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Chapter 18 - THE ENDOCRINE SYSTEM

(figures relate to Tortora/Grabowski 9th edition of Principles of Anatomy and Physiology)

 

I.      INTRODUCTION

A.   Together the nervous and endocrine systems coordinate functions of all body systems.

1.  The nervous system controls body actions through nerve impulses.

2.             The endocrine system controls body activities by releasing mediator
molecules called hormones.

3.             The science concerned with the structure and function of the endocrine glands
and the diagnosis and treatment of endocrine disorders is called
endocrinology.

B.   The nervous and endocrine systems act as a coordinated interlocking supersystem, the
neuroendocrine system.

1.  Parts of the nervous system stimulate or inhibit the release of hormones.

2.             Hormones may promote or inhibit the generation of nerve impulses.

C.   The nervous system causes muscles to contract or glands to secrete. The endocrine
system affects virtually all body tissues by altering metabolism, regulating growth
and development, and influencing reproductive processes.

D.   Table 18.1 compares the characteristics of the nervous and endocrine systems.

II.  ENDOCRINE GLAND

A.   The body contains two kinds of glands: exocrine and endocrine.

1.  Exocrine glands secrete their products into ducts, and the ducts carry the
secretions to the target site.

2.            Endocrine glands secrete their products (hormones) into the interstitial fluid
surrounding the secretory cells from which they diffuse into capillaries to be
carried away by blood.

B.   Endocrine glands constitute the endocrine system and include the pituitary, thyroid,
parathyroid, adrenal, and pineal glands (Figure 18.1).

III. HORMONES

A.   Hormones have powerful effects when present in very low concentrations.

B.   Hormone Receptors

1.   Although hormones travel in blood throughout the body, they affect only specific target cells.

           (p 64)      2.   Target cells have specific protein or glycoprotein receptors to which hormones bind.

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                                3. Receptors are constantly being synthesized and broken down.

a.    When a hormone is present in excess, down-regulation, the decrease in
the number of receptors, may occur.

b.    When a hormone is deficient, up-regulation, an increase in the number
of receptors, may occur.

4.   Synthetic hormones that block the receptors for particular naturally occurring
hormones are available as drugs. (Clinical Application, p.568)

C.   Circulating and Local Hormones

1.  Hormones that travel in blood and act on distant target cells are called
circulating hormones or endocrines.

2.             Hormones that act locally without first entering the blood stream are called
local hormones.

a.    Those that act on neighboring cells are called paracrines.

b.    Those that act on the same cell that secreted them are termed autocrines.

3.   Figure 18.2 compares the site of action of circulating and local hormones.

D.   Chemistry of Hormones

1.  Table 18.2 provides a summary of the hormones. (p. 569)

2.             Lipid-soluble hormones include the steroids, thyroid hormones, and nitric
oxide, which act as a local hormone in several tissues.

3.             Water-soluble hormones include the amines; peptides, proteins, and
glycoproteins; and eicosanoids.

E.   Hormone Transport in Blood

1.  Most water-soluble hormones circulate in plasma in a free, unattached form.

2.             Most lipid-soluble hormones bind to transport proteins to be carried in blood.

3.  The transport proteins improve the transportability of lipid-soluble hormones
by making them temporarily water-soluble, retard passage of the small

hormone molecules through the kidney filter thus slowing the rate of hormone loss in urine, and provide a ready reserve of hormone already present in blood.

IV. MECHANISMS OF HORMONE ACTION

A.   The response to a hormone depends on both the hormone and the target cell; various
target cells respond differently to different hormones.

B.   Action of Lipid-Soluble Hormones:

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1.   Lipid-soluble hormones bind to and activate receptors within cells.(p. 570)

2.             The activated receptors then alter gene expression which results in the
formation of new proteins, (to do this, they must bind to DNA inside nucleus)

3.             The new proteins alter the cells activity and result in the physiological
responses of those hormones.

4.             Figure 18.3 shows this mechanism of action.

C.   Action of Water-Soluble Hormones

1.   Water-soluble hormones alter cell functions by activating plasma membrane receptors, which set off a cascade of events inside the cell.

a.    The water-soluble hormone that binds to the cell membrane receptor is
the first messenger.

b.    A second messenger is then released inside the cell where hormone
stimulated response takes place.

2.    A typical mechanism of action of a water-soluble hormone using cyclic AMP
as the second messenger is seen in Figure 18.4, p. 571.

a.    Hormone binds to receptor on cell membrane

b.   The activated membrane G-protein turns on adenylate cyclase.

c.    Adenylate cyclase converts ATP into cyclic AMP which activates
protein kinases.

d.   Protein kinases phosphorylate enzymes which catalyze reactions that
produce the physiological response.

