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Biology 212
Anatomy & Physiology
Endocrine System
Endocrine System
Isolated organs & tissues (“endocrine glands”) which
secrete chemical messages (“hormones”) into the blood
which cause changes in cells and tissues (“targets”)
elsewhere in the body.
Endocrine glands are
ductless: they have a
rich network of
capillaries, and
hormones are carried to
target tissues & organs
by the blood.
Hormones:
Three major chemical groups:
1) Peptides – Chains / rings of amino acids
Hormones:
Three major chemical groups:
2) Amines – Small molecules containing one or
more amine groups
Hormones:
Three major chemical groups:
3) Steroids – Formed from the lipid cholesterol by adding
specific sidechains
Endocrine System
Target cells & tissues respond to hormones relatively slowly
(seconds, minutes, days) compared to much more rapid
(milliseconds) responses to nervous stimulation
Hormones can produce a variety of changes in their
target cells:
- Stimulate or inhibit cell differentiation
- Stimulate or inhibit cell division
- Activate or deactivate enzymes
- Stimulate or inhibit protein synthesis
- Stimulate or inhibit secretion
- Change permeability of plasma membrane
A specific hormone can only affect cells which have specific
receptors for it on their plasma membranes.
Any single hormone can have only one effect on any specific
target cells, but it may affect different types of target cells in
different ways.
For example, epinephrine can:
Stimulate contraction of smooth muscle cells in
arterioles of intestines
Inhibit contraction of smooth muscle cells in
arterioles of leg muscles
Stimulate cardiac muscle cells to contract more quickly
Stimulate sweat glands to secrete sweat
Stimulate liver cells to release glucose into the blood
Stimulate fat cells to break down and release
fats into the blood
The effect of a hormone on its target cells can be changed
(increased or decreased) by:
- Changing the amount of hormone circulating in the blood
- Changing the number of receptors which the target cells
have for the hormone.
- Changing the specificity and affinity which these receptors
have for the hormone
Upregulation
Downregulation
Pituitary Gland
also called Hypophysis
Located in sella turcica of
sphenoid bone
Two developmentally and
functionally distinct parts,
both under control of
hypothalamus of brain
Anterior pituitary or
adenohypophysis
Secretes 6 peptide hormones
Secretory cells regulated
(stimulated or inhibited) by
hormones produced in the
hypothalamus and carried to
it by hypothalamic –
hypophyseal portal veins
1) Growth Hormone (GH):
Stimulates growth of many organs
2) Thyroid Stimulating Hormone (TSH):
Stimulates thyroid gland to secrete its hormones
3) Adrenocorticotropic Hormone (ACTH):
Stimulates adrenal cortex to secrete its hormones
4) Prolactin (PRL):
Stimulates milk production by breast
5) Follicle Stimulating Hormone (FSH):
Stimulates development or eggs / sperm
6) Luteinizing Hormone (LH):
Stimulates testes to produce testosterone
Stimulates follicle of ovary to change to corpus luteum
Posterior pituitary or
neurohypophysis
Secretes 2 peptide hormones
Secretory cells are really the
axons whose cell bodies are
located in the hypothalamus
and whose axons pass
through the hypothalamichypophyseal tract
1) Oxytocin (OT):
Stimulates contraction of smooth muscle in breast
(lactation) and uterus (labor)
2) Antidiuretic Hormone (ADH):
Increases absorption of water in the kidney to
concentrate urine and decrease its volume
Thyroid Gland:
Located in neck, anterior to trachea.
Thyroid Gland:
Located in neck, anterior to trachea.
Secretes 2 similar amine hormones
Triiodothyronine
Tetraiodothyronine
and 1 peptide hormone
Calcitonin
Thyroid Gland:
Located in neck, anterior to trachea.
Secretes 2 similar amine hormones
Triiodothyronine
Tetraiodothyronine
and 1 peptide hormone
Calcitonin
Secretory cells for T3 and T4
arranged in hollow spheres
called follicles; Stimulated
by thyroid-stimulating hormone
from adenohypophysis.
Secretory cells for calcitonin
located between follicles;
Regulated by concentration of calcium in blood
Triiodothyronine (T3) & tetraiodothyronine (T4)
increase metabolic rates of cells throughout the body by
stimulating energy production by mitochondria.
