Endocrine System
Chapter 11
Endocrine System - Overview
• Endocrine System – the body’s second great
controlling system which influences metabolic
activities of cells by means of hormones
• Endocrine glands – pituitary, thyroid,
parathyroid, adrenal, pineal, and thymus
• The pancreas and gonads produce both
hormones and exocrine products
Endocrine System - Overview
• The hypothalamus has both neural functions
and releases hormones
• Other tissues and organs that produce
hormones – adipose cells, pockets of cells in
the walls of the small intestine, stomach,
kidneys, and heart
Endocrine vs. Exocrine
• Endocrine secretions – secrete hormones
directly into body fluids (ductless)
• Exocrine secretions – reach some internal or
external body surface through ducts (contain
ducts)
Endocrine System
• It is a communication system
• Relies completely on hormones released into
the blood
• Cells have receptors for the hormones to act
on
• Controls rates of certain chemical reactions
• Aids in transport of substances across
membranes
• Helps regulate water & electrolyte balances
• Plays vital roles in reproduction, development,
& growth
Hormone Action
• Hormone – biochemical that a cell secretes
that affects the functions of another cell
• Each particular hormones physiological action
is restricted to the hormone’s target cells cells with specific receptors for the hormone
molecule
• Target cells have receptors that other cells lack
• Made up of proteins or glycoproteins w/
binding sites for a specific hormone
Hormone Action
• Even in extremely low concentrations,
hormones can stimulate changes in target
cells
• Either steroids or steroid-like substances
• Synthesized from cholesterol OR…
• They are amines, peptides, proteins, or
glycoproteins synthesized from amino acids
Control of Hormonal Secretions
• Negative feedback systems control many
hormonal secretions
• The endocrine gland (or system controlling the
release of the hormone) is sensitive to the
concentration of the substance
• When the concentration reaches a certain
level, the gland is inhibited and the secretion
decreases
Control of Hormonal Secretions
• As the concentration of the hormone
decreases, the inhibition of the gland ceases
so the hormone can be secreted again
• Hormone secretion is controlled in 3 ways 
all of which employ negative feedback
Control Sources
• Hypothalamus
– Controls the anterior pituitary gland’s release of
hormones that stimulate other glands to release
hormones
• Nervous system
– Adrenal medulla secretes its hormones in
response to sympathetic nerve impulses
• Changes in the internal environment
– Ex) when blood glucose levels rise, pancreas
secretes insulin; when they fall, glucagon is
secreted
Pituitary Gland
• Also known as the hypophysis, located at the
base of the brain
• Consists of an anterior pituitary & a posterior
pituitary
Anterior Pituitary Hormones
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Growth Hormone
Prolactin
Thyroid-stimulating Hormone
Adrenocorticotropic Hormone
Gonadotropins
Growth Hormone (GH)
• Stimulates cells to increase in size and more
rapidly divide
• Enhances the movement of amino acids
across cell membranes
– Speeds the rate that cells utilize carbohydrates &
fats
Prolactin (PRL)
• Stimulates and sustains a woman’s milk
production following birth
• In males – unknown function
Thyroid-stimulating Hormone (TSH)
• Stimulates the normal development and
secretory activity of the thyroid
• Rising blood levels of thyroid hormones act on
the pituitary and hypothalamus to block the
release of TSH
Adrenocorticotropic Hormone (ACTH)
• Stimulates secretion of glucocorticoids by the
adrenal gland
Gonadotropins
• Follicle-stimulating hormone (FSH)
• Luteinizing hormone (LH)
• In males:
– LH stimulates interstitial cells of the testes to
produce testosterone
– FSH stimulates production of sperm
• In females:
– FSH stimulates follicle development in ovaries
– LH triggers ovulation
– LH promotes synthesis & release of estrogens and
progesterone
Posterior Pituitary Hormones
• Oxytocin (OT)
– Strong stimulant of uterine contraction
– Triggers milk ejection
– Synthetic & natural oxytocic drugs are used to
induce or hasten labor
• Antidiuretic Hormone (ADH)
