Endocrine System
Background Information:
The nervous system sends electrical messages to control and
coordinate the body. The endocrine system has a similar job, but uses
chemicals to “communicate”. These chemicals are known as
hormones.
A hormone is a specific messenger molecule synthesized and secreted by a
group of specialized cells called an
endocrine gland.
These glands are ductless, which means that their secretions
(hormones) are released directly into the bloodstream and travel to elsewhere
in the body to target organs, upon which they act. Note that this is
in contrast to our digestive glands, which have ducts for releasing the
digestive enzymes into the digestive tract.
Pheromones
are also communication chemicals, but are used to send signals to other
members of the same species. Queen bees, ants, and naked mole rats exert
control of their respective colonies via pheromones. One common use for
pheromones is as attractants in mating. Pheromones are widely studied in
insects and are the basis for some kinds of Japanese beetle and gypsy moth
traps. While pheromones have not been so widely studied in humans,
some interesting studies have been done in recent years on pheromonal
control of menstrual cycles in women. It has been found that pheromones in
male sweat and/or sweat from another “dominant” female will both
influence/regulate the cycles of women when smeared above their upper lip,
just below the nose. Also, there is evidence that continued reception of a
given man’s pheromone(s) by a woman in the weeks just after
ovulation/fertilization can significantly increase the chances of successful
implantation of the new baby in her uterus. Pheromones are also used for
things like territorial markers (urine) and alarm signals.
Each hormone’s shape is specific and can be recognized by the
corresponding target cells. The binding sites on the target cells are called
hormone receptors. Many hormones come in antagonistic pairs that have
opposite effects on the target organs. For example, insulin and
glucagon have opposite effects on the liver’s control of blood sugar
level. Insulin lowers the blood sugar level by instructing the liver to take
glucose out of circulation and store it, while glucagon instructs the liver
to release some of its stored supply to raise the blood sugar level. Much
hormonal regulation depends on feedback loops to maintain balance and
homeostasis.
The three types of diabetes are a good illustration of the two
main ways that something can “go wrong” with hormonal control in our bodies.
In type I diabetes, because of the person’s genetic code, his/her pancreas is
unable to make insulin. Without insulin, the liver never
“gets the message” to take glucose out of the bloodstream, so blood glucose
remains too high, while the stores of glucagon in the liver are too low. In
type II diabetes, the person’s pancreas is making enough insulin, but
the insulin receptor sites on the liver cells are “broken” (possibly due to
genetic factors, possibly due to “overuse”) and cannot “get the message.”
Because the liver is unable to receive the instructions (despite the
presence of lots of insulin), it does not take glucose out of the bloodstream,
so blood glucose remains too high, while the stores of glucagon in the liver
are too low. In type III diabetes (AKA Alzheimer’s Disease), it is the
neurons in the brain, specifically, which “don’t get the message,” (though
it sounds like researchers have yet to determine whether that’s due to lack
of the brain-produced insulin upon which they depend, or whether that’s due
to receptors on the neurons that either are or become “broken”) and thus,
cannot take in the sugar that they need, with the result that, without an
alternative fuel source such as medium-chain triglycerides, the neurons will
starve. (Recall that neurons cannot store glycogen like the liver and
muscles, and thus, are second-to-second dependent on receiving the “fuel”
they need. Also, recall that pancreatic insulin (and insulin from
intramuscular injections) cannot/does not cross the blood-brain barrier.)
These same two problems (lack of secretion, lack of reception) are possible
problems for any of our hormones. The
Linked
and Sex-Linked Genes Web Page contains information on Androgen
Insensitivity Syndrome (AIS), a genetic condition in which the testosterone
receptors on the person’s cells are disfunctional, and so cannot receive the
instructions provided by the testosterone, with the result that, even if
her sex chromosomes are XY, she is female.
There are three general classes (groups) of hormones. These
are classified by chemical structure, not function.
- steroid hormones including prostaglandins which function
especially in a variety of female functions (aspirin inhibits synthesis of
prostaglandins, some of which cause “cramps”) and the sex hormones all of
which are lipids made from cholesterol,
- amino acid derivatives (like epinephrine) which are derived from
amino acids, especially tyrosine, and
- peptide hormones (like insulin) which is the most numerous/diverse
group of hormones.
Endocrine Glands:
Endocrine Glands
(clipart edited from Corel Presentations 8)
The major human endocrine glands include:
- Hypothalamus and Pituitary Gland
- The pituitary gland is called the “master gland” but it is under the
control of the hypothalamus. Together, they control many other endocrine
functions. They secrete a number of hormones, especially several which are
important to the female menstural cycle, pregnancy, birth, and lactation
(milk production). These include
- follicle-stimulating hormone (FSH),
which stimulates development and maturation of a follicle in one of a
woman’s ovaries, and
- leutinizing hormone (LH), which causes the
bursting of that follicle (= ovulation) and the formation of a corpus
luteum from the remains of the follicle.
There are a number of other hypothalamus and pituitary hormones which
affect various target organs.