3.             Since water-soluble hormones that bind to plasma membrane receptors initiate a
cascade of events, they can induce their effects at very low concentrations.

4.             The cholera toxin modules G-proteins in epithelial cells lining the intestine so
they become locked in an activated state which results in the massive fluid
loss this toxin causes. (Clinical Application) (Cl- Na+ Loss …H2O loss)

D.   Hormonal Interactions

1.  The responsiveness of a target cell to a hormone depends on the hormone's concentration, the abundance of the target cell's hormone receptors, and influences exerted by other hormones.

2.   Three hormonal interactions are the permissive effect, the synergistic effect, and the antagonist effect.

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V.         CONTROL OF HORMONE SECRETIONS

A.   Most often, negative feedback systems regulate hormonal secretions. Regulation of
hormone secretion normally maintains homeostasis and prevents overproduction or
underproduction of a particular hormone; when these regulating mechanisms do not
operate properly, disorders result.

B.   Hormone secretion is controlled by signals from the nervous system, by chemical
changes in the blood (chemoreceptors), and by other hormones.

VI. HYPOTHALAMUS AND PITUITARY GLAND

A.   The hypothalamus is the major integrating link between the nervous and endocrine systems.

B.   The hypothalamus and the pituitary gland (hypophysis) regulate virtually all aspects
of growth, development, metabolism, and homeostasis.

C.   The pituitary gland is differentiated into the anterior pituitary and the posterior
pituitary (Figure 18.5).

1.   Anterior Pituitary Gland (Adenohypophysis)

a.    Hormones of the anterior pituitary are controlled by releasing or
inhibiting hormones produced by the hypothalamus.

b.    Hormones of the anterior pituitary are as follows, (see Tbl. 18.3) (Seven)

1)  Human growth hormone (hGH)

2)           Thyroid-stimulating hormone (TSH)

3)       Follicle-stimulating hormone (FSH) and luteinizing hormone (LH)

4)           Prolactin (PRL)

5)           Adrenocorticotrophic hormone (ACTH) and melanocyte stimulating hormone (MSH)

d.   Secretion of anterior pituitary gland hormones is regulated by
hypothalamic regulating hormones (see l.a. above) and by negative
feedback mechanisms (see V.A. above) (Figure 18.6, Table 18.3).

e.   Human Growth Hormone and Insulinlike Growth Factors

1)  Human growth hormone (hGH) is the most plentiful anterior pituitary hormone.

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2)        It acts indirectly on tissues by promoting the synthesis and
secretion of small protein hormones called insulinlike growth
factors (IGFs).

3)        IGFs stimulate general body growth and regulate various
aspects of metabolism.

(norm. bld. Glu.= ~90mg/100ml)           4)  One symptom of excess hGH is hyperglycemia. (Clin.App)p577

5)  In adults, it is secreted ~1.5 hrs. after sleep to assist in sleep and body repair.

f.    Thyroid-stimulating hormone (TSH) regulates thyroid gland activities and is
controlled by TRH (thyrotropin releasing hormone, from hypothalamus).

g.    Follicle-Stimulating Hormone (FSH)

1)   In females, FSH initiates follicle development and secretion of
estrogens in the ovaries.

2)   In males, FSH stimulates sperm production in the testes.
h.   Luteinizing Hormone (LH)

1)  In females, LH stimulates secretion of estrogen by ovarian cells to
result in ovulation and stimulates formation of the corpus luteum and
secretion of progesterone.

2)          In males, LH stimulates the interstitial cells of the testes to
secrete testosterone.

i.    Prolactin (PRL), together with other hormones, initiates and maintains

milk secretion by the mammary glands.

j.    Adrenocorticotrophic hormone (ACTH) controls the production and

secretion of hormones called glucocorticoids by the cortex of the

adrenal gland,.

 k.   Melanocyte-stimulating hormone (MSH) increases skin pigmentation

although its exact role in humans is unknown.

 1.    Table 18.4 summarizes the principal actions of the anterior pituitary

gland hormones. (p. 578)

2.         Posterior Pituitary Gland (Neurohypophysis )

a.   Although the posterior pituitary gland does not synthesize hormones, it

does store and release two hormones. (See Tbl. 18.5)

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b.    Hormones made by the hypothalamus and stored in the posterior pituitary
are oxytocin (0T) and antidiuretic hormone (ADH).

1)  Oxytocin stimulates contraction of the uterus and ejection (let­
down) of milk from the breasts.

2)          Antidiuretic hormone stimulates water reabsorption by the
kidneys and arteriolar constriction.

