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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Ch20 Endocrine System

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