Endocrine System - The Endocrine System Function like the...

The Endocrine System
Function - like the nervous system it is a controlling system. It's effects are slower and
more prolonged.
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Influences metabolic activities of cells by means of hormones.
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Controls and integrates: reproduction; growth and development; mobilization of
body defenses against stressors; maintenance of electrolyte, water and nutrient
balance of blood; regulation of cellular metabolism and energy balance.
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Endocrine organs are scattered about the body and are quite small.
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Endocrine glands are ductless, release hormones into surrounding tissue rich in
vascular and lymph vessels which circulate these chemical messengers.
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Endocrine glands include: pituitary, thyroid, parathyroid, adrenal, pineal, pancreas
and thymus glands.
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Several organs produce hormones as well as other products; pancreas,
hypothalmus, and gonads (ovaries and testes).
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A few organs have pockets of hormone producing cells; small intestine, stomach,
kidneys and heart.
HORMONES
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Definition: chemical substances secreted by cells into extra-cellular fluids, that
regulate the metabolic function of other cells in the body.
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Classification:
1. Amino acid based - from simple amino acid derivatives to peptides to
proteins.
Cannot enter lipid bi-layer of cell membrane, they do not enter the cell,
rather they act through second messenger systems.
Examples: most all hormones.
2. Steroid - synthesized from cholesterol. They can enter lipid bi-layer of cell
membrane, binds to DNA associated receptor and turns on a gene. Result
of synthesis of a protein in the cell.
Examples: gonadal hormones, adrenocortical hormones.
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Mechanisms of Hormone Action
1. Changes in plasma membrane permeability and or electrical state by
opening or closing ion channels.
2. Synthesis of proteins or certain regulatory molecules within the cell.
3. Enzyme activation or deactivation
4. Induction of secretory activity

5. Stimulation of mitosis
Hormone Target Cell Specificity
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Target cells - hormones circulate to virtually all tissues but influence activity of
only certain cells, referred to as its target cells.
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Receptors - located on plasma membrane or in interior of cell, hormone can bind
to it. For a target cell to respond to a hormone it must have receptors specific for
the hormone.
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Target cell activation by hormone depends upon three factors:
Control of Hormone Release
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Endocrine glands are stimulated to manufacture and release their hormones by
three major types of stimuli:
1. Humoral - stimulated by changing blood levels of certain ions and
nutrients.
EX: parathyroid glands release PTH when blood calcium declines.