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Endocrinology
A System of Blood-Born
Chemical Messengers
Endocrine System
• What is an endocrine gland?
Tissue that secretes a chemical messenger directly into the blood
stream.
• How does it differ from an exocrine gland?
Exocrine glands have ducts (internal tubes) that store secretions
and deliver the secretion to a distinct target.
• What is a hormone? (traditional definition)
A chemical messenger secreted by a tissue and acting on a distant
target tissue.
• What is a target tissue?
A tissue that has a receptor molecule that can bind with, and retain
the hormone.
• What does a hormone receptor do?
After binding with a hormone, the receptor initiates a biochemical
change in the target tissue at the membrane, the cytoplasm or the
nucleus.
Melatonin: Biorhythms
Pineal Gland
11 different hormones that are direct acting or
tropic hormones
Parathormone – regulates blood calcium
Thyroid hormone – regulates metabolic rate;
calcitonin - regulates blood calcium
Pituitary
Parathyroid
Thyroid
Thymus
Thymosin – important for development of
The immune system.
Epinephrine and norepinephrine
Regulate response to short-term stress
Steroids regulate glucose, fat, and mineral Adrenal Gland
Levels in blood
Insulin and Glucagon - both help
Regulate the use or storage of glucose
Pancreas
Ovary
Ovaries and testes produce various steroid hormones
that regulate reproduction and cause sex
differentiation.
Testes
Classes of Hormones
• Steroids (estrogen, testosterone, cortisol)
All synthesized from cholesterol
All are lipid soluble and may pass through membranes
• Proteins (insulin, glucagon, prolactin)
Large molecules, generally water soluble
• Peptides and amino acid (thyroid hormone, hormones from
the posterior pituitary)
• Prostaglandins (made from membrane lipids) – usually
function as very short range hormones, involved with blood
clotting, regulation of reproductive system.
How Does Endocrine Control Differ
From Nervous Control?
Nervous System
• uses neurons to reach
target tissue
• very fast
• short duration
• typically controls
muscles and glands
Endocrine System
• uses hormones and blood
stream
• slower onset (minutes-days)
• longer duration (hoursmonths)
• Typically regulates
metabolism, growth,
reproduction
Steroid and Thyroid Receptor Systems
Steroid Hormone
Cell Membrane
Receptor
RNA
Messenger RNA
Ribosome
DNA
Nucleus
Newly Made Protein
Amino Acids
Protein and Peptide Receptor Systems
Protein or Peptide Hormone
Hormone Receptor
Enzyme (adenyl cylase)
Cell Membrane
ATP
Active Enzyme
C
AMP
Inactive Enzyme
Endocrine Pancreas - Islet Cells
Regulation of Blood Glucose
Alpha cells secrete glucagon
stimulates the breakdown of
glycogen to glucose at liver
and muscle
stimulates the production of
glucose from protein
(gluconeogenesis)
Beta Cells secrete insulin
stimulates uptake of glucose
from blood into muscle,
epithelium and adipose tissue
stimulates the formation of
glycogen in muscle and liver.
Stimulates formation of
glycogen from glucose
at liver and muscle
Promotes uptake of glucose
at muscle, adipose and
epithelial tissues
Insulin
When blood
glucose is high
Blood Glucose
Decreases
b
Pancreas
a
Inhibits insulin
secretion
Inhibits glucagon secretion
Blood Glucose
Increases
Stimulates change of
glycogen into glucose
at liver and muscle
Stimulates gluconeogenesis
When blood
Glucose is low
Glucagon
Diabetes Mellitus – high blood glucose
• Type I: - patient produces very little, or no insulin
- glucose levels remain high following meals
- liver and muscle do not store glycogen
- patient rapidly loses weight (wasting disease)
- autoimmune disease
- requires daily insulin injections
• Type II: - patient produces normal levels (or even high)
levels of insulin, but tissues do not respond.
- glucose levels remain high following meals
- enough insulin functions so that glycogen and
lipids are stored, but blood glucose remains
high.
