41.1.
Chemical Signals
A. Categories of Signals
1. Chemical signals are used: between individuals, between body
parts, and between cells.
2. Pheromones are
environmental signals that act at a distance between individual organisms.
a. Ants lay
down a pheromone trail for other members to find food.
b. Female
silkworm moth releases pheromone to lure a male moth from miles away.
c. Dog urine
serves as a territorial marker.
3. Endocrine secretions
or hormones are environmental signals that act at a distance
between body parts.
4. A hormone is an
organic chemical produced by one set of cells that affects a different set.
5. A hormonetravels
through the circulatory system to its target organ.
6. Cells respond to a hormone
depending on their receptors; they combine in a lock-and-key manner.
7. This also includes the secretions
of neurosecretory cells into the hypothalamus.
8. Environmental signals can act
locally between adjacent cells.
a.
Neurotransmitters released by neurons belong to this category.
b.
Prostaglandins and growth factors are also called local hormones.
c. Tumor
angiogenesis factor promotes formation of capillaries; antagonists can stop
this growth.
B. Action of Hormones
1. A hormonedoes not seek out a target organ; the organ is awaiting
the arrival of the hormone.
2. Steroid hormones
have the same complex of four carbon rings but have different side chains.
a. Steroid
hormones are lipids and cross cell membranes freely.
b. Inside a
nucleus, hormones such as estrogen and progesterone bind to a specific
receptor.
c.
Hormone-receptor complex binds to DNA resulting in activation of genes that
produce enzymes.
3. Peptide hormones
a. Peptide
hormones never enter a cell so they bind to a receptor protein in plasma
membrane.
b.
Epinephrine binds to receptor protein; relay system leads to conversion of ATP
to cyclic AMP.
c. Cyclic
AMP (cAMP) is made from ATP; it has one phosphate group attached to
adenosine at two locations.
d. Peptide
hormones are the first messenger; cAMP and calcium are
often the second messenger.
e. cAMP sets an enzyme cascade in motion.
f. Activated
enzymes can be used repeatedly, resulting in a thousand-fold response.
41.2. Human
Endocrine System
A. Endocrine Glands
1. Endocrine glands are ductless glands in contrast to exocrine
glands with ducts.
2. Endocrine system
consists of endocrine glands that coordinate body activities through hormones.
3. Their hormones that are secreted
directly into bloodstream.
4. A hormone may have a different
function in different species.
a. Prolactin
stimulates breasts to secrete milk but stimulates the gut in pigeons.
b. Thyroxine
in human stimulates metabolism but induces metamorphosis of tadpoles to frogs.
5. Principal human endocrine
glands include:
a.
Hypothalamus, pineal, and pituitary glands located in brain
b. Thyroid
and parathyroid glands located in neck.
c. Ovaries
located in abdomen, and testes in scrotum.
d. Thymus
located in thorax.
6. Endocrine system is especially
involved with homeostasis.
7. Effect of hormones is controlled
by negative feedback and contrary hormone action.
a. Endocrine
glands can be sensitive to the condition monitored or to level of hormone
produced.
1) Several hormones affect blood glucose, calcium, and sodium levels.
2) Others are involved in maturation and function of organs (i.e. gonads, etc.)
b. Negative
feedback control is one mechanism.
1) Pancreas produces insulin when blood glucose rises; this causes liver to
store glucose.
2) When glucose is stored, level goes down and pancreas stops insulin
production.
c. Contrary
actions of hormones can control hormonal regulation.
1) Effect of insulin is offset by production of glucagon by pancreas.
2) Thyroid lowers blood calcium level but the parathyroids raise blood calcium
level.
B. Hypothalamus and Pituitary Gland
1. Hypothalamus regulates the internal environment through the
autonomic system.
2. It controls heart beat,
temperature, water balance, as well as glandular secretions of pituitary gland.
3. Pituitary Gland
a. Pituitary
gland is connected to hypothalamus by a stalk-like structure.
b. It is
about 1 cm in diameter and lies just below the hypothalamus.
c. It is
comprised of two portions: posterior pituitary and anterior pituitary.
4. Posterior Pituitary
a. This
portion of pituitary gland is connected to hypothalamus by a stalk-like
structure.
b. It
contains portions of neurosecretory cells that originate in
hypothalamus and respond to
neurotransmitters and produce hormones.
c.
