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ENDOCRINOLOGY
Function
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The nervous system coordinates rapid and precise
responses to stimuli using action potentials.
The endocrine system maintains homeostasis and longterm control using chemical signals.
The endocrine system works in parallel with the nervous
system to control growth and maturation along with
homeostasis.
Locations
Hormones
 The endocrine system is a collection of glands
(ductless gland) that secrete chemical messengers hormones.
 Hormones -chemical messengers secreted into blood
or extracellular fluid by one cell that affect the
functioning of other cells.
 These signals are passed through the blood to arrive
at a target organ to produce its effect.
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hormones circulate in blood
Affects only a limited number of cells-called target
cells
Target cell contain receptor on it.
Receptors may be on the cell membranes or
cytoplasm,
Cells without receptor does not respond.
Some hormones-more than one target eg. Insulinskeletal muscle and liver.
Hormones are grouped into three classes
based on their structure:
1. Steroids
2. Peptides
3. Amines
Synthesis, Storage, and Secretion
Steroid hormones
 derived from cholesterol
 Lipid soluble-can diffuse through cell membranes.
 Receptors in cytoplasm or nucleus.
 Steroid hormone are not stored in the gland
 Soon after synthesis they are released into blood
 Production is based on its need
 Acts slowler but last longer.
 Secreted by gonads and adrenal cortex
 Testosterone, estrogen
Peptide hormones
 are synthesized as precursor molecules
 processed by endoplasmic reticulum & Golgi
 are stored in secretory granules.
 When needed granules are released
into bloodstream.
Receptors on cell membrane
Secreted by pituitary, parathyroid, kidneys.
most hormones are peptides.
Amine hormones
 are stored as granules in cytoplasm until needed.
 Are derived from the amino acid – tyrosine
 Are secreted by thyroid and adrenal medulla.
Endocrine Systems Feedback regulations
Mechanism that controls release and plasma levels of
hormones
1. Negative Feedback regulation
2. Positive Feedback regulation
Negative:
The rising level of hormone bring about biologic
response that inhibits further hormonr release.
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Eg. Increased glucose concentration in the blood-beta
cells produces insulin-more glucose uptake by skeletal
muscles- blood glucose level decreases-stimulus for
insulin release is removed-less insulin released-normal
blood glucose level.
Positive
Less common.
Presence of hormone produces about biologic response
that produces further release of the hormone.
Eg. Entry of fetus in the birth canal-Increased release of
oxytocin-contraction of uterine wall-further dilatation of
cervix-further release of oxytocin-Parturition.
Mechanisms of Hormone Action
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Hormones act by binding to receptor
Hormone-receptor complex cause – response
Based on hormone type 2 mechanisms seen
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2.
Nonsteroid (peptides and Amines)
Steroid hormone
Nonsteroid Hormones
 Nonsteroid hormones-water soluble
 Do not enter the cell but bind to plasma membrane
receptors, generating a chemical signal (second
messenger) inside the target cell.
 Five different second messenger chemicals, have been
identified.
 Second messengers activate other intracellular chemicals
to produce the target cell response.
Steroid Hormones
 Hormones pass through the plasma membrane
 act in a two-step process.
 Steroid hormones bind, once inside the cell, to the
nuclear membrane receptors, producing an activated
hormone-receptor complex.
 The activated hormone-receptor complex binds to DNA
and activates specific genes, increasing production of
proteins.
The Nervous and Endocrine Systems
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The pituitary gland (often called the master gland) is
located in a small bony cavity at the base of the brain.
A stalk links the pituitary to the hypothalamus, which
controls release of pituitary hormones.
Two lobes: the anterior and posterior lobes.
Anterior pituitary is glandular.
The hypothalamus contains neurons that control releases
from the anterior pituitary.
Seven hypothalamic hormones are released into a portal
system connecting the hypothalamus and pituitary, and
cause targets in the pituitary to release eight hormones.
The Adrenal Glands
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Each kidney has an adrenal gland located above it.
Divided into an inner medulla and an outer cortex.
The medulla synthesizes amine hormones, the cortex
secretes steroid hormones.
The Thyroid Gland
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The thyroid gland is located in the neck.
Follicles in the thyroid secrete thyroglobulin, a
storage form of thyroid hormone.
Calcitonin is also secreted by large cells in the
thyroid; it plays a role in regulation of calcium
The Pancreas
 The pancreas contains exocrine cells that secrete
digestive enzymes into the small intestine and clusters
of endocrine cells (the pancreatic islets).
 The islets secrete insulin (uptake and storage of
glucose) and glucagon (converts into glucose), which
regulate blood glucose levels.
 Diabetes results from inadequate levels of insulin.
Type I diabetes -inadequate levels of insulin secretion,
often due to a genetic cause.
 Type II -loss of response of targets to insulin rather
than lack of insulin causes this type of diabetes.
Other endocrine glands and hormones
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Other organs with some hormonal activity as part of
their function-the thymus, stomach, small intestines,
heart, and placenta.
Thymosin, produced by the thymus gland, plays an
important role in the development of the body's
immune system.
The gastric mucosa, produces a hormone-gastrin, in
response to the presence of food in the stomachStimulates the production of hydrochloric acid and
the enzyme pepsin, which are used in the digestion of
food.
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The mucosa of the small intestine -secretin and
cholecystokinin. Secretin- stimulates the pancreas to
produce a bicarbonate-rich fluid that neutralizes the
stomach acid. Cholecystokinin stimulates contraction
of the gallbladder, which releases bile. It also
stimulates the pancreas to secrete digestive enzyme.
The placenta - human chorionic gonadotropin, which
signals the mother's ovaries to secrete hormones to
maintain the uterine lining so that it does not
degenerate and slough off in menstruation.