Calcitonin (CT) stimulates osteoblasts to store additional
calcium in the extracellular matrix of bone, thus lowering
calcium levels in the blood
Parathyroid Glands:
Four small glands on
posterior surface of thyroid
Secretes one peptide:
Parathyroid Hormone (PTH)
PTH increases concentration of calcium in blood:
- Stimulates osteocytes & osteoclasts to reabsorb calcium
from bone
- Inhibits kidneys from losing calcium through urine
Adrenal Glands:
Located in abdomen
Superior to kidneys
Each adrenal actually two
functionally distinct glands,
one (adrenal cortex)
wrapped around the other
(adrenal medulla)
Adrenal Medulla:
Actually a ganglion of the
sympathetic division of the
autonomic nervous system
Secretory cells stimulated by
preganglionic sympathetic neurons
and
Secrete two related amine
hormones called catecholamines
Since epinephrine and
norepinephrine are the
neurotransmitters normally
secreted by postganglionic
sympathetic neurons, they
produce all of the effects of
sympathetic nerve stimulation
on various organs
Adrenal Cortex:
Secretes more than twenty
different steroid hormones,
collectively called the
corticosteroids
Secretory cells arranged in
three concentric layers, each
of which secretes a specific
group of corticosteroids.
From superficial to deep:
- Zona glomerulosa
- Zona fasciculata
- Zona reticularis
Cells in zona glomerulosa
secrete mineralocorticoids
which
regulate concentrations of
various minerals (ions,
electrolytes) in the blood by
controlling their retention and
removal by the kidneys.
Most common mineralocorticoid = Aldosterone
Cells in zona fasciculata
secrete glucocorticoids
which
Regulate metabolism in many cells by:
a) stimulating breakdown of fat
b) stimulating breakdown of proteins
c) increasing concentrations of fatty acids, amino acids,
and glucose in the blood
Stimulate repair of damaged tissue
Most common glucocorticoids = cortisol & corticosterone
Cells in zona reticularis secrete
gonadocorticoids, or
sex hormones, which supplement those secreted by testes
and ovaries.
Mostly androgens (male hormones)
Some estrogens and progesterone (female hormones)
Most common = dehydroepiandrosterone & testosterone
Pancreas:
Located in abdomen
Posterior & inferior to stomach
Body of pancreas anterior to
aorta & inferior vena cava
Tail of pancreas extends left
Pancreas:
Primarily an exocrine gland
99% of cells secretes digestive enzymes into small intestine
but
1% = groups of endocrine
cells called pancreatic islets
or islets of Langerhans
Secrete two peptide hormones
Insulin
Glucagon
which are antagonistic to each
other in regulating blood glucose
Pancreatic Islets:
Alpha cells produce glucagon
Stimulated by low blood sugar
Causes increased blood sugar
by glycogen breakdown
gluconeogenesis
Beta cells produce insulin
Stimulated by high blood sugar
Causes decreased blood sugar
by glycogen synthesis
gluconeogenesis
Diabetes Mellitus
Two forms:
INSULIN DEPENDENT (Type I; Juvenile-onset)
- Beta cells destroyed by body’s immune system,
not producing enough insulin
NON-INSULIN DEPENDENT (Type II; Adult-onset)
- Beta cells still producing insulin, but:
Insulin is defective & can’t act on target cells; or
- Target cells are defective & can’t recognize insulin
Hyperglycemia: Beta cells not producing enough insulin
Alpha cells producing too much glucagon
Hypoglycemia: Beta cells producing too much insulin
Alpha cells not producing enough glucagon
Gonads:
Produce most of the steroid sex hormones
Gonads:
Produce most of the steroid sex hormones
Testes:
Interstitial (Leydig) cells
produce androgens
mostly testosterone
Stimulated by luteinizing
hormone from adenohypophysis
Causes development of male
sexual characteristics & their
maintenance
Gonads:
Produce most of the steroid sex hormones
Ovaries:
As oocyte develops, it is
surrounded by follicular cells
which produce estrogens,
primarily estradiol
Stimulated by Follicle
Stimulating Hormone from
adenohypophysis.
Causes development of female sexual characteristics
Prepares uterus for pregnancy
Prepares breast for lactation
Gonads:
Produce most of the steroid sex hormones
Ovaries:
After ovulation, follicle changes
to corpus luteum, secretes
progesterone.
Stimulated by luteinizing
hormone from adenohypophysis
Maintains thick lining with uterus for pregnancy
Many other organs also contain cells & tissues
which secrete hormones.
Examples:
Kidney:
Renin (blood pressure)
Erythropoietin (erythrocyte development)
Calciferol (blood calcium)
Heart:
Atrial natriuretic hormone (blood sodium)
Liver:
Angiotensin (blood pressure)
Thymus:
Thymopoietin (lymphocyte development)
Stomach: Cholecystokinin (gall bladder contraction)
&
Secretin (pancreas secretion)
Intestine: Gastrin (stomach function)