Hypothalamus produces antidiuretic hormone (ADH or vasopressin)
and oxytocin, which pass through
axon endings in posterior pituitary and are stored until released.
d. Antidiuretic
hormone (ADH) promotes reabsorption of water from collecting ducts in
kidneys.
1) Nerve cells in the hypothalamus determine when blood is too concentrated;
ADH is released
and kidneys respond by reabsorbing water.
2) As blood becomes dilute, ADH is no longer released; this is a case of
negative feedback.
e. Oxytocin
is also made in hypothalamus and stored in posterior pituitary.
1) Oxytocin stimulates uterine muscle contraction in response to uterine wall
nerve impulses.
2) It also stimulates release of milk from mammary glands.
3) This positive feedback increases intensity; positive feedback does not
maintain homeostasis.
5. Anterior Pituitary
a.
Stimulation by hypothalamus controls release of anterior pituitary hormones;
direct stimulation of
pituitary does not.
b.
Hypothalamus produces hypothalamic-releasing and hypothalamic-release-inhibiting
hormones which
pass to anterior pituitary by portal system of two capillary beds and one vein.
1) Thyroid-releasing hormones released from hypothalamus act on
cells in anterior pituitary to
stimulate production and secretion of a specific hormone.
2) Thyroid-inhibiting hormones produced in and released from
hypothalamus act on cells in anterior
pituitary to inhibit production and secretion of a specific hormone.
c. Anterior
pituitary produces six different hormones, each by a distinct cell type.
d. Three
anterior pituitary hormones affect other glands.
1) Thyroid-stimulating hormone (TSH) stimulates thyroid to
produce and secrete thyroxin.
2) Adrenocorticotropic hormone (ACTH) stimulates the adrenal
cortex to release cortisol.
3) Gonadotropic hormones (follicle-stimulating hormone
[FSH] and luteinizing hormone [LH]) act
on gonads (ovaries and testes) to secrete sex hormones.
e. Three
hormones have direct effects on the body.
f. Prolactin
(PRL) is produced in quantity only after childbirth.
1) Prolactin causes mammary glands to produce milk.
2) It also plays a role in carbohydrate and fat metabolism.
g. Melanocyte-stimulating
hormone (MSH) causes skin color changes in fishes, amphibians, and
reptiles with melanophores, special skin cells.
h. Growth
Hormone (GH or somatotropic hormone)
1) GH promotes skeletal and muscular growth.
2) GH acts to stimulate transport of amino acids into cells and to increase
activity of ribosomes.
3) GH promotes fat metabolism rather than glucose metabolism.
4) Too little GH during childhood makes an individual a pituitary dwarf.
5) Too much forms a giant; life expectancy is less-GH affects blood glucose
levels and promotes
diabetes mellitus.
6) Overproduction of GH in adults results in acromegaly; only
feet, hands, and face grow.
C. Thyroid Glands
1. Thyroid gland is in neck and attached to trachea just below
larynx. (Fig. 49.3)
2. The two hormones produced by many
follicles of the thyroid both contain iodine.
a. Thyroxine
(T4) contains four iodine atoms.
b. Triiodothyronine
(T3) contains three iodine atoms.
3. Iodine, actively transported into
thyroid, may reach concentrations 25 times greater than in blood.
4. Lack of iodine causes enlargement
(goiter).
a. The
anterior pituitary stimulates the thyroid to secrete thyroxine.
b. An
increase in size (goiter) is ineffective since the thyroxine level is low due
to iodine shortage.
c. Goiter is
prevented by supplementing iodine in salt.
5. Thyroid hormones increase
metabolic rate; there is no one target organ, all organs respond.
6. Cretinism occurs in
individuals who have suffered from low thyroid function since birth.
a. They are
short and stocky and have had hypothyroidism since infancy.
b. Thyroid
treatment helps but unless begun in first two months, mental retardation
occurs.
7. Myxedema is
hypothyroidism in adults; thyroid hormones restore normal function.
8. Hyperthyroidism (Grave's disease)
occurs when thyroid gland is enlarged or overactive.
a. The eyes
protrude because of edema in eye socket tissue; called exophthalmic goiter.
b. Removal
or destruction of some thyroid tissue by surgery or radiation often cures it.
9. Thyroid gland also
produces calcitonin.
a. Calcitonin
lowers calcium level in blood and increases deposit in bone by reducing
osteoclasts.
b. If blood
calcium is normal, release of calcitonin is inhibited; low calcium levels
stimulate release
of parathyroid hormone (PTH).
D. Parathyroid Glands
1. Four parathyroid glands are embedded in posterior surface of
thyroid gland.
