Download Endocrine System Endocrine Glands

Endocrine System
Endocrine Glands
- unlike exocrine glands (which secrete their secretory products into ducts), endocrine
gland secrete their products directly into the blood.
- Some glands have both an endocrine and exocrine function and are termed
heterocrine glands. Some heterocrine glands we will study are:
- pancreas
- ovaries
- testes
Effects of Hormones
1. regulates chemical composition of extracellular fluids.
2. regulate metabolism and energy balance
3. function in the contraction of the heart muscle and visceral muscles.
4. Helps the body maintain homeostasis and cope with:
- stress
- dehydration - starvation - hemorrhage - temperature extremes
5. helps regulate the immune system.
6. Growth and Development.
7. Reproductive processes.
Target Cells - any cell that responds to an individual hormone
Receptors - target cells have integral proteins that are receptive to certain hormones
- different cells with receptors for the same hormone may respond differently to that
hormone .
Feedback Control
Negative Feedback - the bodies response is opposite to stimulus
Positive Feedback - the bodies response intensifies the stimulus
Hormonal secretion is stimulated by:
1. the nervous system - example: the sympathetic nervous system causes the
release of norepinephrine from the adrenal medulla.
2. chemical changes in the blood - example: a decrease in the level of calcium
in the blood will trigger the release of PTH
3. other hormones - example: releasing hormones from the hypothalamus will
in turn cause the release of hormones from the adenohypophysis
THE PITUITARY GLAND
- often referred to as the Master Gland
- attached to the hypothalamus via the infundibulum
- divided into two lobes - Anterior and Posterior
Adenohypophysis (Pars Distalis)
- the anterior lobe
- derived from an outgrowth of ectoderm in the pharyngeal region called
Rathke's pouch
- glandular portion of the pituitary
- Releasing Hormones and Inhibiting Hormones from the hypothalamus
control the hormone secretions from this part of the pituitary
Neurohypophyses (Pars Nervosa)
- the posterior lobe
- is derived from ectodermal neurohypophseal bud
- contains axons or secretory cells that originate in the hypothalamus
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The figure below illustrates the fact that the pituitary gland is located on the base of
the brain and consists of two parts:
(1) The anterior lobe or adenohypophysis.
(2) The posterior lobe or neurohypophysis.
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The Adenohypophysis
- the release of hormones from the adenohypophysis is controlled by regulating
hormones (or factors) from the hypothalamus via the blood supply
Note on the figure above that the blood supply to the anterior pituitary passes through the
hypothalamus first where it picks up regulating hormones that control the release of
hormones from the anterior pituitary.
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Human Growth Hormone
- causes body cells to grow (mainly muscle and bone)
- causes cells to:
- increase their uptake of amino acids
- increase their rate of protein synthesis.
- decrease the use of proteins for energy (promotes fat catabolism
- stop burning glucose, and stop uptaking glucose, leading to hyperglycemia
of the blood (this is referred to as the Diabetogenic effect)
Note: the effects shown above are not produced directly by the hormone hGH but
indirectly by Somatomedins (insulin like growth factors IGFs) released from the liver
(which are much more potent)
Pituitary Dwarfism - hyposecretion of hGH in growth years
Giantism - hypersecretion of hGH in growth years
Acromegaly - hypersecretion of hGH in adult years
THYROID STIMULATING HORMONE (TSH) - (Thyrotropin)
- stimulates the synthesis and secretion of the thyroid hormones T3 and T4
- release of TSH from the adenohypophysis is controlled by:
- Thyrotropin Releasing Hormone (TRH) - in response to:
- low blood glucose levels
- a decrease in the metabolic rate
- low levels of blood TSH
- negative feedback system
ADRENOCORTICOTROPIC HORMONE (ACTH) (Adrenocorticotropin)
- controls the synthesis and secretion of hormones from the Adrenal Cortex
- release of ACTH from the adenohypophysis is regulated by:
- Corticotropin Releasing Hormone (CRH) - in response to:
- stress related symptoms such as low blood sugar
- negative feedback system
