Download Chapter 32: Chemical Control of the Animal Body: The Endocrine

Chapter 32: Chemical Control of the Animal Body: The Endocrine System
I. Characteristics of Animal Hormone
a. hormone – chemical that is secreted by specialized cells in one part of the body then transported in the bloodstream to
another part of the body, where it affects particular cells called target cells
b. endocrine glands or organs release hormones
c. a chemical is called a hormone on the basis of the kind of function it performs, but hormone molecules assume a wide
variety of forms of 4 general types
1. Peptide hormones – Most animal hormones are composed of peptides, which are chains of amino acids. The term
peptide technically refers to short amino acid chains, whereas longer chains are proteins. Both can serve as hormones, but
all hormones are composed of amino acid chains are described as peptide hormones.
2. Amino-acid derivatives – A few hormones consist of comparatively simple molecules derived from single amino acids.
For example, tyrosine forms the basis for the hormones epinephrine (adrenaline) and norepinephrine (noadrenaline).
3. Steroid hormones – Sometimes known as steroids, have a chemical structure resembling cholesterol, from which most
of them are synthesized. Steroid hormones are secreted by the ovaries, placenta, testes, and adrenal cortex.
4. Prostaglandins – Molecules in this category consist of 2 fatty acid carbon chains attached to a 5-carbon ring. They don’t
fit neatly into our definition of hormones, because they are produced by nearly every type of cell in the body (instead of
endocrine glands or organs exclusively), and act mainly on nearby cells (instead of traveling through the bloodstream to
act on more-distant cells). The effects of prostaglandins can be similar to those of hormones, and they often act in concert
with the endocrine system. Hence, they are considered to be a special type of hormone.
d. hormones function by binding to specific receptors on target cells
1. A hormone in the bloodstream will reach nearly all cells of the body because all cells have blood supply. But to their
precise control, they only act on certain target cells, which have receptors for particular hormone molecules; cells that lack
the appropriate receptor will not respond to the hormonal message.
2. A given hormone may have several different effects depending on the nature of the target cell it contacts.
3. Receptors are found in 2 general locations on target cells—plasma membrane and the nucleus.
e. Some hormones bind to surface receptors –
1. Most peptide and amino acid-based hormones are soluble in water, but not in lipid, which hinders them from entering
the plasma membrane. Instead, these hormones will react with protein receptors that protrude from the outside surface of
the target cell’s plasma membrane.
2. Some of these surface receptors are linked t plasma membrane channels that open in response to the binding of the
hormone.
3. When adrenaline binds to receptors on heart muscle cells, calcium channels open, allowing more calcium to flow into
the muscle cells, which increases the contraction strength.
4. Direct mechanism that involved in adrenaline’s effect on heart muscles are relatively uncommon among hormones that
bind to surface receptors in the endocrine system. The mechanism requires a second messenger that transfers molecules
within the cell.
5. In second-messenger systems, a hormone binding to a receptor on the cell’s surface triggers the release inside the cell of
a chemical (the second messenger) that initiates other biochemical reactions.
6. The binding of receptor activities activates an enzyme that catalyzes the conversion of ATP to cyclic AMP (cAMP), a
nucleotide that regulates many cellular activities. It acts as a second messenger and initiates a chain of reactions inside the
cell. Each reaction in the chain involves an increasing # of molecules, amplifying the original signal.
7. The end result varies with the target cell—channels may be open in the plasma membrane or substances may be
synthesized or secreted.
f. Other hormones bind to intracellular receptors –
1. Steroid hormones and others produced in the thyroid gland are lipid soluble so they can pass through the plasma
membrane.
2. Once inside the cell, these hormones bind to receptors inside the cell—to protein receptors in the nucleus. The receptorhormone complex binds to DNA and stimulates particular genes to transcribe mRNA, which moves to the cytoplasm and
directs the synthesis of proteins that alter the cell’s activity.