2. Parathyroid glands produce parathyroid
hormone (PTH).
3. Under influence of PTH, calcium
level in blood increases and phosphate level decreases.
4. PTH stimulates the absorption
of Ca2+ by activating vitamin D, the retention of Ca2+ (and excretion
of
phosphate) by the kidneys, and demineralization
of bone by promoting activity of osteoclasts.
5. When blood calcium level reaches
the right level, parathyroid glands no longer produce PTH.
6. If PTH is not produced in
response to low blood Ca2+, tetany results because Ca2+ plays an important
role in both
nerve conduction and muscle contraction.
7. In tetany, the body
shakes from continuous muscle contraction due to increased excitability of
nerves
that fire
spontaneously and without rest.
E. Adrenal Glands
1. Each of two adrenal glands sit atop
each kidney.
2. Each gland consists of two parts:
an outer adrenal cortex and an inner adrenal medulla.
3. Hypothalamus exerts control over both portions.
a. Nerve
impulses travel via brain stem to spinal cord to sympathetic nerve fibers to
medulla.
b.
Hypothalamus uses ACTH-releasing hormone to control anterior
pituitary's secretion of ACTH.
4. Adrenal hormones increase during
times of physical and emotional stress.
F. Adrenal Medulla
1. Epinephrine and norepinephrine are produced by adrenal
medulla.
2. Both hormones bring about body
changes corresponding to an emergency.
a. Blood
glucose level rises and metabolic rate increases.
b.
Bronchioles dilate and breathing rate increases.
c. Blood
vessels to digestive tract and skin constrict; those to skeletal muscles
dilate.
d. Cardiac
muscle contracts more forcefully and heart rate increases.
G. Adrenal cortex hormones provide a sustained response to stress.
1. Adrenal cortex secretes two types of hormones: glucocorticoids
and mineralocorticoids.
a. Glucocorticoids
help regulate blood glucose levels.
b. Mineralocorticoids
regulate levels of minerals in blood.
c. It also
secretes a small amount of both male and female sex hormones in
both sexes.
2. Cortisol is a
biologically significant glucocorticoid.
a. Cortisol
promotes breakdown of muscle protein to amino acids taken up by liver from
blood.
b. Cortisol
breaks down of fatty acids rather than carbohydrates; cortisol raises blood
glucose levels.
c. Cortisol
counteracts the inflammatory response; it helps medicate arthritis and
bursitis.
3. Aldosterone is the
most important of the mineralocorticoids.
a. Primary
target organ is kidney where it promotes reabsorption of Na+ and excretion of
K+.
b.
Mineralocorticoid secretion is controlled by
renin-angiotensin-aldosterone system.
1) Under low blood volume and sodium levels, kidneys secrete renin.
2) The enzyme renin converts plasma protein angiotensinogen to angiotensin I;
this becomes
angiotensin II by a converting enzyme in lungs.
3) Angiotensin II stimulates adrenal cortex to release aldosterone.
4) Angiotensin I constricts arterioles directly; aldosterone causes kidneys to
absorb calcium.
5) When blood sodium rises, water is reabsorbed as hypothalamus secretes ADH;
blood pressure
increases to normal.
c. Atrial
natriuretic hormone (ANH) causes excretion of sodium.
1) When atria of heart are stretched due to increased blood volume, cardiac
cells release ANH.
2) ANH inhibits secretion of renin by kidneys and secretion of aldosterone from
adrenal cortex.
3) When sodium is excreted, so is water; blood volume
and pressure return to normal.
H. Malfunction of the Adrenal Cortex
1. Low levels of adrenal cortex hormones (hyposecretion) results in
a. ACTH is
in excess; like MSH, it can lead to buildup of melanin and bronzing of skin.
b. Lack of
cortisol results in low glucose levels; a stressed person has insufficient
energy.
c. Lack of
aldosterone drops blood sodium levels; a person has low blood pressure and
dehydration.
d.
Untreated,
2. High levels of adrenal cortex
hormones from hypersecretion result in Cushing syndrome.
a. Excess
cortisol causes a tendency toward diabetes mellitus.
b. Muscular
protein decreases; subcutaneous fat forms an obese trunk but normal arms and
legs.
c. Other
symptoms: high blood sodium level, basic blood pH, hypertension, and edema of
the face.
d. Women
may have masculinization from oversecretion of adrenal male sex hormone.