- usually due to obesity
- may be “cured” through diet and exercise
Consequences of High Blood Glucose
• High Glucose levels are toxic to small blood
vessels.
• Diabetic retinopathy
• Kidney failure (need of dialysis)
• Neuropathy
• Loss of peripheral blood flow
(gangrene and amputations)
Hypothalamus
Neurosecretory
cells
Infundibulum
Posterior
Pituitary
Anterior
Pituitary
Antidiuertic
Hormone
Oxytocin
Uterus
Mammary
(milk ejection)
Kidney
(water retention)
Neurosecretory
cells
RH
RH
Portal capillaries
Anterior Pituitary
(pars distalis)
(adenohypophysis)
Tropic or
Direct Acting
Hormone
Releasing
hormone
Mini-Review
1. What type of cell produces oxytocin and antidiuretic hormone?
2. What are the target tissues for oxytocin and antidiuretic hormone?
3. What physiological effects do they have?
4. What type of cell produces releasing hormones?
5. What is the target tissue for a releasing hormone and how does the
releasing hormone get there?
6. What happens when a releasing hormone binds to receptors on its target
tissue?
7. What do tropic hormones do?
8. What do direct acting hormones do?
Thyroid Hormone and Regulation
of Metabolism
• Thyroid Hormone is made from the amino
acid called tyrosine. It is the only hormone
that requires iodine, and so small amounts of
iodine are required in the diet. Two different
forms T3 and T4
• Thyroid hormone is stored in the thyroid gland
in chambers called follicles – attached to a
protein called thyroglobulin.
• Goiter: enlargement of the thyroid gland.
Physiological Effects of Thyroid
Hormone
• Development of the nervous system; effects
sensitivity of the nervous system
• Regulation of metabolic rate
• Hyperthyroidism: person is nervous, high
energy, higher resting body temperature,
often tends to lose weight. May have
emotional instability.
• Hypothyroidism: person lacks energy, has
difficulty focusing, often gains weight
Regulation of Thyroid Hormone Secretion
Hypothalamus
T3 and T4 also
provide negative
feedback to the
hypothalamus
and inhibit TRH
secretion
TRH
Thyrotropic Releasing Hormone
(TRH) is carried to the anterior
pituitary by the portal capillaries
Anterior
Pituitary
TSH
Thyroid Stimulating Hormone
(TSH) is carried to the Thyroid
gland by the general circulation
Thyroid Gland
T3 and T4 increase metabolic
rate, increase body
temperature and effect
nervous tissue
T3 and T4
The Thyroid Gland releases T3
andT4 into the general
circulation.
Cycles of Releasing Hormones, Tropic
Hormones and Direct Acting Hormones
Hormone
Level
TRH
TSH
Time
T3 & T4
The Adrenal Gland
Adrenal Medulla
• Secretes Epinephrine and Norepinephrine
directly into the blood stream during times of
immediate threat (stress).
• Regulated directly by
the nervous system.
When the sympathetic
nervous system is
activated, sympathetic
nerves stimulate the
adrenal medulla.
Adrenal Cortex
• Produces a wide variety of steroid hormones
that regulate sodium levels in the blood
(Aldosterone), sex hormones (low levels), and
glucocorticoids (cortisol) which plays an
important role in managing long-term stress.
• Effects of Cortisol - increases blood glucose
levels, increases blood lipids, increases blood
pressure, suppresses the immune response.
Regulation of Cortisol Secretion
Hypothalamus
Cortisol also
provides negative
feedback to the
hypothalamus
and pituitary
CRH
Corticotropic Releasing Hormone
(CRH) is carried to the anterior
pituitary by the portal capillaries
Anterior
Pituitary
ACTH
Adrenocorticotropic Hormone
(ACTH) is carried to the Adrenal
gland by the general circulation
Adrenal Cortex
Increased blood glucose,
increased blood fatty acids,
increased blood pressure,
suppression of immune system
Cortisol
The Adrenal Cortex releases
cortisol into the general
circulation.