CHAPTER 18 ENDOCRINE SYSTEM
The
organs of this system are the various endocrine glands. There are 2 basic kinds
of glands in the body:
1.
Exocrine
glands—secrete products into ducts
Sweat glands Sebaceous glands
Digestive glands Mucous
glands
2.
Endocrine
glands—secrete products (hormones) directly into the bloodstream (no ducts)
a.
Pituitary
gland
b.
Thyroid
c.
Parathyroids Exclusively endocrine glands
d.
Adrenals No other job in the body
e. Pineal
f.
Hypothalamus
g.
Thymus
h.
Pancreas
i.
Ovaries
j.
Testes Contain endocrine tissue and
k.
Kidneys secrete essential hormones but
l.
Stomach have other jobs also
m.
Liver
n.
Small
intestine
o.
Skin
p.
Heart
q.
Adipose
tissue
r.
Placenta
Endocrinology
is the study of the endocrine system.
The
2 control systems of the body work together to maintain homeostasis:
1.
Nervous
system—messengers are nerve impulses and effects are rapid
a.
Causes
muscles to contract
b.
Causes
glands to secrete more or less of their product
2.
Endocrine
system—messengers are hormones and effects are slower
a.
Alters
metabolic activity
b.
Regulates
growth and development
c.
Regulates
activity of smooth and cardiac muscle
d.
Regulates
activity of some glands
e.
Influences
reproductive processes
Since
hormones are carried in the blood, they reach all cells. However, a hormone
causes an effect only in certain cells and not in others. The affected cells
are called target cells and each hormone has its own specific target cells.
When a hormone reaches a target cell, it binds to specific receptors which only
the target cell will have—no receptor, no effect.
The
number of receptors a target cell has may vary.
Up-regulation—the number of receptors may
increase when the amount of hormone is low---this causes target tissue to
become more sensitive to the hormone
Down-regulation—the number of receptors may decrease when the amount
of hormone is high---this causes target tissue to become
less sensitive to the hormone
Only
tiny quantities of a hormone are required to produce the effect.
Circulating
hormones—carried by the blood to act on cells located some distance from the
secreting cells. Their effects generally last from minutes to hours, and these
eventually are inactivated by the liver and excreted by the kidney. If the
effect needs to be prolonged, the hormone must be steadily secreted.
Local hormones—act near the cells that
secrete them and are quickly inactivated.
Paracrines—act on cells located very
near the secreting cells
CHEMICAL CLASSES OF HORMONES:
1.
Lipid-soluble hormones
a. Steroids—derived from cholesterol and
synthesized on smooth ER. These have a complicated carbon ring structure.
Sex hormones
Cortisol
b. Thyroid hormones (T3 and T4)---synthesized by attaching iodine to an amino acid
(tyrosine)
c. Nitric oxide gas
2.
Water soluble hormones
a. Amine hormones---synthesized by modifying
an amino acid molecule.
Epinephrine
Norepinephrine
Histamine
Serotonin
b. Peptides and proteins—chains of amino
acids, synthesized by ribosomes associated with rough ER. Many of the hormones are in
this group.
1) Peptides—3 – 49 amino
acids—antidiuretic hormone, oxytocin
2) Proteins—50 – 200 amino
acids—human growth hormone, insulin, thyroid
stimulating
hormone
c. Eicosanoids—recently classified group of
hormones—derived from arachidonic acid (a fatty acid) and various ones are
produced by all cells of the body except RBC. 2 major families:
Prostaglandins
Leukotrienes
Hormone
molecules are released into the bloodstream, where they travel in 2 ways:
Free form if they are water soluble
Attached to plasma
proteins called transport proteins if they are lipid soluble. Transport
proteins are synthesized by the liver—the combination of hormone plus transport
protein is water soluble, although the hormone molecule itself is not. The
combination is not permanent—small amounts of the hormone are released by the
transport protein and diffuse out to body tissues.
Hormones
may cause target cells to:
1.
Synthesize
new molecules
2.
Change
the permeability of the plasma membrane
3.
Stimulate
transport of a substance in or out of the cell
4.
Alter the
rate of specific metabolic reactions
5.
Contract
(smooth or cardiac muscle fibers)
Hormones
begin causing their activity with the binding of the hormone to receptors.
1.
