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SYSTEM PHYSIOLOGY-ANIMAL
Part J: ENDOCRINOLOGY
Introduction
The branch of physiology and medicine concerned with endocrine glands and
hormones. Circulating or local hormones of the endocrine system contribute to homeostasis
by regulating the activity and growth of target cells in your body. Your metabolism is also
controlled by hormones.
Definitions:
EXOCRINE GLAND: (exo-outside) secrete their products into ducts that carry the secretions
into body cavities, into the lumen of an organ, or to the outer surface of the body. Exocrine
glands include sudoriferous (sweat), sebaceous (oil), mucous, and digestive glands
ENDOCRINE GLAND: (endo- within) secrete their products (hormones) into the interstitial
fluid surrounding the secretory cells rather than into ducts
ENDOCRINOLOGY: Study of endocrine glands and their secretions.
HORMONE: is a mediator molecule that is released in one part of the body but regulates the
activity of cells in other parts of the body.
AUTOCRINE: Relates to hormone having an effect on its
own immediate source.
PARACRINE: Relates to hormone’s action on nearby target
cells e.g. within immediate area around source.
ENDOCRINE: Relates to hormone’s action on target cells
at a distance from source.
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SYSTEM PHYSIOLOGY-ANIMAL
Part J: ENDOCRINOLOGY
HORMONE FUNCTIONS:
Functions of Hormones
1.
4.
Help regulate
•
Chemical composition and volume of internal environment
•
Metabolism and energy balance
•
Contraction of smooth and cardiac muscle fibres
•
Glandular secretions
•
Some immune system activities
2.
Control growth and development
3.
Regulate operation of reproductive systems
Help establish circadian rhythms.
Chemical Classes of Hormones:
Chemically, hormones can be divided into two broad classes: those that are soluble in lipids,
and those that are soluble in water.
1. Lipid-soluble Hormones
1.1. Steroid hormones are derived from cholesterol. Each
steroid hormone is unique due to the presence of
different chemical groups attached at various sites on
the four rings at the core of its structure.
1.2. Thyroid hormone (T3 and T4) are synthesized by
Figure 1 Examples of steroid hormones
attaching iodine to the amino acid tyrosine. The
benzene ring of tyrosine plus the attached iodine make T3 and T4 very lipid soluble.
1.3. NO (Gas) hormones NO are both a hormone and a neurotransmitter. Its synthesis is
catalysed by the enzyme nitric oxide synthase.
2. Water-soluble Hormones
2.1. Amine hormones are synthesized by decarboxylation (removing a molecule of CO2)
and otherwise modifying certain amino acids. They are called amines because they
retain an amino group (NH3). The catecholamine, epinephrine, norepinephrine,
serotonin and dopamine.
2.2. Peptide hormones and protein hormones are amino acid polymers. May be smaller
peptide hormones consist of chains of 3 to 49 amino acids or may be the larger protein
hormones include 50 to 200 amino acids. Examples are all hormones from Pituitary
Gland and Insulin and Glucagon.
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SYSTEM PHYSIOLOGY-ANIMAL
Part J: ENDOCRINOLOGY
2.3. Eicosanoid hormones (eicos-twenty forms; -oid-resembling) are derived from
arachidonic acid, a 20-carbon fatty acid. The two major types of eicosanoids are
prostaglandins and leukotrienes. The eicosanoids are important local hormones, and
they may act as circulating hormones as well.
Important facts:
Love and hormones: falling in love and having babies change hormonal levels in both men and women! When women fall in love,
their testosterone levels spike but they produce less estrogen. For men, it's the other way around. This is nature's way of reducing
the differences between the sexes, making them more compatible, and the hormone levels return to their normal state about one to
three years into the sexual desire. When women give birth, their estrogen levels plummet. This can cause them to develop
depressions that disappear only once the hormone level is restored. The dads on the other hand will have less testosterone than
usual: that makes them more caring and loving towards the new-born baby.
Action of Lipid-soluble Hormones:

Hormone molecule diffuses from the blood, through
the lipid bilayer of the plasma membrane into a cell.
 If the cell is a target cell, the hormone binds to and
activates receptors located within the cytosol or nucleus.
The activated receptor–hormone complex then alters
gene expression: It turns specific genes of the nuclear
DNA on or off.
 After transcription of genes, new messenger RNA
(mRNA) directs synthesis of a new protein, often an
enzyme, on the ribosomes.
 The new proteins alter the cell’s activity and cause the
Figure 2 Action of Lipid Soluble Hormone : taken from
th
Principle of Anatomy and Physiology Tortora 12 Edition
responses typical of that hormone.
Action of Water-soluble Hormones:

A water-soluble hormone (the first messenger) diffuses from the blood through interstitial
fluid and then binds to its receptor at the exterior surface of a target cell’s plasma
membrane.
 The hormone–receptor complex activates a membrane protein called a G protein. The
activated G protein in turn activates adenylate cyclase.

Adenylate cyclase converts ATP into cyclic AMP (cAMP). Because the enzyme’s active
site is on the inner surface the plasma membrane, this reaction occurs in the cytosol of cell.
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