CHEMICAL COORDINATION AND INTEGRATION 331
C HAPTER 22
C HEMICAL C OORDINATION
AND INTEGRATION
22.1 Endocrine You have already learnt that the neural system pr ovides a
Glands and point-to-point rapid coordination among organs. The neural
Hormones 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
22.2 Human
regulated; a special kind of coordination and integration has to be
Endocrine
provided. This function is carried out by hormones. The neural system
System
and the endocrine system jointly coordinate and regulate the
22.3 Hormones of physiological functions in the body.
Heart, Kidney
and
22.1 ENDOCRINE GLANDS AND HORMONES
Gastrointestinal
Tract
Endocrine glands lack ducts and are hence, called ductless glands. Their
22.4 Mechanism of secretions are called hormones. The classical definition of hormone as a
Hormone Action 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.
2020-21
,332 BIOLOGY
22.2 HUMAN ENDOCRINE SYSTEM
The endocrine glands and hormone
Hypothalamus Pineal producing diffused tissues/cells located
Pituitary in different parts of our body constitute
the endocrine system. Pituitary, pineal,
Thyroid and thyroid, adrenal, pancreas, parathyroid,
Parathyroid
thymus and gonads (testis in males and
ovary in females) are the organised
Thymus endocrine bodies in our body
(Figure 22.1). In addition to these, some
other organs, e.g., gastrointestinal tract,
liver, kidney, heart also produce
Pancreas
hormones. A brief account of the
Adrenal structure and functions of all major
endocrine glands and hypothalamus
of the human body is given in the
following sections.
Ovary
(in female)
Testis 22.2.1 The Hypothalamus
(in male)
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
Figure 22.1 Location of endocrine glands
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 are 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).
2020-21
, CHEMICAL COORDINATION AND INTEGRATION 333
22.2.2 The Pituitary Gland Hypothalamus
The pituitary gland is located in a bony cavity Hypothalamic
neurons
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), Portal circulation
adrenocorticotrophic hormone (ACTH),
luteinizing hormone (LH) and follicle
stimulating hormone (FSH). Pars intermedia Posterior
pituitary
secretes only one hormone called melanocyte
stimulating hormone (MSH). However, in
humans, the pars intermedia is almost merged Anterior
with pars distalis. Neurohypophysis (pars pituitary
nervosa) also known as posterior pituitary, stores Figure 22.2 Diagrammatic representation of
and releases two hormones called oxytocin and pituitary and its relationship with
hypothalamus
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
2020-21
C HAPTER 22
C HEMICAL C OORDINATION
AND INTEGRATION
22.1 Endocrine You have already learnt that the neural system pr ovides a
Glands and point-to-point rapid coordination among organs. The neural
Hormones 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
22.2 Human
regulated; a special kind of coordination and integration has to be
Endocrine
provided. This function is carried out by hormones. The neural system
System
and the endocrine system jointly coordinate and regulate the
22.3 Hormones of physiological functions in the body.
Heart, Kidney
and
22.1 ENDOCRINE GLANDS AND HORMONES
Gastrointestinal
Tract
Endocrine glands lack ducts and are hence, called ductless glands. Their
22.4 Mechanism of secretions are called hormones. The classical definition of hormone as a
Hormone Action 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.
2020-21
,332 BIOLOGY
22.2 HUMAN ENDOCRINE SYSTEM
The endocrine glands and hormone
Hypothalamus Pineal producing diffused tissues/cells located
Pituitary in different parts of our body constitute
the endocrine system. Pituitary, pineal,
Thyroid and thyroid, adrenal, pancreas, parathyroid,
Parathyroid
thymus and gonads (testis in males and
ovary in females) are the organised
Thymus endocrine bodies in our body
(Figure 22.1). In addition to these, some
other organs, e.g., gastrointestinal tract,
liver, kidney, heart also produce
Pancreas
hormones. A brief account of the
Adrenal structure and functions of all major
endocrine glands and hypothalamus
of the human body is given in the
following sections.
Ovary
(in female)
Testis 22.2.1 The Hypothalamus
(in male)
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
Figure 22.1 Location of endocrine glands
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 are 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).
2020-21
, CHEMICAL COORDINATION AND INTEGRATION 333
22.2.2 The Pituitary Gland Hypothalamus
The pituitary gland is located in a bony cavity Hypothalamic
neurons
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), Portal circulation
adrenocorticotrophic hormone (ACTH),
luteinizing hormone (LH) and follicle
stimulating hormone (FSH). Pars intermedia Posterior
pituitary
secretes only one hormone called melanocyte
stimulating hormone (MSH). However, in
humans, the pars intermedia is almost merged Anterior
with pars distalis. Neurohypophysis (pars pituitary
nervosa) also known as posterior pituitary, stores Figure 22.2 Diagrammatic representation of
and releases two hormones called oxytocin and pituitary and its relationship with
hypothalamus
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
2020-21