Lipid
soluble hormones enter the target cell because they pass easily through the
plasma membrane. Once inside the target cell, the hormone binds to an
intracellular receptor in either the cytosol or the nucleus. The activated
receptor then alters gene expression (turns specific genes on or off). If a
gene is turned on, mRNA is produced and directs the synthesis of new proteins,
which are often enzymes. These new proteins alter the cell’s activity. Or, if
the effect is to turn off a gene, mRNA synthesis (transcription) is halted and
levels of that gene's protein decline. Either way, this mode of action is
slower.
2.
Water-soluble
hormones cannot easily enter the cell. Their receptors are located on the
outside of the plasma membrane. The hormone binds to the receptor and never
enters the cell. These are integral proteins of the plasma membrane which stick
out from the from the cell.
Hormones
which act this way are called first messengers. They deliver their message only
as far as the plasma membrane. A second messenger inside the cell must relay
the message and bring about the response. Second messengers include:
a.
Cyclic
AMP (cAMP)—most common one—synthesized from ATP by an enzyme, adenylate
cyclase, which is attached to the inside of the plasma membrane. Molecules
within the plasma membrane called G proteins link receptors on the outside to
enzymes molecules on the inside. Binding of a hormone molecule to its receptor
causes activation of the enzyme and cyclic AMP is produced.
Inside the cell, one or more protein
kinases (more enzymes) are activated by the higher cyclic AMP level and add a
phosphate group to other enzymes within the cell. This will either turn on or
turn off the activity of these other enzymes. What kind of enzymes are affected
and the results of their change in activity varies from one type of cell to
another.
End results could be:
Thyroid cell—secretes thyroid
hormones
Cell of adrenal cortex—secretes
steroid hormones
Cells of kidney tubules—increase
permeability to water
In some cells, rising cAMP causes the
cell to INHIBIT glycogen synthesis
In other cells, higher cAMP levels
cause the cell to INCREASE glycogen synthesis
Cyclic AMP is quickly inactivated by an
enzyme, phosphodiesterase, so hormone molecules must continue to bind to extra
cellular receptors for a long-term effect.
To show how hormone action can vary, a
few hormones bring about their effect by decreasing the level of cAMP in
the target cell.
b.
Other
second messengers include Ca2+ ions, cyclic GMP, and several
substances—the mechanisms of these are less clearly understood.
Tiny
amounts of this type of hormone may cause a great response because the binding
of one hormones molecule to one receptor may activate about 100 G proteins.
Each G protein activates an adenylate cyclase, which then produces cyclic AMPs,
which activates large numbers of protein kinases. Hundreds or thousand of
substrate molecules are then quickly affected.
Three
things influence the response of a target cell to a hormone:
1.
Concentration of the hormone
2.
Number of receptors
3.
Influence by other hormones:
a. Permissive effect—the effect of one
hormone on a target cell requires the previous or simultaneous exposure to
another hormone. The permissive hormone may cause and increase in the number of
receptors for the other hormone or promote synthesis of an enzyme needed for
the expression of the other hormone.
Estrogen exposure in the early part of
the monthly cycle causes an increase in the number of progesterone receptors in
cells of the uterine wall, which allows progesterone to have a greater effect.
b. Synergistic effect—2 or more hormones
complement each other and both are needed for full expression. No one hormone
can produce the effect alone.
Milk production (lactation)—estrogens,
progesterone, prolactin, oxytocin, somatomammotropin
3.
Antagonistic
effect—effect of 1 hormone on a target cell is opposed by another.
Insulin/glucagon—lower/raise blood
sugar
Hormone
secretion by endocrine glands is stimulated and inhibited by:
1.
Signals
from the nervous system
Adrenal medulla à Epinephrine
2. Chemical changes in the blood
Blood Ca2+
level and CT/PTH
3. Other hormones
ACTH à Cortisol
Most often negative feedback systems act to regulate hormone
secretion and maintain homoestasis, although several examples of beneficial
positive feedback systems are also found.
Hypersecretion—too much
Hyposecretion—too little
Hormones that influence another endocrine gland are called
tropins or tropic hormones.
