330 BIOLOGY
CHAPTER 22
CHEMICAL COORDINATION
AND INTEGRATION
22.1 Endocrine You have already lear nt that the neural system provides 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 regulation 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.
, CHEMICAL COORDINATION AND INTEGRATION 331
22.2 HUMAN ENDOCRINE SYSTEM
The endocrine glands and hormone
producing diffused tissues/cells located
in different parts of our body constitute Hypothalamus Pineal
the endocrine system. Pituitary, pineal,
Pituitary
thyroid, adrenal, pancreas, parathyroid,
thymus and gonads (testis in males and
Thyroid and
ovary in females) are the organised Parathyroid
endocrine bodies in our body
(Figure 22.1). In addition to these, some
Thymus
other organs, e.g., gastrointestinal tract,
liver, kidney, heart also produce
hormones. A brief account of the
structure and functions of all major Pancreas
endocrine glands and hypothalamus of Adrenal
the human body is given in the following
sections.
22.2.1 The Hypothalamus
As you know, the hypothalamus is the Ovary
Testis
(in female)
basal part of diencephalon, forebrain (in male)
(Figure 22.1) and it regulates a wide
spectrum of body functions. It contains
several groups of neurosecretory cells
called nuclei which produce hormones. Figure 22.1 Location of endocrine glands
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
hypothlamic 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).
CHAPTER 22
CHEMICAL COORDINATION
AND INTEGRATION
22.1 Endocrine You have already lear nt that the neural system provides 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 regulation 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.
, CHEMICAL COORDINATION AND INTEGRATION 331
22.2 HUMAN ENDOCRINE SYSTEM
The endocrine glands and hormone
producing diffused tissues/cells located
in different parts of our body constitute Hypothalamus Pineal
the endocrine system. Pituitary, pineal,
Pituitary
thyroid, adrenal, pancreas, parathyroid,
thymus and gonads (testis in males and
Thyroid and
ovary in females) are the organised Parathyroid
endocrine bodies in our body
(Figure 22.1). In addition to these, some
Thymus
other organs, e.g., gastrointestinal tract,
liver, kidney, heart also produce
hormones. A brief account of the
structure and functions of all major Pancreas
endocrine glands and hypothalamus of Adrenal
the human body is given in the following
sections.
22.2.1 The Hypothalamus
As you know, the hypothalamus is the Ovary
Testis
(in female)
basal part of diencephalon, forebrain (in male)
(Figure 22.1) and it regulates a wide
spectrum of body functions. It contains
several groups of neurosecretory cells
called nuclei which produce hormones. Figure 22.1 Location of endocrine glands
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
hypothlamic 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).