3. Steroid and thyroid hormones act by stimulating gene expressions that are otherwise switched off. These hormones may
take minutes or even days to exert their full effects (in contrast to the relatively rapid and short-lived effects of hormones
that bind to cell-surface receptors).
g. Hormones are regulated by feedback mechanisms
1. A signal that’s always turned on is incapable of carrying an instruction for change. If a hormone provides a useful signal
for physiological control, there must be a way to turn its message on and off. This “switch” involves negative feedback—
the secretion of a hormone stimulates a response in target cells that inhibits further secretion of the hormone.
2. Positive feedback controls hormone releases in few cases—the contraction of the uterus early in childbirth cause the
release of oxytocin by the posterior pituitary. Oxytocin stimulates stronger contraction of the uterus, which cause more
oxytocin release, creating a positive feedback cycle. It also causes uterine cells to release prostaglandins, which further
enhance uterine contractions, another example of positive feedback.
II. Structures and Functions of the Mammalian Endocrine System
a. Mammals have 2 types of glands in general—exocrine and endocrine.
b. Exocrine secrete outside the body or into the digestive tract. The secretions are released through tubes or openings—
ducts. They include sweat glands, tear glands, salivary glands, and pancreas.
c. Endocrine – ductless glands that release hormones into the body. They consist of clusters of hormone-producing cells
embedded within a network of capillaries. The cells secrete their hormones into the extracellular fluid surrounding the
capillaries. They then enter the capillaries by diffusion and are distributed throughout the body by the bloodstream.
Endocrine Gland
Hormone
Chemical
Function
Hypothalamus (via ADH
Peptide
Promotes reabsorption of water from kidneys; constricts
posterior pituitary)
arterioles
Oxytocin
Peptide
In females, stimulates contraction of uterine muscles
during childbirth, milk ejection, and maternal behaviors;
in males, causes sperm ejection
Hypothalamus (to
Releasing and inhibiting Peptide
At least 9 hormones; releasing hormones stimulate the
anterior pituitary)
hormones
release of hormones from anterior pituitary; inhibiting
hormones inhibit release of hormones from anterior
pituitary
FSH
Peptide
In females, stimulate growth of follicle, secretion of
estrogen, and perhaps ovulation; in males, stimulates
spermatogenesis
LH
Peptide
In females, stimulates ovulation, growth of corpus
luteum, and secretion of estrogen and progesterone; in
males, stimulates secretion of testosterone
Thyroid-stimulating
Peptide
Stimulates thyroid to release thyroxine
hormone (TSH)
Growth hormone
Peptide
Stimulates growth, protein synthesis, and fat
metabolism, inhibits sugar metabolism
Andrenocorticotropic
Peptide
Stimulates adrenal cortex to release hormones,
hormone (ACTH)
especially glucocorticoids
Prolactin
Peptide
Stimulates milk synthesis in and secretion from
mammary glands
Thyroid
Thyroxine
Amino acid
Increases metabolic rate of most body cells; increases
derivative
body temperature; regulates growth and development
Calcitonin
Peptide
Inhibits release of Ca from bones
Parathyroid
Parathormone
Peptide
Stimulates release of Ca from bone; promotes
absorption of Ca by intestines; promotes reabsorption of
Ca by kidneys
Pancreas
Insulin
Peptide
Decreases blood glucose levels by increase uptake of
glucose into cells and converting glucose to glycogen,
especially in liver; regulates fat metabolism
Glucagon
Peptide
Converts glycogen to glucose, raising blood glucose
levels
Ovaries
Estrogen
Steroid
Causes development of female secondary sexual
characteristics and maturation of eggs; promotes growth
of uterine lining
Progesterone
Steroid
Stimulates development of uterine lining and formation
of placenta
Testes
Testosterone
Steroid
Adrenal medulla
Adrenaline and
noadrenaline
Amino acid
derivatives
Adrenal cortex
Glucocorticoids
Steroid
Endocrine
Organ
Stimulates development of genitalia and male secondary
characteristics; stimulates spermatogenesis