Gonadotropin—gonads
Adrenocorticotropic hormone—adrenal cortex
BE SURE TO SEE THE CHART FOR A SUMMARY OF ENDOCRINE GLANDS, HORMONES, AND THEIR EFFECTS
ENDOCRINE
GLANDS/HORMONES
I. HYPOTHALAMUS---this
small region of the brain acts as the major link between the nervous system and
the endocrine system and plays a major role in maintaining homeostasis. It
receives input from other parts of the brain and also from various internal
organs. In many situations, the hypothalamus initiates autonomic nerve impulses
in response to this input, but it may also act to produce a response by the
endocrine system. Hormones of the hypothalamus act on the nearby pituitary
gland to turn production and release of anterior pituitary hormones on or off:
1. Growth hormone releasing hormone (GHRH)--turns hGH
secretion on
2. Growth hormone inhibiting hormone (GHIH)--turns hGH
secretion off
3. Thyrotropin releasing hormone (TRH)--1st step in turning on thyroid
gland
4. Gonadotropin releasing hormone (GRH)--1st step in turning on the
ovaries or
testes
5. Prolactin inhibiting hormone (PIH)--turns off production of prolactin
6. Prolactin releasing hormone (PRH)--turns on production of prolactin
7. Corticotropin releasing hormone--turns on the production of ACTH,
which acts on
the adrenal glands
II.
PITUITARY GLAND (HYPOPHYSIS)--called
the master gland of the body because its hormones control other endocrine
glands. It is the size of a pea and lies in the sella turcica of the sphenoid
bone. It dangles from the hypothalamus by a stalk called the infundibulum. It
is divided into 2 lobes:
A.
Anterior pituitary (adenohypophysis)—makes
up 95% of the gland by weight and secretes 7 hormones that regulate a wide
range of body activities. Secreting cells are called neurosecretory cells and
are specialized neurons of five different types. The cell bodies produce the
hormones, which are released from the axon terminals. Release of these hormones
occurs in response to releasing hormones and some are suppressed by inhibiting
hormones, all produced by the hypothalamus. Blood vessels (the hypophyseal
portal system) directly connect the hypothalamus to the anterior pituitary to
deliver the controlling hormones quickly and in concentrated form. The 7
hormones of the anterior pituitary are:
1. Human growth hormone (hGH,
somatotropin)--general metabolic hormone. Secreted by somatotrophs, it causes
body cells to produce small protein hormones called insulinlike growth factors
(IGFs) which have several effects:
a. Growth of skeletal muscles and
long bones in children
b. Maintenance of muscles and bones
in adults
c. Encourage the building of amino
acids into proteins (anabolism)
d. Stimulation of target cells to
grow and divide
e. Decrease in glucose uptake by
most body cells
f. Increase in blood sugar
The IGFs may act locally in the body
cells that produce them or circulate to affect other cells.
CONTROL: Secretion increases or
decreases in response to GHRH or GHIH of the hypothalamus.
ABNORMALITIES: During growth years
hyposecretion results in decreased growth and very short stature;
hypersecretion results in giantism. Hypersecretion beginning in adulthood
results in acromegaly, a thickening of the bones and soft tissues of the hand,
feet, cheeks and jaws.
2. Thyroid-stimulating hormone (TSH)—secreted by thryotrophs and
stimulates cells of the thyroid gland to produce thyroid hormones (T3
and T4).
CONTROL: Thyrotropin releasing hormone
from the hypothalamus, which is released when levels of TSH and T3
in the blood drop.
3. Follicle-stimulating hormone (FSH)--in females causes egg-containing
follicles to begin to develop in the ovary each month. FSH also causes the
follicular cells to secrete estrogens. In males FSH stimulates sperm
production.
CONTROL: Gonadotropin releasing hormone
from the hypothalamus causes secretion of FSH. GnRH is secreted when
estrogen/testosterone level drops.
4. Luteinizing hormone (LH)--in females aids in stimulation of estrogen
production by the ovaries and brings about the release of the egg (ovulation).
LH also stimulates the formation of the corpus luteum, which develops from the
follicular tissue after the egg is released. The corpus luteum secretes
progesterone.
In males, LH is also referred to as
interstitial-cell stimulating hormone (ICSH), because it stimulates the
interstitial cells of the testes to produce testosterone.
CONTROL: Also by GnRH of the
hypothalamus.
Both FSH and LH are secreted by cells
called gonadotrophs. Together these 2 hormones are often called gonadotropic
hormones.
