NCERT Textbook - Chemical Coordination and Integration NEET Notes | EduRev

Biology Class 11

Created by: Sushil Kumar

NEET : NCERT Textbook - Chemical Coordination and Integration NEET Notes | EduRev

 Page 1


330 BIOLOGY
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.
22.1 ENDOCRINE GLANDS AND HORMONES
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.
CHEMICAL COORDINATION
AND INTEGRATION
CHAPTER  22
22.1 Endocrine
Glands and
Hormones
22.2 Human
Endocrine
System
22.3 Hormones of
Heart, Kidney
and
Gastrointestinal
Tract
22.4 Mechanism of
Hormone Action
2015-16(19/01/2015)
Page 2


330 BIOLOGY
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.
22.1 ENDOCRINE GLANDS AND HORMONES
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.
CHEMICAL COORDINATION
AND INTEGRATION
CHAPTER  22
22.1 Endocrine
Glands and
Hormones
22.2 Human
Endocrine
System
22.3 Hormones of
Heart, Kidney
and
Gastrointestinal
Tract
22.4 Mechanism of
Hormone Action
2015-16(19/01/2015)
CHEMICAL COORDINATION AND I NTEGRATION 331
22.2 HUMAN ENDOCRINE SYSTEM
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 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).
Figure 22.1 Location of endocrine glands
Testis
(in male)
Ovary
(in female)
Adrenal
Pancreas
Thyroid and
Parathyroid
Thymus
Pineal
Pituitary
Hypothalamus
2015-16(19/01/2015)
Page 3


330 BIOLOGY
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.
22.1 ENDOCRINE GLANDS AND HORMONES
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.
CHEMICAL COORDINATION
AND INTEGRATION
CHAPTER  22
22.1 Endocrine
Glands and
Hormones
22.2 Human
Endocrine
System
22.3 Hormones of
Heart, Kidney
and
Gastrointestinal
Tract
22.4 Mechanism of
Hormone Action
2015-16(19/01/2015)
CHEMICAL COORDINATION AND I NTEGRATION 331
22.2 HUMAN ENDOCRINE SYSTEM
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 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).
Figure 22.1 Location of endocrine glands
Testis
(in male)
Ovary
(in female)
Adrenal
Pancreas
Thyroid and
Parathyroid
Thymus
Pineal
Pituitary
Hypothalamus
2015-16(19/01/2015)
332 BIOLOGY
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 melanocyte
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. 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 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
Posterior
pituitary
Anterior
pituitary
Hypothalamus
Hypothalamic
neurons
Portal circulation
Figure 22.2 Diagrammatic representation of
pituitary and its relationship with
hypothalamus
2015-16(19/01/2015)
Page 4


330 BIOLOGY
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.
22.1 ENDOCRINE GLANDS AND HORMONES
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.
CHEMICAL COORDINATION
AND INTEGRATION
CHAPTER  22
22.1 Endocrine
Glands and
Hormones
22.2 Human
Endocrine
System
22.3 Hormones of
Heart, Kidney
and
Gastrointestinal
Tract
22.4 Mechanism of
Hormone Action
2015-16(19/01/2015)
CHEMICAL COORDINATION AND I NTEGRATION 331
22.2 HUMAN ENDOCRINE SYSTEM
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 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).
Figure 22.1 Location of endocrine glands
Testis
(in male)
Ovary
(in female)
Adrenal
Pancreas
Thyroid and
Parathyroid
Thymus
Pineal
Pituitary
Hypothalamus
2015-16(19/01/2015)
332 BIOLOGY
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 melanocyte
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. 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 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
Posterior
pituitary
Anterior
pituitary
Hypothalamus
Hypothalamic
neurons
Portal circulation
Figure 22.2 Diagrammatic representation of
pituitary and its relationship with
hypothalamus
2015-16(19/01/2015)
CHEMICAL COORDINATION AND I NTEGRATION 333
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).
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
4
) and
triiodothyronine (T
3
). 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 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.
Figure 22.3 Diagrammatic view of the
position of Thyroid and
Parathyroid
(a) Ventral side
(b) Dorsal side
2015-16(19/01/2015)
Page 5


330 BIOLOGY
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.
22.1 ENDOCRINE GLANDS AND HORMONES
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.
CHEMICAL COORDINATION
AND INTEGRATION
CHAPTER  22
22.1 Endocrine
Glands and
Hormones
22.2 Human
Endocrine
System
22.3 Hormones of
Heart, Kidney
and
Gastrointestinal
Tract
22.4 Mechanism of
Hormone Action
2015-16(19/01/2015)
CHEMICAL COORDINATION AND I NTEGRATION 331
22.2 HUMAN ENDOCRINE SYSTEM
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 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).
Figure 22.1 Location of endocrine glands
Testis
(in male)
Ovary
(in female)
Adrenal
Pancreas
Thyroid and
Parathyroid
Thymus
Pineal
Pituitary
Hypothalamus
2015-16(19/01/2015)
332 BIOLOGY
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 melanocyte
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. 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 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
Posterior
pituitary
Anterior
pituitary
Hypothalamus
Hypothalamic
neurons
Portal circulation
Figure 22.2 Diagrammatic representation of
pituitary and its relationship with
hypothalamus
2015-16(19/01/2015)
CHEMICAL COORDINATION AND I NTEGRATION 333
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).
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
4
) and
triiodothyronine (T
3
). 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 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.
Figure 22.3 Diagrammatic view of the
position of Thyroid and
Parathyroid
(a) Ventral side
(b) Dorsal side
2015-16(19/01/2015)
334 BIOLOGY
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
2+
 levels in the blood. PTH
acts on bones and stimulates the process of bone resorption (dissolution/
demineralisation). PTH also stimulates reabsorption of Ca
2+
 by the renal
tubules and increases Ca
2+
 absorption from the digested food. It is, thus,
clear that PTH is a hypercalcemic hormone, i.e., it increases the blood
Ca
2+
 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.
22.2.7 Adrenal Gland
Our body has one pair of adrenal glands, one at the anterior part of each
kidney (Figure 22.4 a). The gland is composed of two types of tissues.
The centrally located tissue is called the adrenal medulla, and outside
this lies the adrenal cortex (Figure 22.4 b).
2015-16(19/01/2015)
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