I. Pancreas
1. Pancreas lies transverse in abdomen between kidneys and near
duodenum.
2. Pancreas is composed of two types
of tissue.
a. Exocrine
tissue produces and secretes digestive juices into small
intestine by way of ducts.
b. Endocrine
tissues called pancreatic islets (of Langerhans) produce insulin
and glucagon.
3. All body cells utilize glucose;
therefore, its level must be closely regulated.
4. Insulin is secreted
when blood glucose level is high after eating; insulin has three actions.
a. Insulin
stimulates liver, fat, and muscle cells to take up glucose.
b. Insulin
stimulates liver and muscles to store glucose as glycogen.
c. Insulin
promotes buildup of fats and proteins and inhibits their use as an energy
source.
5. Glucagon is
secreted between meals in response to low blood glucose level.
a. Liver and
adipose tissue are main targets.
b. Adipose
tissue cells break fat into glycerol and fatty acids.
c. Liver
uses glycerol and fatty acids as substrates for glucose, raising blood glucose
levels.
J. Diabetes Mellitus
1. Diabetes mellitus is a common disease where body cells do not
take up or metabolize sugar.
2. Sugar in urine is common
laboratory test; blood glucose level is high enough kidneys excrete glucose.
3. Liver is not storing glucose as
glycogen and cells are not utilizing glucose for energy.
4. Since carbohydrate is not being
metabolized, the body breaks down protein and fat for energy.
5. Ketones build up in blood;
resulting reduced blood volume and acidosis can lead to coma and death.
6. In type I
(insulin-dependent) diabetes, pancreas does not produce insulin.
a. Viral
infection causes cytotoxic T cells to destroy pancreatic islets.
b. Treated
with daily administration of insulin, overdose or lack of eating results in
hypoglycemia.
c. Brain has
constant sugar requirements; low blood sugar can result in unconsciousness.
d. An
immediate intake of sugar is simple treatment.
7. Of 16 million diabetics in
a. This form
of diabetes usually occurs in obese and inactive individuals of any age.
b. Pancreas
does produce insulin but cells do not respond to it.
c. Initially,
this is a result of cells lacking receptors for insulin.
d.
Untreated, type II can have serious symptoms: blindness, kidney disease,
circulatory disorders, strokes, etc.
e. Low fat
diet and regular exercise help; oral drugs make cells more sensitive to insulin
or stimulate higher
levels of insulin production by pancreas.
K. Testes and Ovaries
1. Male testes located in scrotum function as gonads and produce androgens
(e.g., testosterone).
a. Testosterone
is male sex hormone.
b. It
stimulates development of male secondary sex characteristics: large vocal
cords, pubic hair, etc.
c.
Testosterone is largely responsible for the sex drive and probably
aggressiveness.
d. Anabolic
steroids are supplemental testosterone or similar chemicals with serious side
effects. (Fig. 49.15)
e.
Testosterone also affects sweat glands, expression of baldness genes, and other
effects.
2. Female sex hormones are estrogen
and progesterone.
a. Estrogens
secreted at puberty stimulate maturation of ovaries and other sexual organs.
b. Estrogen
is necessary for oocyte development.
c. It is
responsible for development of female secondary sex characteristics: a layer of
fat beneath skin,
larger pelvic girdle, etc.
d. Estrogen
and progesterone are required for breast development and regulation of uterine
cycle.
L. Thymus Gland
1. Thymus Gland is a lobular gland that lies in the upper
thoracic cavity. (Fig. 49.3)
2. It reaches its largest size and
is most active during childhood; with age, it shrinks and becomes fatty.
3. Some lymphocytes that originate
in bone marrow pass through thymus and change into T cells.
4. Thymus produces thymosins
which aid differentiation of T cells and may stimulate immune cells.
M. Pineal Gland
1. Pineal gland produces melatonin, primarily at
night.
2. In fishes and amphibians, pineal
gland is near surface and is a "third eye" receiving light directly.
3. In mammals, it is located in
third ventricle and cannot receive light directly; it receives nerve impulses
from the
eyes, by way of the optic tract.
4. Pineal gland and melatonin help
establish circadian rhythms, daily physiological cycles.
5. Pineal gland may also be involved
in human sexual development.
a. Some
animals go through a yearly cycle of enlargement of reproductive organs when
melatonin levels are low.
b. Children
in whom a brain tumor has destroyed the pineal gland experience puberty
earlier.
6. Melatonin may cause seasonal
affective disorder where persons are depressed at onset of winter.