5. Prolactin (PRL)—secreted by lactotrophs. In females, together with
other hormones, PRL initiates and maintains milk secretion by the mammary
glands (lactation). Function in males is uncertain.
CONTROL: Prolactin inhibiting hormone
(PIH) secreted by the hypothalamus inhibits the release of PRL. In non-pregnant
females fluctuating levels of estrogens and progesterone during normal cycles
cause the level of PRL to vary, but any rise does not last long enough to bring
about lactation.
During pregnancy the hormones present
initiate the release by the hypothalamus of prolactin releasing hormone (PRH),
which causes the release of PRL from the anterior pituitary and initiates
lactation.
6. Adrenocorticotropic hormone (ACTH)—secreted by corticotrophs and
controls the production and secretion of hormones called glucocorticoids by the
adrenal cortex.
CONTROL: Produced in response to
corticotropin releasing hormone (CRH) of the hypothalamus and also directly in
response to stress
7. Melanocyte-stimulating hormone (MSH)--increases skin pigmentation.
Purpose in humans is unknown. This hormone is secreted by slightly modified
corticotrophs.
CONTROL: Excessive levels of CRH can
increase MSH release; dopamine inhibits release
TABLE 18.3 P. 627
B. Posterior pituitary gland
(neurohypophysis)--does not actually produce hormones but stores and
releases 2 hormones that are synthesized in the hypothalamus. Two different
types of neurosecretory cells have their cell bodies in the hypothalamus with
their axons extending down a special tract into the posterior pituitary. The
cell bodies secrete the 2 hormones, which are then transported down the axons
to the posterior pituitary. Also present in the posterior pituitary are
specialized glial cells called pituicytes.
1. Oxytocin (OT)--this hormone has 2 kinds of target cells and 2
effects:
a. During labor and delivery the target cells are the smooth muscle
fibers of the uterus, which contract in response. (Positive feedback cycle)
b. After delivery OT is involved in the ejection ("let-down")
of milk from the breasts. The target cells are smooth muscle fibers of which
surround the milk-secreting glands.
2. Antidiuretic hormone (ADH, vasopressin)--function is to decrease the
urine production. It causes the kidneys to reabsorb more of the water passing
through and also decreases the activity of the sweat glands. It causes
vasoconstriction (decrease in diameter) of arterioles if present in large amounts,
and is also known as vasopressin because this tends to raise blood pressure.
CONTROL: In case of low water intake or
high output, the body becomes dehydrated. The concentration of water in the
blood drops below normal. Osmoreceptors in the hypothalamus detect this change
and activate the neurosecretory cells that produce and relesase ADH.
In case of high water intake, water concentration in the blood becomes
higher than normal and osmoreceptors detect this and cause ADH production to
slow or stop, so the excess water can be removed by the kidneys.
Other things that can alter ADH
production:
ADH secretion increases (urine output decreases) due to pain, stress,
trauma and certain drugs
ADH secretion decreases (urine output increases) due to alcohol
ABNORMALITIES: If ADH is not secreted
the condition is called diabetes insipidus. It results in constant production
of very large amounts of urine (up to 20 liters/day).
III.
THYROID GLAND--located
just below the larynx and consists of 2 lateral lobes connected by a mass
called the isthmus that runs over the front of the trachea. The gland is filled
with microscopic spherical sacs called thyroid follicles. The walls of the
follicles consist of 2 kinds of cells:
1. Follicular cells extend to the lumen of the follicle and secrete the
2 thyroid hormones, thyroxine (T4) and triiodothyronine (T3).
2. Parafollicular cells ( C cells)--fewer in
number and lie between the follicles. These secrete another hormone,
calcitonin.
A. Thyroid hormones--these are normally stored in fairly large
quantities in the thyroid, which is unusual for endocrine glands. T3
and T4 are the hormones and they are formed by attaching iodine
atoms to an amino acid, tyrosine. Follicular cells trap iodine from the blood
and at the same time form a glycoprotein called thyroglobulin. The
thyroglobulin is moved into the lumen of the follicle, where it is called
colloid. Iodine molecules in the colloid react with tyrosine molecules in the
colloid to form thyroid hormones. They travel in the blood combined with
transport proteins.
Thyroid hormones regulate:
1.
Oxygen
use and the basal metabolic rate
2.
Cellular
metabolism
3.