FOLLICLE STIMULATING HORMONE (FSH)
- release of FSH from the adenohypophysis is regulated by:
- Gonadotropic Releasing Hormone (GnRH) - in response to:
- blood levels of sex hormones
- FSH stimulates the development of follicles and ova in the ovaries and the
secretion of estrogen
- FSH stimulates the development of sperm in the testes
LUTEINIZING HORMONE (LH)
- release of LH from the adenohypophysis is regulated by:
- Gonadotropic Releasing Hormone (GnRH) - in response to:
- low levels of the sex hormones
- LH in females:
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- stimulates the secretion of estrogen by the ovarian cells
- stimulates ovulation
- stimulates the formation of the corpus luteum
- stimulates the secretion of progesterone from the corpus luteum
- in males LH:
- stimulates the secretion of testosterone from the interstitial cells of Leydig
- for this reason in males LH called: Interstitial Cell-stimulating Hormone
(ICSH)
PROLACTIN (PRL) (Lactogenic hormone)
- controls the production of milk by the mammary glands
- release of PRL from the adenohypophysis is regulated by:
- Prolactin Releasing Hormone (PRH) - in response to:
- other hormonal changes during a pregnancy
- inhibition of PRL from the adenohypophysis is regulated by:
- Prolactin Inhibiting Hormone (PIH) (dopamine) - in response to:
- low blood levels of estrogen and progesterone
- NOTE: PRL works in conjunction with estrogen, progesterone, corticosteroids,
hGH, thyroxine, insulin, oxytocin, and hCS. All of these hormones are necessary for
milk production and secretion to occur
MELANOCYTE STIMULATING HORMONE (MSH)
- release of MSH from the adenohypophysis is regulated by:
- MSH Releasing Hormone (MRH) - in response to:
- high UV levels
- inhibition of MSH from the adenohypophysis is regulated by:
- MSH Inhibiting Hormone (MIH) - response to:
- low UV levels
THE NEUROHYPOPHYSIS (Pars Nervosa)
- the posterior lobe of the pituitary gland
- it is not an endocrine gland because it does not synthesize hormones
- the hormones released from the neurohypophysis are produced in the hypothalamus
by neurosecretory cells and transported to the neurohypophysis by the axons of the
cells in the Hypothalamic-Hypophyseal Tract
- two hormones are produced - Oxytocin and ADH
OXYTOCIN (OT)
- stimulates the contraction of smooth muscle in the uterus and mammary glands
- release of oxytocin from the neurohypophysis is regulated by:
- nervous system input - for example: when the baby is suckling at the nipple
nerve endings are stimulated which tell the hypothalamus to release oxytocin
so milk can be ejected from the mammary glands (this is called a
neuroendocrine reflex or milk ejection (let-down) reflex)
- positive feedback system
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ANTIDIURETIC HORMONE (ADH) - (Vasopressin)
- controls the volume of urine by causing the removal of water from the urine
(antidiuresis) in the distal convoluted tubules of the kidney
- raises blood pressure by constricting arterioles.
- release of ADH from the neurohypophysis is regulated by:
- osmoreceptors in the hypothalamus (that detect low blood water content)
- negative feedback
THE THYROID GLAND
- consists of two lobes on either side of the trachea joined by an isthmus
- histologically
- composed of follicles (made up of follicular cells) and interstitial
parafollicular cells
- the follicular cells secrete the hormones:
- thyroxine (T4)
- triiodothyronine (T3)
- the parafollicular cells secrete the hormone
- calcitonin
Effects of Hormones T3 and T4 - three main effects
1. Metabolism (calorigenic effect).
2. Growth and development
3. Nervous system activity
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Calcitonin (CT)
- homeostatasis of blood calcium (lowers blood calcium)
- release of calcitonin from the parafollicular cells is regulated by:
- high calcium levels in the blood
- negative feedback
- antagonistic to PTH
The PARATHYROID GLAND (see thyroid figure above)
- there are two pairs of glands embedded in the lateral lobes of the thyroid gland
- secretes the hormone PTH
Parathyroid Hormone (PTH)
- controls the homeostatis of ions (mainly calcium and phosphate) in the blood
- release of PTH from the parathyroid gland is regulated by:
- low blood levels of calcium and phosphate
- negative feedback
- in the presence of vitamin D, PTH causes an increase in the absorption of
calcium and phosphate from the G.I.. tract.