– Helps to avoid dehydration or water overload
Thyroid Gland
• Largest endocrine gland, located in the
anterior neck (around trachea & below neck)
• Produces and releases:
– Thyroid hormone
– Calcitonin
Thyroid Hormone (TH)
• Regulates tissue growth
• Developing skeletal & nervous systems
Calcitonin
• Lowers blood calcium levels in children
• Targets the skeleton where it:
– Inhibits osteoclast activity and release of calcium
from the bone matrix
– Stimulates calcium uptake and incorporation in
the bone matrix
Parathyroid Glands
• Tiny glands embedded in the posterior side of
the thyroid, usually there are four (superior &
inferior gland on each of the thyroid’s lobes)
• Secretes parathyroid hormone (PTH)
– Increases Ca2+ in the blood
– Helps break down bone to add Ca2+ into the blood
Adrenal Glands
• Paired, pyramid-shaped organs above the
kidneys
• Structurally & functionally, they are two
glands in one
– Adrenal medulla
– Adrenal cortex
Hormones of the Adrenal Medulla
• Epinephrine (adrenaline)
• Norepinephrine (noradrenaline)
• Secretions of these hormones causes:
– Blood glucose levels to rise
– Blood vessels to constrict
– The heart to beat faster
– Blood to be diverted to the brain, heart, and
skeletal muscle
Hormones of the Adrenal Cortex
• Mineralocorticoids – regulate electrolytes in
extracellular fluids (salt concentration)
– Aldosterone: maintains Na+ balance
• Glucocorticoids (Cortisol)
– Helps the body resist stress by:
• Keeping blood sugar levels relatively constant
• Maintaining blood volume and preventing water shift
into tissue
Hormones of the Adrenal Cortex
• Gonadocorticoids (Sex Hormones)
– Most secreted are androgens (male sex
hormones), and most important one is
testosterone
– Androgens contribute to:
• Onset of puberty
• Sex drive in females
• Can be converted into estrogens after menopause
Pancreas
• Has both exocrine and endocrine cells
• Located behind the stomach
• Produces an enzyme-rich juice used for
digestion (exocrine product)
• Pancreatic islets produce hormones
(endocrine products)
• Islets contain 2 major cell types:
– Alpha (α) cells – produce glucagon
– Beta (β) cells – produce insulin
Glucagon
• Major target is the liver where it promotes:
– Breakdown of glycogen to glucose
– Release of glucose to the blood
– Increases blood-glucose levels
Insulin
• Lowers blood-glucose levels
• Acts on the liver
• Enhances the transport of glucose into body
cells
Pineal Gland
• Small gland hanging from the roof of the third
ventricle of the brain
• Secretory product is melatonin
• Melatonin is involved with:
– Day/night cycles
– Physiological processes that show rhythmic
variations (body temperature, sleep, appetite)
Thymus Gland
• Located deep to the sternum, above the heart
• Major hormonal products are thymopoietins
& thymosins
• These are essential in the development of the
T lymphocytes (T cells) of the immune system
Gonads: Female
• Paired ovaries produce estrogens and
progesterone
• Responsible for:
– Maturation of the reproductive organs
– Appearance of secondary sexual characteristics
(Only 1 ovary produces hormones at a time, so if 1 is
damaged, the other will take over)
Gonads: Male
• Testes that produce testosterone
• Testosterone:
– Initiates maturation of male reproductive organs
– Necessary for sperm production
– Appearance of secondary sexual characteristics
Stress
• The result of the body’s response to a change
in the internal environment that threatens life
• Stressor – a factor that can stimulate such a
response
Types of Stress
• Physical factors
– Exposure to extreme heat or cold, decreased
oxygen concentration, infections, injuries, and
loud sounds
• Psychological factors
– Thoughts about real or imagined dangers,
personal losses, & unpleasant social interactions
– Feelings of anger, fear, grief, anxiety, depression,
and guilt can also produce psychological stress
Responses to Stress
• The hypothalamus activates mechanisms that
prepare the body for “fight or flight”
– Raising blood concentrations of glucose, glycerol,
and fatty acids
– Increasing heart rate, blood pressure, and
breathing rate
– Dilating air passages
– Shunting blood from the skin and digestive organs
to the skeletal muscles
– Increasing epinephrine secretions from the
adrenal medulla