Growth
and development
Both 1 & 2 increase as the level of
thyroid hormones increases. All body cells are target cells for TH. As cells
use oxygen and produce ATP, heat is given off (the calorigenic effect) and this
is essential to maintaining normal body temperature.
CONTROL: Negative feedback systems
involving the hypothalamus and anterior pituitary. Low blood levels of
triiodothyronine stimulate the hypothalamus to secrete thyrotropin releasing
hormone, which in turn causes production and release of TSH. TSH stimulates the
thyroid gland to secrete thyroid hormones.
ABNORMALITIES:
1. Cretinism--severe deficiency of thyroid hormones during fetal
development or early infancy causes a combination of failure of skeletal growth
and severe mental retardation. Treatment is oral thyroid medication, but this
will not reverse severe damage.
2. Myxedema--severe deficiency beginning in adult years. Symptoms are
facial edema (puffiness), low heart rate and body temp, sensitivity to cold and
a tendency to gain weight. Retardation does not occur, but mental dullness may
result. Treatment is oral thyroid medication.
3. Hypersecretion (in general) causes increased metabolic rate, heat
intolerance, sweating, weight loss, nervousness.
4. Graves disease--most common type of hypersecretion, autoimmune
disorder that causes the thyroid to continuously secrete. In addition to other
symptoms of hypersecretion, the thyroid may enlarge and a pop-eyed appearance
(exophthalmus) may develop. Treatment may include removal of part of the gland,
drugs or radiation.
5. Goiter--enlarged thyroid gland, may occur
due to severe dietary iodine deficiency.
B. Calcitonin--produced by parafollicular cells and involved in
homeostasis of blood calcium and phosphate levels. Calcitonin inhibits the
action of osteoclasts and accelerates the uptake of calcium and phosphates by
bones, which tends to lower blood calcium and phosphate levels.
CONTROL: High blood calcium stimulates
secretion, low blood calcium inhibits
IV.
PARATHYROID GLANDS--these
are masses of tissue attached to the back of the thyroid gland, usually a
superior and inferior parathyroid attached to each lateral lobe of the thyroid.
The parathyroids contain 2 kinds of cells:
1. Principal (chief) cells--produce parathyroid hormone (PTH)
2. Oxyphil cells--fewer in number, function uncertain
Involved in
regulating levels of calcium, magnesium, and phosphates in the blood. PTH acts to raise blood calcium levels
in several ways:
1.Increases the number and activity of
osteoclasts (bone breakdown cells). Bone matrix is broken down and the calcium
and phosphates are released into the blood.
2. PTH also causes the kidneys to reabsorb more of the calcium from the
blood
3. Encourages the formation of calcitriol, the activated form of Vitamin
D, which increases the absorption of calcium from the digestive tract.
PTH also raises blood levels of
magnesium and lowers blood phosphates, but we think of it mainly in relation to calcium.
CONTROL: PTH is released when blood
calcium drops; high blood calcium inhibits secretion.
ABNORMALITIES: Hyposecretion of PTH may
cause a severe drop in blood calcium levels. Neurons depolarize with much less
stimulus than normal, leading to muscle twitches, spasms and convulsions. This
is called tetany and most commonly occurs due to damage during thyroid surgery.
Hypersecretion causes extreme
demineralization of bone. The most common cause is a benign tumor of the
parathyroid.
V.
ADRENAL GLANDS--one lies
superior to each kidney. The outer layer of the gland is called the adrenal
cortex and surrounds a small center area, the adrenal medulla. The gland is
covered by a CT capsule.
A. Adrenal cortex--this layer is divided into 3 zones, each of which secretes
different steroid hormones:
1. Zona glomerulosa--just beneath the capsule and secretes the
mineralocorticoids. These hormones (aldosterone is the main one) help control
the mineral content of the blood. Aldosterone causes certain cells in the
kidneys to increase the reabsorption of sodium into the blood and as sodium is
reabsorbed water follows. Aldosterone also causes the kidneys to excrete
potassium and H+ ions.
CONTROL: Decreased blood volume causes
release of an enzyme, renin, by the kidney. This is the beginning of a series
of reactions which result in production of the hormone, angiotensin II, which causes
the adrenals to secrete aldosterone. Increased blood potassium also causes the
secretion of aldosterone.