- PTH increases the number and activity of osteoclasts (which break down the
matrix of bone and release the ions into the blood)
- PTh increases the rate at which the kidneys reabsorb these ions from the
urine and stops the secretion of these ions by the kidneys
- PTH is antagonistic to calcitonin
THE ADRENAL GLAND (Suprarenals)
- two glands in humans - one on top of each kidney
- structurally and functionally divided into two regions
1. Adrenal Cortex - formed from mesoderm
2. Adrenal Medulla - formed from ectoderm
- highly vascular gland
The Adrenal Cortex
- subdivided into three zones
1. Zona Glomerulosa - mineralocorticoids
2. Zona Fasciculata - glucocorticoids
3. Zona Reticularis - gonadocorticoids
Mineralocorticoids
- help control water and electrolyte homeostasis, especially sodium and potassium
- the main hormone is Aldosterone
- the release of aldosterone is regulated by the:
1. Renin-Angiotensin pathway
- renin is released by juxtaglomerular cells in the kidney in response to low
blood pressure
- renin converts angiotensinogen from the liver into Angiotensin I
- when angiotensin I reaches the lungs in the blood it is converted into
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Angiotensin II - which stimulates the adrenal cortex to release
Aldosterone.
2. the potassium levels in the interstitial fluids.
This figure details the renin-angiotensin pathway
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The figure below illustrates both the adrenal cortex and adrenal medulla.
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Glucocorticoids
- control metabolism and resistance to stress
- three major glucocorticoids:
1. Cortisol (Hydrocortisone) - most abundant
2. Corticosterone
3. Cortisone
- effects of glucocorticoids
1. Promotes normal metabolism
2. Resistance to stress by:
- increasing blood glucose
- raising blood pressure
3. Anti-inflammatory responses
Gonadocorticoids
- concentrations are too low for effects to be significant
The Adrenal Medulla
- directly innervated by the ANS
- two hormones are produced:
1. Epinephrine
2. Norepinephrine
- these hormones are Sympathomimetic (they imitate the effects of the
sympathetic nervous system and produce fight or flight responses)
Pancreas
- is both an endocrine and a exocrine gland (heterocrine)
- has special areas called islets of Langerhans with four types of cells:
1. Alpha Cells - secrete glucagon
2. Beta cells - secrete insulin
3. Delta cells -secrete growth hormone inhibiting hormone (somatostatin)
4. F-cells - secrete pancreatic polypeptide
Glucagon
- increased blood sugar levels
Insulin
- lowers blood sugar levels
Somatostatin
- inhibits the secretion of insulin and glucagon
Pancreatic Polypeptide
- regulates the release of pancreatic digestive enzymes. This prevents the
breakdown and absorption of carbohydrates from the small intestine if the
blood sugar level is to high, and increases the digestion and absorption of
carbohydrates if the blood sugar level is to low.
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The figure below illustrates the fact that the pancreas is both an endocrine and
exocrine gland (and therefore is a heterocrine gland). The pancreas has a duct system
(C below) that empties digestive enzymes into the duodenum. The pancreas also has
islets (D below) that secrete hormones into the blood.
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Ovaries and Testes
Ovaries
- produce estrogen and progesterone which are responsible for the development,
and maintenance of female sexual characteristics, regulation of the menstrual
cycle, and lactation
- produces the hormone relaxin which relaxes the symphysis pubis and cervix
during the birth process
- produces the hormone inhibin that inhibits the secretion of FSH during
pregnancy so that a new menstrual cycle does not begin while there is
a fetus in the uterus
Testes
- produces testosterone which is responsible for the development and
maintenance of the male sexual characteristics
- produces inhibin with effects similar to that in females
Pineal Gland
- secretes
- Melatonin which inhibits reproductive activities
Thymus
- secretes hormones:
- Thymosin, Thymic Humoral Factor, Thymic factor, Thymopoietin
which promote the proliferation and maturity of T cells (immunity)
Other Endocrine tissues
Gastrointestinal Tract
- stomach gastrin
- enteric gastrin
- secretin
- cholecystokinin
- enterocrinin
- gastric inhibiting peptide
Placenta
- human chorionic gonadotropin
- estrogen and progesterone
- relaxin
- human chorionic somatomammotropin
Kidneys
- renal erythropoietic factor
- vitamin D activation
Skin - vitamin D
Cardiac Muscle - atrial natiuretic factor
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