ch20 endocrine system
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Human Anatomy, First EditionMcKinley & O'Loughlin
Chapter 20 :
EndocrineSystem
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Endocrine System Major control system
Works with the nervous system
Function: to maintain homeostasis
Both use
specific communication methods
affect specific target organs
Their methods and effects differ.
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Endocrine Glands & Hormones Exocrine glands: ducted
secretions released into ducts
open onto an epithelial surface
Endocrine glands: ductless
secrete product directly into the bloodstream
All endocrine cells are located within highlyvascularized areas
ensure that their products enter the bloodstreamimmediately.
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Overview of Hormones Molecules that have an effect on specific
organs
called target organs Only cells with specific receptors for the
hormone respond to that hormone called target cells
Organs, tissues, or cells lacking the specificreceptor do not respond to its stimulatingeffects.
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Classification of Hormones Peptide hormones (Hydrophilic: polar)
formed from chains of amino acids
most hormones are peptide hormones
longer chains are called protein hormones
Example: growth hormone
Steroid hormones (Hydrophobic: nonpolar) type of lipid derived from cholesterol
Example: testosterone
Biogenic amines (Hydrophobic: nonpolar) small molecules produced by altering the structure of a
specific amino acid
Example: thyroid hormone
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Negative Feedback Loop Major mechanism of hormone action
Mechanism:A stimulus starts a process
Process causes release of a hormone
Either the hormone or a product of its
effects causes the process to slow down orturn off.
Example: the regulation of the bloodglucose level in the body
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Positive Feedback Loop Called positive because it accelerates the
original process
can ensure that the pathway continues to run
can speed up its activities.
Few positive feedback loops in the humanendocrine system.
Example: milk release from the mammary glands
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Hypothalamic Control of the
Endocrine System Master control center of the endocrine
system
Hypothalamus oversees most endocrineactivity: special cells in the hypothalamus secrete
hormones that influence the secretory activityof the anterior pituitary gland called regulatory hormones releasing hormones (RH) inhibiting hormones (IH)
Hypothalamus has indirect control overthese endocrine organs.
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Hypothalamic Control of the
Endocrine System Hypothalamus produces two hormones that are
transported to and stored in the posteriorpituitary. oxytocin (paraventicular nucleus)
antidiuretic hormone (ADH) (supraoptic nucleus)
Hypothalamus directly oversees the stimulationand hormone secretion of the adrenal medulla. An endocrine structure that secretes its hormones in
response to stimulation by the sympathetic nervoussystem.
Some endocrine cells are notunder direct controlof hypothalamus.
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Pituitary Gland (Hypophysis) lies inferior to the hypothalamus.
Small, slightly oval gland housed within the
hypophyseal fossa of the sphenoid bone. Connected to the hypothalamus by a thin
stalk, the infundibulum.
Partitioned both structurally and functionally
into an anterior pituitary and a posteriorpituitary. (called anterior lobes and posterior lobes)
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Control of Anterior Pituitary
Gland Secretions Anterior pituitary gland is controlled by
regulatory hormones secreted by the
hypothalamus. Hormones reach the anterior pituitary via
hypothalamo- hypophyseal portal system.
essentially a shunt
takes venous blood carrying regulatoryhormones from the hypothalamus directly to theanterior pituitary
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Thyroid Gland Located immediately inferior to the thyroid cartilage of the
larynx and anterior to the trachea.
Distinctivebutterfly shape due to its left and right lobes, which
are connected at the anterior midline by a narrow isthmus.
Both lobes of the thyroid gland are highly vascularized, giving itan intense reddish coloration.
Regulation of thyroid hormone secretion depends upon acomplex thyroid glandpituitary gland negative feedback
process.
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Thyroid Gland Follicle cells:
Produce and secrete thyroid hormone Precursor is stored in colloid
Thyroid hormone Increases metabolic rate Important in growth and development.
Parafollicular cells Produce and secrete calcitonin
Calcitonin Secreted in response to elevated calcium levels Reduces blood calcium levels Acts on osteoblasts.
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Parathyroid Glands Small, brownish-red glands
located on the posterior surface of the thyroid gland
Usually four small nodules
may have as few as two or as many as six. Two different types of cells in the parathyroid gland:
chief cells oxyphil cells
Chief cells are the source ofparathyroid hormone (PTH). stimulates osteoclasts to resorb bone and release calcium
ions from bone matrix into the bloodstream stimulates calcitriol hormone synthesis in the kidney promotes calcium absorption in the small intestine prevents the loss of calcium ions during the formation of
urine The function of oxyphil cells is not known.