 

a)             The effect of ADH is to decrease urine volume and
conserve body water.

b)            ADH is controlled primarily by osmotic pressure of the
blood (Figure 18.9).

c.    Table 18.5 lists the posterior pituitary gland hormones and summarizes their
principal actions and the control of their secretions.

VII. THYROID GLAND

A.   The thyroid gland is located just below the larynx and has right and left lateral lobes
(Figure 18.10).

B.   Histologically, the thyroid consists of the thyroid follicles composed of follicular
cells, which secrete the thyroid hormones thyroxine (T4} and triiodothyronine (T3),
and parafollicular cells, which secrete calcitonin (CT).

C.   Formation, Storage, and Release of Thyroid Hormones

1.  Thyroid hormones are synthesized from iodine and tyrosine within a large
glycoprotein molecule called thyroglobulin (TGB) and are transported in the
blood by plasma proteins, mostly thyroxine-binding globulin (TBG).

2.            The formation, storage, and release steps include iodide trapping, synthesis of
thyroglobulin, oxidation of iodide, iodination of tyrosine, coupling of T1 and
T2, pinocytosis and digestion of colloid, secretion of thyroid hormones, and
transport in blood (Figure 18.11).

D.   Thyroid hormones regulate oxygen use and basal metabolic rate (burning energy),
cellular metabolism, growth and development, and temperature regulation.

E.   Secretion of thyroid hormone is controlled by the level of iodine in the thyroid gland
and by negative feedback systems involving both the hypothalamus and the anterior
pituitary gland (Figure 18.12).

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F.    Calcitonin lowers the blood level of calcium. Secretion is controlled by calcium
levels in the blood.

G.   Table 18.6 summarizes the hormones produced by the thyroid gland, their principal
actions, and control of secretion.

VIII.  PARATHYROID GLANDS

A.   The parathyroid glands are embedded on the posterior surfaces of the lateral lobes of
the thyroid and contain principal cells, which produce parathyroid hormone, and
oxyphil cells, whose function is unknown (Figure 18.13).

B.   Parathyroid hormone (PTH) regulates the homeostasis of calcium and phosphate by
increasing blood calcium level and decreasing blood phosphate level.

1.  PTH increases the number and activity of osteoclasts, increases the rate of Ca+2
and Mg+2 reabsorption from urine, and inhibits the reabsorption of HPO4-2 so more
is secreted in the urine.

2.             Blood calcium level directly controls the secretion of calcitonin and
parathyroid hormone via negative feedback loops that do not involve the
pituitary gland (Figure 18.14), instead involve thyroid and parathyroid.

C.   Table 18.7 summarizes the principal actions and control of secretion of parathyroid hormone.

IX.  ADRENAL GLANDS

A.   The adrenal glands are located superior to the kidneys (Figure 18.15); they
consist of an outer cortex and an inner medulla.

B.   Adrenal Cortex

1.  The adrenal cortex is divided into three zones, each of which secretes
different hormones (Figure 18.15).

2.             Mineralocorticoids

a.    Mineralocorticoids (e.g., aldosterone) increase sodium and water
reabsorption and decrease potassium reabsorption, helping to regulate
sodium and potassium levels in the body.

b.    Secretion is controlled by the renin-angiotensin pathway (Figure
18.16) and the blood level of potassium.

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3.    Glucocorticoids

a.. Glucocorticoids (e.g., cortisol, ‘wake up hormone’) promote breakdown of proteins, formation of glucose, lipolysis, resistance to stress, anti-inflammatory effects, and depression of the immune response.

b.  Secretion is controlled by CRH (corticotropin releasing hormone)
and ACTH (adrenocorticotropic hormone) from the anterior pituitary (Figure 18.17).

4.  Androgens secreted by the adrenal cortex usually have minimal effects.

C.   Adrenal Medulla

1.  Medullary secretions are epinephrine and norepinephrine (NE), which
produce effects similar to sympathetic responses. (p. 559)

2.             They are released under stress by direct innervation from the autonomic
nervous system. Like the glucocorticoids of the adrenal cortex, these
hormones help the body resist stress. However, unlike the cortical hormones,
the medullary hormones are not essential for life.

D.   Table 18.8 summarizes the hormones produced by the adrenal glands, the principal
actions, and control of secretion.

X. PANCREAS

A.   The pancreas is a flattened organ located posterior and slightly inferior to the stomach and
can be classified as both an endocrine and an exocrine gland (Figure 18.18).

B.   Histologically, it consists of pancreatic islets or islets of Langerhans (Figure 18.19)
and clusters of cells called acini (exocrine cells which we won't focus on here).