Increase levels of sugar and fatty acid of blood; increase
metabolic rate; increase rate and force of contractions of
the heart; constrict some blood vessels
Increase blood sugar; regulate sugar, lipid, and fat
metabolism; anti-inflammatory effects
Hormone
Chemical
Function
Pineal Gland
Aldosterone
Testosterone
Melatonin
Thymus
Kidney
Thymosin
Renin
Steroid
Steroid
Amino acid
derivative
Peptide
Peptide
Heart
Erythropoietin
Atrial natriuretic
peptide (ANP)
Increases reabsorption of salt in kidney
Causes masculinization of body features, growth
Regulates seasonal reproductive cycles and sleep-wake
cycles; may regulate onset of puberty
Stimulates maturation of cells of immune system
Acts on blood proteins to produce hormone (angiotensin)
that regulates blood pressure
Stimulates red blood cell synthesis in bone marrow
Increases salt and water excretion by kidneys; lowers blood
pressure
Peptide
Peptide
d. The hypothalamus controls the secretion of the pituitary gland –
1. The hypothalamus is a part of the brain that contains neurosecretory cells that synthesize peptide hormones, store them,
and release them when stimulated.
2. The pituitary is a pea-sized gland that dangles from the hypothalamus by a stalk which has 2 lobes, the anterior
pituitary and the posterior pituitary that are controlled by the hypothalamus. The anterior is a true endocrine gland
whereas the posterior is derived from an outgrowth of the hypothalamus.
3. An improperly functioning anterior pituitary can produce either too much or too little growth hormone. Too little can
result in dwarfism; too much causes gigantism.
e. Thyroid gland – lies at the front of the neck, nestled around the larynx.
1. A diet deficient in iodine can reduce the production of thyroxine and trigger a feedback mechanism that acts to restore
normal hormone levels by dramatically increasing the number of thyroxine-producing cells. This will lead to excessive
growth of the thyroid gland; the enlarged gland may bulge from the neck and produce goiter.
f. Parathyroid glands – embedded in the back of the thyroid gland.
g. Pancreas – both an exocrine and endocrine organ.
1. Exocrine portion synthesizes digestive secretions that are released into the pancreatic duct and flow into the small
intestines. Endocrine portion consist of clusters of cells called islet cells, which produce peptide hormones.
2. Defects in insulin production, release, or reception by target cells result in diabetes mellitus, a condition in which blood
glucose levels are high and fluctuate wildly with sugar intake. The lack of functional insulin in diabetics cause the body to
rely more on fats as an energy source, leading to high levels of lipid, including cholesterol. Severe diabetes causes fat
deposits in the blood vessels, resulting in high blood pressure and heart disease; diabetes is an important cause of heart
attacks in the US. These fatty deposits can also damage the retina of the eye, leading to blindness.
h. Testes and Ovaries – secrete androgens
i. Adrenal Glands – Adrenal medulla and adrenal cortex
j. Prostaglandins – produced by almost all types of cells in the body.
1. Hormones that are modified fatty acids synthesized by the cell from membrane phospholipids.
2. Different types: causes arteries to constrict and stop bleeding from the umbilical cords of newborn infants, works in
conjunction with oxytocin during labor, and stimulates uterine contractions. Those soaked in vaginal suppositories are used
to induce labor. Menstrual cramps are caused by the overproduction of uterine prostaglandins, stimulating uterine
contractions.
III. Evolution of Hormones
a. In recent years, physiologists have discovered that hormones are evolutionarily ancient.
b. Insulin – found not only in vertebrates but also in protists, fungi, and bacteria, although research has not yet determined
the function of insulin in most of those organisms.
c. Protists manufacture ACTH, even though they have no adrenal glands to stimulate.
d. Yeasts have receptors for estrogen but no ovaries.
e. Thyroid hormones have been found in certain invertebrates, such as worms, insects, and mollusks, as well as in
vertebrates.