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Adrenal Glands (suprarenal) Paired, pyramid-shaped endocrine
glands anchored on the superior surface
of each kidney. Retroperitoneal and embedded in fat
and fascia to minimize their movement.
Outer adrenal cortex and an innercentral core called the adrenal medulla. secrete different types of hormones
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Adrenal Cortex Distinctive yellow color due to stored lipids in its cell.
Synthesize more than 25 different steroid hormones, collectivelycalled corticosteroids.
corticosteroid synthesis is stimulated by the ACTH produced by theanterior pituitary
corticosteroids are vital to our survival; trauma to or removal of theadrenal glands requires corticosteroid supplementation throughoutlife
Partitioned into the zona glomerulosa, the zona fasciculata, and
the zona reticularis. Different functional categories ofsteroid hormones are
synthesized and secreted in the separate zones.
Regulates salt, sugar, and sex!
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Adrenal Cortex Partitioned into
zona glomerulosa
Mineralocordicoids
Aldosterone
Regulates ratio of sodium and potassium
zona fasciculata
Glucocorticoids
Cortisol and corticosterone
Stimulate metabolism of lipids, proteins, glucose
Resist stress, repair tissues
zona reticularis.
gonadocorticoids
androgens
Regulates salt, sugar, and sex!
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Adrenal Medulla Forms the inner core of each adrenal gland.
Pronounced red-brown color due to its extensivevascularization.
Primarily consists of clusters of large, spherical cells calledchromaffin cells.
When innervated by the sympathetic division of the ANS, onepopulation of cells secretes the hormone epinephrine(adrenaline).
The other population secretes the hormone norepinephrine(noradrenaline).
Hormones work with the sympathetic nervous system toprepare the body for an emergency or fight-or-flight situation.
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Pancreas Elongated, spongy, nodular organ
between the duodenum and the spleen posterior to the stomach.
Both exocrine and endocrine considered a heterocrine (mixed) gland.
Mostly composed of cells called pancreatic acini. produce an alkaline pancreatic juice that aids digestion
Scattered among the pancreatic acini are small clusters ofendocrine cells called pancreatic islets (islets of Langerhans)composed offour types of cells: two major types (called alpha cells and beta cells) two minor types (called delta cells and F cells) each type produces its own hormone
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Pancreas Alpha cells secrete glucagon when blood glucose levels drop.
Beta cells secrete insulin when blood glucose levels are elevated.
Delta cells are stimulated by high levels of nutrients in the
bloodstream. synthesize somatostatin, also described as growth hormone-inhibiting
hormone, or GHIH, which slows the release of insulin and glucagonand slows the rate of nutrient entry into the bloodstream
F cells are stimulated by protein digestion.
secrete pancreatic polypeptide to suppress and regulate
somatostatin secretion from delta cells
Pancreatic hormones provide for orderly uptake and processing ofnutrients.
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Pineal Gland Pineal gland or pineal body, is a small, cone-shaped structure
attached to the posterior region of the epithalamus.
Secretes melatonin.
helps regulate a circadian rhythm (24-hour body clock)
also appears to affect the synthesis of the hypothalamicregulatory hormone responsible for FSH and LH synthesis
role in sexual maturation is not well understood
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Thymus A bilobed structure located within the mediastinum superior to
the heart and immediately posterior to the sternum. Size of the thymus varies between individuals.
it is always relatively large in infants and children as with the pineal gland, the thymus diminishes in size and
activity with age, especially after puberty Functions principally in association with the lymphatic system to
regulate and maintain body immunity. Produces complementary hormones thymopoietin and
thymosins. hormones act by stimulating and promoting the
differentiation, growth, and maturation of a category oflymphocytes called T-lymphocytes (thymus-derivedlymphocytes)
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Endocrine Functions of the Kidneys,
Heart, GI Tract, and Gonads Organs of the urinary, cardiovascular, digestive, and
reproductive systems contain their own endocrinecells, which secrete their own hormones.
help regulate electrolyte levels in the blood
red blood cell production, blood volume, and blood pressure
digestive system activities
sexual maturation and activity
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Aging and the Endocrine
System Secretory activity of endocrine glands wanes,
especially secretion ofgrowth hormone and sexhormones.
Reduction in GH levels leads to loss of weightandbody mass.
Testosterone or estrogen levels decline
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