2. Zona fasciculata--below the zona glomerulosa and secretes the
glucocorticoids, which regulate metabolism and resistance to stress. Specific
hormone names are cortisol (main one), corticosterone and cortisone. Effects
include making sure enough ATP is generated to meet the body's needs, providing
resistance to stress and inhibition of the inflammatory response. High levels
of glucocorticoids depress immune responses.
CONTROL: Low levels of cortisol
stimulate the hypothalamus to secrete corticotropin releasing hormone, which
leads to the release of ACTH which causes release of glucocorticoids. CRH is
also produced in response to stress.
ABNORMALITIES: Hyposecretion of
glucocorticoids and aldosterone is known as Addison's disease. Symptoms are a
bronze pigmentation of skin, problems with electrolyte and water imbalance,
hypoglycemia, and decreased resistance to stress.
Hypersecretion of glucocorticoids is
called Cushing's syndrome. Symptoms include development of spindly arms and
legs with a rounded face and protruding abdomen, osteoporosis, poor resistance
to stress, hypertension. May result from oversecretion by the gland or when
steroids are given in large doses for prolonged periods in treating autoimmune
disease, transplant recipients, etc.
3. Zona reticularis--this zone is next to the medulla and produces small
amounts of a weak androgen (male hormone), DHEA. Some of the DHEA is converted
to estrogen, so all males have some female hormones and all females have some
male hormones.
Another abnormality of the adrenals is
adrenogenital syndrome (adrenal hyperplasia). The person is unable to produce
cortisol due to a genetic defect. ACTH levels are high and precursors of
cortisol accumulate. These have the effect of a weak androgen. They cause
little effect in adult males, but in women and female children cause virilism. In male children, very early puberty results.
B. Adrenal medulla--this area consists of chromaffin cells, which are
modified neurons directly attached to the sympathetic nervous system neurons. They secrete the
hormones epinephrine and norepinephrine (catecholamines). These hormones are
sympathomimetic (mimic the effect of the sympathetic nervous system). They help
the body resist stress but are not essential for life (hormones of the cortex
are).
VI.
PANCREAS--both an
endocrine and an exocrine gland. The gland is a flattened organ located
posterior and inferior to the stomach. Most of the tissue of the pancreas is
exocrine tissue (pancreatic acini) but scattered among the acini are tiny
clusters of endocrine tissue called pancreatic islets or islets of Langerhans.
The islets contain 4 kinds of cells:
1. Alpha cells--secrete glucagon
2. Beta cells--secrete insulin
3. Delta cells--secrete growth hormone inhibiting hormone (identical to that
of
hypothalamus)
4. F cells--secrete pancreatic polypeptide (inhibits certain digestive
activities)
A. Glucagon--acts to increase blood glucose level when it falls below
normal. Main target tissue is the liver. Actions (all raise blood sugar):
1. Conversion of glycogen to glucose (glycogenolysis)
2. Formation of glucose from non-carbohydrate sources (gluconeogenesis)
3. Release of glucose from cells into the blood
B. Insulin--necessary for normal use of glucose by body cells, this is
the only hormone that lowers blood sugar. Actions:
1. Transport of glucose from blood into cells
2. Conversion of glucose to glycogen (glycogenesis)
3. Aids the entry of amino acids into cells and protein synthesis
CONTROL: Increased blood glucose
stimulates insulin secretion
ABNORMALITIES: Diabetes
mellitus--disease characterized by elevated blood glucose (hyperglycemia),
glucose in urine (glycosuria), increased urine volume (polyuria) and excessive
thirst (polydipsia).
Type I (insulin dependent)--previously called juvenile diabetes. Most
commonly develops before the age of 20 and is apparently an autoimmune disease
in which the body destroys the beta cells. There seems to be a genetic
predisposition to the disease. Without insulin the body cannot use glucose
normally, so it burns large amounts of fats for energy, with the formation of
by-products called ketone bodies (diabetic ketoacidosis). Other complications
include weight loss, cardiovascular problems, blindness and kidney damage.
Current treatment is insulin injections and careful monitoring of diet.
Type II--more common (90% of cases) and previously called adult onset
diabetes. Most frequently occurs in people who are over 40 and overweight.
Symptoms are similar to type I but usually milder. It is often controlled by
diet, weight loss and exercise. Oral medications are sometimes used but insulin
is usually not required. Many type II diabetics have a normal or even elevated
insulin level but apparently cells have fewer insulin receptors.