C.   Cell Types in the Pancreatic Islets (we focus on #1 & 2, but ALL are endocrine)

1.  Alpha cells secrete the hormone glucagon which increases blood glucose levels.

2.            Beta cells secrete the hormone insulin which decreases blood glucose levels.

3.            Delta cells secrete growth hormone inhibiting hormone or somatostatin,
which acts as a paracrine to inhibit the secretion of insulin and glucagon.

4.            F-cells secrete pancreatic polypeptide, which regulates release of pancreatic
digestive enzymes.

D.   Regulation of glucagon and insulin secretion is via negative feedback mechanisms
(Figure 18.19).

E.   Table 18.9 summarizes the hormones produced by the pancreas, their principal
actions, and control of secretion.

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XL OVARIES AND TESTES

A.  Ovaries are located in the pelvic cavity and produce sex hormones (estrogens and
progesterone) related to development and maintenance of female sexual
characteristics, reproductive cycle, pregnancy, lactation, and normal reproductive
functions. The ovaries also produce inhibin and relaxin.

B.  Testes lie inside the scrotum and produce sex hormones (primarily testosterone) related to the
development and maintenance of male sexual characteristics and normal reproductive
functions. The testes also produce inhibin.

C.  Table 18.10 summarizes the hormones produced by the ovaries and testes and their
principal actions.

XII.  PINEAL GLAND - resets hypothalamus (clock), called "third eye", it’s a CVO.

A. The pineal gland is attached to the roof of the third ventricle, inside the brain (Fig. 18.1). (p.597)   

B. The pineal secrets melatonin in a diurnal rhythm linked to the dark(more)-light(less) cycle (Figure 18.20).

C. Seasonal affective disorder (SAD), a type of depression that arises during the winter
months when day length is short, is thought to be due, in part, to over-production of
melatonin. Bright light therapy, repeated doses of several hours of exposure to artificial
light as bright as sunlight, may provide relief for this disorder and for jet lag. (Clin. App.)

XIII.  THYMUS GLAND

A. The thymus gland secretes several hormones related to immunity that promote the proliferation and maturation of T cells, a type of white blood cell involved in immunity.

XIV.   MISCELLANEOUS HORMONES - Table 18.11 summarizes these hormones and their actions.

A.   Eicosanoids

1.  Eicosanoids. (prostaglandins [PGs] and leukotrienes[LTs]   and
thromboxanes act as paracrines and autocrines in most body tissues by
altering the production of second messengers, such as cyclic AMP.

2.             Prostaglandins have a wide range of biological activity in normal physiology
and pathology.

5.   Aspirin and related nonsteroidal anti-inflammatory drugs (NSAIDS), such as

ibuprofen and acetaminophen, inhibit a key enzyme in prostaglandin and thromboxanes synthesis and are used to treat a wide variety of inflammatory disorders.

B.   Growth Factors

1.   Growth factors are hormones that stimulate cell growth and division.

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2.             Examples include epidermal growth factor (EGF), platelet-derived growth factor
(PDGF), fibroblast growth factor (FGF), nerve growth factor (NGF), tumor
angiogenesis factors (TAFs), insulinlike growth factor (IFG), and cytokines, (see p.
757 - immune cells secrete cytokines)

3.             Table 18.12 presets a summary of sources and actions of six important growth
factors.

XV. STRESS AND THE GENERAL ADAPTATION SYNDROME

A.   Homeostatic mechanisms attempt to counteract the everyday stresses of living. If
successful, the internal environment maintains normal physiological limits of
chemistry, temperature, and pressure. If a stress is extreme, unusual, or long-lasting,
however, the normal mechanisms may not be sufficient, triggering a wide-ranging set
of bodily changes called the stress response or general adaptation syndrome (GAS):

1.  Unlike the homeostatic mechanisms, this syndrome does not maintain a
constant internal environment. It does just the opposite to prepare the body to
meet an emergency.

2.              Productive stress is termed eustress; whereas, harmful stress is termed distress.

3.             The stimuli that produce the general adaptation syndrome are called stressors.

4.             Stressors include almost any disturbance: heat or cold, surgical operations,
poisons, infections, fever, and strong emotional responses.

B.   Stages of the General Adaptation Syndrome

1.    The Alarm Reaction

a.    The alarm reaction is initiated by nerve impulses from the
hypothalamus to the sympathetic division of the autonomic nervous
system and adrenal medulla (Figure 18.2la).

b.    Responses are the immediate and brief flight-or-flight reactions that
increase circulation, promote catabolism for energy production, and
decrease nonessential activities.

2.   The Resistance Reaction

a.   The resistance reaction is initiated by regulating hormones secreted by the hypothalamus (Figure 18.21b).

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