
through axons and are released from their nerve endings. These hormones
reach the pituitary gland through a portal circulatory system and regulate
the functions of the anterior pituitary. The posterior pituitary is under
the direct neural regulation of the hypothalamus (Figure 22.2).
Figure 22.1 Location of endocrine glands
(in male)
(in female)
Thyroid and
22.2.2 The Pituitary Gland
The pituitary gland is located in a bony cavity
called sella tursica and is attached to
hypothalamus by a stalk (Figure 22.2). It is
divided anatomically into an adenohypophysis
and a neurohypophysis. Adenohypophysis
consists of two portions, pars distalis and pars
intermedia. The pars distalis region of pituitary,
commonly called anterior pituitary, produces
growth hormone (GH), prolactin (PRL), thyroid
stimulating hormone (TSH),
adrenocorticotrophic hormone (ACTH),
luteinizing hormone (LH) and follicle
stimulating hormone (FSH). Pars intermedia
secretes only one hormone called
stimulating hormone (MSH). However, in
humans, the pars intermedia is almost merged
with pars distalis. Neurohypophysis (pars
nervosa) also known as posterior pituitary, stores
and releases two hormones called oxytocin and
vasopressin, which are actually synthesised by
the hypothalamus and are transported axonally to neurohypophysis.
Over-secretion of GH stimulates abnormal growth of the body leading
to gigantism and low secretion of GH results in stunted growth resulting
in pituitary dwarfism. Excess secretion of growth hormone in adults
especially in middle age can result in severe disfigurement (especially of
the face) called Acromegaly, which may lead to serious complications,
and premature death if unchecked. The disease is hard to diagnose in
the early stages and often goes undetected for many years, until changes
in external features become noticeable. Prolactin regulates the growth of
the mammary glands and formation of milk in them. TSH stimulates the
synthesis and secretion of thyroid hormones from the thyroid gland. ACTH
stimulates the synthesis and secretion of steroid hormones called
glucocorticoids from the adrenal cortex. LH and FSH stimulate gonadal
activity and hence are called gonadotrophins. In males, LH stimulates
the synthesis and secretion of hormones called androgens from testis. In
males, FSH and androgens regulate spermatogenesis. In females, LH
induces ovulation of fully mature follicles (graafian follicles) and maintains
the corpus luteum, formed from the remnants of the graafian follicles
Portal circulation
Figure 22.2 Diagrammatic representation of
pituitary and its relationship with
after ovulation. FSH stimulates growth and
development of the ovarian follicles in females. MSH
acts on the melanocytes (melanin containing cells) and
regulates pigmentation of the skin. Oxytocin acts on
the smooth muscles of our body and stimulates their
contraction. In females, it stimulates a vigorous
contraction of uterus at the time of child birth, and milk
ejection from the mammary gland. Vasopressin acts
mainly at the kidney and stimulates resorption of water
and electrolytes by the distal tubules and thereby
reduces loss of water through urine (diuresis). Hence,
it is also called as anti-diuretic hormone (ADH).
An impairment affecting synthesis or release of ADH
results in a diminished ability of the kidney to conserve
water leading to water loss and dehydration. This
condition is known as
Diabetes Insipidus.
22.2.3 The Pineal Gland
The pineal gland is located on the dorsal side of
forebrain. Pineal secretes a hormone called melatonin.
Melatonin plays a very important role in the regulation
of a 24-hour (diurnal) rhythm of our body. For
example, it helps in maintaining the normal rhythms
of sleep-wake cycle, body temperature. In addition,
melatonin also influences metabolism, pigmentation,
the menstrual cycle as well as our defense capability.
22.2.4 Thyroid Gland
The thyroid gland is composed of two lobes which are
located on either side of the trachea (Figure 22.3). Both
the lobes are interconnected with a thin flap of connective
tissue called isthmus. The thyroid gland is composed of
follicles and stromal tissues. Each thyroid follicle is
composed of follicular cells, enclosing a cavity. These
follicular cells synthesise two hormones,
tetraiodothyronine or thyroxine (T
) and
triiodothyronine (T
). Iodine is essential for the normal
rate of hormone synthesis in the thyroid. Deficiency of
iodine in our diet results in hypothyroidism and
enlargement of the thyroid gland, commonly called
goitre. Hypothyroidism during pregnancy causes
defective development and maturation of the growing
Figure 22.3 Diagrammatic view of the
position of Thyroid and
(a) Ventral side
(b) Dorsal side
baby leading to stunted growth (cretinism), mental retardation, low
intelligence quotient, abnormal skin, deaf-mutism, etc. In adult women,
hypothyroidism may cause menstrual cycle to become irregular. Due to
cancer of the thyroid gland or due to development of nodules of the thyroid
glands, the rate of synthesis and secretion of the thyroid hormones is
increased to abnormal high levels leading to a condition called
hyperthyroidism which adversely affects the body physiology.
Exopthalmic goitre is a form of hyperthyroidism, characterised by
enlargement of the thyroid gland, protrusion of the eyeballs, increased
basal metabolic rate, and weight loss, also called Graves’ disease.
Thyroid hormones play an important role in the regulation of the basal
metabolic rate. These hormones also support the process of red blood cell
formation. Thyroid hormones control the metabolism of carbohydrates, proteins
and fats. Maintenance of water and electrolyte balance is also influenced by
thyroid hormones. Thyroid gland also secretes a protein hormone called
thyrocalcitonin (TCT) which regulates the blood calcium levels.
22.2.5 Parathyroid Gland
In humans, four parathyroid glands are present on the back side of the
thyroid gland, one pair each in the two lobes of the thyroid gland (Figure
22.3b). The parathyroid glands secrete a peptide hormone called
parathyroid hormone (PTH). The secretion of PTH is regulated by the
circulating levels of calcium ions.
Parathyroid hormone (PTH) increases the Ca
levels in the blood. PTH
acts on bones and stimulates the process of bone resorption (dissolution/
demineralisation). PTH also stimulates reabsorption of Ca
by the renal
tubules and increases Ca
absorption from the digested food. It is, thus,
clear that PTH is a hypercalcemic hormone, i.e., it increases the blood
levels. Along with TCT, it plays a significant role in calcium balance
in the body
22.2.6 Thymus
The thymus gland is a lobular structure located between lungs behind
sternum on the ventral side of aorta. The thymus plays a major role in
the development of the immune system. This gland secretes the peptide
hormones called thymosins. Thymosins play a major role in the
differentiation of T-lymphocytes, which provide cell-mediated
immunity. In addition, thymosins also promote production of antibodies
to provide humoral immunity. Thymus is degenerated in old individuals
resulting in a decreased production of thymosins. As a result, the immune
responses of old persons become weak.