VII.
OVARIES--female
gonads, paired oval bodies located in the pelvic cavity. As well as producing
eggs, the ovaries produce the female sex hormones, estrogens and progesterone,
which:
1. Cause development & maintenance of female sexual characteristics
2. Regulate female reproductive cycle
3. Maintain pregnancy
4. Prepare the mammary glands for lactation
The ovaries also produce inhibin, a
hormone that inhibits the secretion of FSH late in the menstrual cycle. Late in
pregnancy the ovaries and the placenta also produce the hormone relaxin, which
relaxes the symphysis pubis and aids in dilation of the cervix.
VIII.
TESTES--male gonads that
as well as producing sperm also produce testosterone, the primary male sex
hormone, which:
1. Stimulates the development and maintenance of male sexual
characteristics
2. Regulates the production of sperm
The testes also produce inhibin.
IX.
PINEAL GLAND
(epiphysis cerebri)--attached to the roof of the third ventricle and covered by
pia mater. It consists of masses of neuroglia and secretory cells called
pinealocytes. Its role is not fully understood in humans, but it secretes the
hormone melatonin, which is produced during darkness and inhibited when light
enters the eyes. In animals that breed seasonally, melatonin alters
reproductive capacity. In humans it may help establish the sleeping/waking
cycle. It seems to be involved in a form of depression, seasonal affective
disorder (SAD), that arises in winter months when days
are short. It is thought to be due to overproduction of melatonin and treatment
is exposure to bright artificial light.
X.
THYMUS GLAND--several
hormones produced by this gland promote the production of certain types of white blood cells which are
important in the immune response. They are:
Thymosin
Thymic humoral factor
Thymic factor
Thymopoietin
These hormones promote proliferation
and maturation of certain white blood cells necessary for the immune response.
Various miscellaneous hormones will be
covered in chapters on the system which they influence most. Two groups will be
briefly discussed here.
EICOSANOIDS—many of these hormones have been
discovered fairly recently and their classification as hormones is also recent.
They include 2 families, the prostaglandins (PGs) and the leukotrienes (LTs).
Both are synthesized by taking a fatty acid, arachidonic acid, from the plasma membraneand changing it by
enzyme activity to either a prostaglandin or a leukotriene. All body cells
except red blood cells produce and release one or more of these hormones in
response to various chemical and mechanical stimuli. They act mainly as
paracrines and autocrines and ar3e quickly inactivated.
Leukotrienes stimulate chemotaxis of
white blood cells and mediate inflammation and allergic responses.
Prostaglandins have a wide range of
effects throughout the body including:
1. Alteration of smooth muscle contraction, glandular secretion, blood
flow, reproduction, nerve impulse transmission, platelet function, fat
metabolism, and
immune response.
2. Roles in inflammation, fever, and pain
Some prostaglandins have desirable
effects and there is considerable interest in developing these as drugs. Other
undesirable effects can be controlled by anti-prostaglandin drugs, so this is
another area of pharmaceutical research. Aspirin, Tylenol and ibuprofen are
such drugs that are already in use.
Some of the hormones already discussed stimulate cell growth
and division (insulinlike growth factors, thyroid hormones, etc.). We have
recently discovered other hormones which mostly act locally to stimulate cell
division in nearby cells. Since they encourage mitosis, they are also called
mitogenic factors. These are simply called growthfactors.
TABLE 18.12 P. 652
Any
stimulus that produces a stress response is called a stressor. Some stresses
are beneficial—prepare us to meet challenges. These are called eustress. Other
stress is harmful and is called distress.
This
may be almost any disturbance---heat, cold, toxins, blood loss, emotional
reaction, etc. The body responds to stressors in these ways:
1.
Alarm reaction (fight-or-flight response)---initiated
by nerve impulses from the hypothalamus, happens very quickly and is relatively
short in duration. The sympathetic nervous system and adrenal medulla bring
about the following:
a. Increase in rate and force of heartbeat
b. Blood vessels to skin and viscera
contract
c. Blood vessels to skeletal muscle, heart
and lungs dilate
d. Capsule of spleen contracts and releases
stored blood into circulation
e. Liver converts glycogen to glucose, which
raises blood sugar
f. Rate of respiration increases and
respiratory passages widen