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An Introduction to Endocrinology

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ENDOCRINOLOGY
BPH, 1 st semester

An Introduction to Endocrinology
E ndocrine system : N ervous system being the first it is the
second great control system of the body, which interacts with the
nervous system to coordinate and integrate the activity of body
cells.
 The nervous system regulates the activity of muscles and
glands via electrochemical impulses delivered by neurons ,
and those organs respond within milliseconds .
 The endocrine system influences metabolic activity by means
of hormones (hormon =to excite ), which are chemical
messengers released into the blood to be transported
throughout the body. Binding of a hormone to cellular receptors
initiates responses that typically occur after a lag period of
seconds or even days.
 But, once initiated, those responses tend to be much more
prolonged than those induced by the nervous system .

The Endocrine System: An Overview
• T wo kinds of glands: Endocrine & Exocrine
• Exocrine glands:
– P roduce nonhormonal substances , such as sweat and saliva.
– H ave ducts through which these substances are moved to a
membrane surface.
• Endocrine glands ,
– A lso called ductless glands
– produce hormones and lack ducts .
– They release their hormones into the surrounding tissue fluid
(endo = within; crine = to secrete ),
– T ypically have a rich vascular and lymphatic drainage that
receives their hormones.
Most of the hormone -producing cells in endocrine glands are arranged
in cords and branching network, a situation that maximizes contact
between them and the capillaries surrounding them.

Fig. Location of the major
endocrine organs of the body .
• The hypothalamus, along
with its neural functions,
produces and releases
hormones, so we can consider
the hypothalamus a
neuroendocrine organ
• Besides the major endocrine
organs, various other tissues
and organs produce
hormones, adipose cells
release leptin , and pockets
of hormone -producing cells
are found in the walls of the
small intestine, stomach,
kidneys, and heart

Functional division of endocrine glands
1. Endocrine Glands :
 Endocrine hormones are long -distance chemical signals
that travel in blood or lymph throughout the body.
2 . Autocrine Glands :
 Autocrine hormones are chemicals that exert their effects
on the same cells that secrete them .
 For example, certain prostaglandins released by smooth
muscle cells cause those smooth muscle cells to contract .
3. Paracrine Glands:
 A ct locally on the adjacent cells but affect cell types
other than those releasing the paracrine chemicals .
 For example, somatostatin released by one population
of pancreatic cells inhibits the release of insulin by a
different population of pancreatic cells .
Note: Certain tumor cells, such as those of some cancers of the lung or
pancreas, synthesize hormones identical to those made in normal endocrine
glands

Objectives of endocrinology

• Identify the gland or tissue that is producing the
hormone.
• Identify the hormone that is produced.
• Identify the target for that hormone (where is that
hormone going in the body?).
• Identify the function of that hormone (what is that
hormone going to cause the body to do?).

• For maintaining homeostasis, two major system come into play.
• The autonomic nervous system is concerned with rapid
changes, while hormones of the endocrine system are mainly
involved in slower and more precise adjustments.
• The endocrine system consists of glands widely separated from
each other with no direct anatomical links.
• Endocrine glands consist of groups of secretory cells surrounded
by an extensive network of capillaries which facilitates diffusion
of hormones (chemical messengers) directly from the secretory
cells into the bloodstream; ductless gland.
• Two major types of hormone based on chemical nature:
– A mino acids based (amines , polypeptides and proteins).
– C holesterol -based lipids (steroids ).

Position of major
endocrine glands
The receptors for water -soluble
hormones are situated on the cell
membrane and those for lipid -
soluble hormones
are inside the cell.
Cholesterol derived Aminoacid derived

1.The Pituitary Gland ( Hypophysis )

Regulations of hormone
secretions :

Negative feedback
mechanism is the primary
regulatory mechanism for
secretion of almost all
hormones in the body.

The axis termed as
Hypothalamo -pituitary -adrenal
axis is one of the example for
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Introduction to Endocrinology

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