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The Endocrine System
What is it?
 Includes all the endocrine cells and tissues of the
body
 Endocrine cells: glandular secretory cells that
release secretions into extracellular fluid.
 The released chemicals may affect only adjacent cells, or
cells throughout the body.
Organs in the Endocrine System
 Organs of the endocrine system: Hypothalamus,
Pituitary Gland, Thyroid Gland, Thymus, Adrenal
Glands, Pineal Gland, Parathyroid Glands, Heart,
Kidney, Digestive Tract, Pancreatic Islets, and
Gonads.
 Release hormones, which are chemical messengers
that are released in one tissue and transported in the
blood stream to reach specific cells in other tissues.
Communication
 Direct Communication: occurs between cells of the
same type that are in physical contact
 Paracrine Communication: The use of chemical
messengers to transfer information from cell to cell
within a single tissue
 Endocrine Communication: The use of hormones
to coordinate cellular activities in tissues in distant
portions of the body
 Synaptic Communication: Chemical
communication used by the nervous system
Hormones
 Hormones can be divided into three groups based
on chemical structure:
 Amino Acid Derivatives
 Peptide hormones
 Lipid derivatives
Amino Acid Derivatives
 Structurally similar to Amino Acids
 Sometimes called biogenic amines
 Includes epinephrine, norepinephrine, dopamine,
thyroid hormones, and melatonin.
Peptide hormones
 Chains of amino acids
 Usually produced as Prohormones: inactive
molecules that are converted to active hormones
before or after they are secreted.
 2 groups
 Group 1: All hormones secreted by the
hypothalamus, posterior pituitary gland, heart,
thymus, digestive tract, pancreas, and most
hormones in the anterior pituitary gland.
 Group 2: Glycoproteins: proteins that are over 200
amino acids long and have carbohydrate side chains.
Most of these hormones are secreted by the anterior
pituitary gland and other organs
Lipid Derivatives
 2 classes
 Steriod Hormones: derived from cholesterol
 Released by reproductive organs, adrenal glands, and
kidneys
 Bound to specific transport proteins in plasma and
remain in circulation longer than peptide hormones
 Eicosanoids: derived from arachidonic acid, a 20carbon fatty acid
 Small molecules with a five-carbon ring at one end
 Coordinate cellular activities that affect enzymatic
processes that occur in extracellular fluids
Hormone distribution
and transport
 Hormones may circulate freely in the blood or they
may be bound to special carrier proteins
 A freely circulating hormone is only functional for
up to one hour, and possibly for only 2 minutes
 It becomes inactive when it diffuses out of the blood
stream, it is absorbed or broken down by cells of the
liver or kidneys, or it is broken down by plasma
 Thyroid and steroid hormones remain in circulation
for much longer because they become attached to
special transport proteins after being released.
Hormone Function
 Stimulate the synthesis of enzymes or structural
proteins by activating appropriate genes in the cell
nucleus
 Turn existing enzymes “on” or “off” by changing
shape and structure
 Increase or decrease the rate of synthesis of a
particular enzyme or other protein by changing the
rate of transcription or translation
Endocrine Reflexes
 Functional counterparts of neural reflexes
 Triggered by:
 Humoral stimuli: changes in composition of
extracellular fluid
 Hormonal stimuli: the arrival or removal of a specific
hormone
 Neural stimuli: arrival of neurotransmitter at
neuroglandular junctions
 Controlled by negative feedback responses
 A stimulus triggers the production of a hormone
whose effects reduce the intensity of the stimulus.
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