- One non-sex hormone secreted by the posterior pituitary is
antidiuretic hormone
or ADH. This hormone helps prevent excess water excretion by the
kidneys. Ethanol inhibits the release of ADH and can, thus, cause excessive
water loss. That’s also part of the reason why a group of college students
who go out for pizza and a pitcher of beer need to make frequent trips to
the restrooms. Diuretics are chemicals which interfere with the
production of or action of ADH so the kidneys secrete more water. Thus
diuretics are often prescribed for people with high blood pressure, in an
attempt to decrease blood volume.
- Another group of non-sex hormones that many people have heard of is the
endorphins, which belong to the category of chemicals known as
opiates and serve to deaden our pain receptors. Endorphins, which
are chemically related to morphine, are produced in response to pain. The
natural response to rub an injured area, such as a pinched finger, helps to
release endorphins in that area. People who exercise a lot and push their
bodies “until it hurts” thereby stimulate the production of endorphins. It
is thought that some people who constantly over-exercise and push themselves
too much may actually be addicted to their own endorphins which that severe
exercise regime releases.
- Thyroid Gland
- Thyroid hormones regulate metabolism, therefore body temperature
and weight. The thyroid hormones contain iodine, which the thyroid needs
in order to manufacture these hormones. If a person lacks iodine in his/her
diet, the thyroid cannot make the hormones, causing a deficiency. In
response to the body’s feedback loops calling for more thyroid hormones,
the thyroid gland then enlarges to attempt to compensate (The body’s plan
here is if it’s bigger it can make more, but that doesn’t help if there
isn’t enough iodine.). This disorder is called goiter. Dietary
sources of iodine include any “ocean foods” because ocean-dwelling organisms
tend to accumulate iodine from the seawater, and would include foods like
ocean fish (tuna) and seaweeds like kelp. Because of this, people who live
near the ocean do not have as much of a problem with goiter as people who
live inland and don’t have access to these foods. To help alleviate this
problem in our country, our government began a program encouraging salt
refiners to add iodine to salt, and encouraging people to choose to consume
this iodized salt.
- Pancreas
- This organ has two functions. It serves as a ducted gland, secreting
digestive enzymes into the small intestine. The pancreas also serves as a
ductless gland in that the islets of Langerhans secrete insulin
and glucagon to regulate the blood sugar level. The
α-islet cells secrete glucagon, which tells the liver to take
carbohydrate out of storage to raise a low blood sugar level. The
β-islet cells secrete insulin to tell the liver to take excess
glucose out of circulation to lower a blood sugar level that’s too high.
If a person’s body does not make enough insulin (and/or there is a reduced
response of the target cells in the liver), the blood sugar rises, perhaps
out of control, and we say that the person has diabetes
mellitus.
Adrenal Gland
- Adrenal Glands
- These sit on top of the kidneys. They consist of two parts, the outer
cortex and the inner medulla. The medulla secretes
epinephrine
(= adrenaline)
and other similar hormones in response to stressors such as fright, anger,
caffeine, or low blood sugar. The cortex secretes corticosteroids
such as cortisone. Corticosteroids are well-known as being
anti-inflammatory, thus are prescribed for a number of conditions. However,
these are powerful regulators that should be used with caution. Medicinal
doses are typically higher than what your body would produce naturally,
thus the person’s normal feedback loops suppress natural secretion, and it
is necessary to gradually taper off the dosage to trigger the adrenal glands
to begin producing on their own again. Because the corticosteroids suppress
the immune system, their use can lead to increased susceptibility to
infections, yet physicians typically prescribe them for people whose immune
systems are hard at work trying to fight off some pathogen. For example,
back when I was in grad school, I was diagnosed with mono, and the campus
doctor prescribed penicillin and cortisone. Since mono is a virus and
penicillin only is effective against some bacteria, about all it did was
kill off the friendly bacteria in my body, therefore causing me to develop
a bad case of thrush. At the same time, the cortisone was suppressing my
immune system so my body could not as efficiently fight off the mono and
the thrush. People with high blood pressure should be leery of taking
prescription corticosteroids: they are known to raise blood pressure, thus
can cause things like strokes. My mother-in-law had high blood pressure
and was being treated with diuretics. Her physician also had her on large
doses of cortisone for her arthritis. While he was on vacation, she started
having significant back pain and was referred to an orthopedic surgeon. This
man decided the back pain was just due to arthritis, and without carefully
checking on what dosage she was already taking, prescribed more cortisone.
Simultaneously, because of difficulty walking due to her arthritis, she
decided to decrease the amount of diuretics she was taking so she didn’t
have to make as many “long” trips to the other end of the house. The
combination of lowered dose of diuretics and high dose of cortisone raised
her blood pressure to the point where a blood vessel in her brain burst,
causing a stroke. When the EMTs took her blood pressure, as I recall the
systolic was way over 200 mm Hg.
- Gonads or Sex Organs
- In addition to producing gametes, the female ovaries and male
testes (singular = testis) also secrete hormones. Therefore, these
hormones are called sex hormones. The secretion of sex hormones by
the gonads is controlled by pituitary gland hormones such as FSH and LH.
While both sexes make some of each of the hormones, typically male testes
secrete primarily
androgens
including testosterone. Female ovaries make estrogen and
progesterone in varying amounts depending on where in her cycle a
woman is. In a pregnant woman, the baby’s placenta also secretes hormones
to maintain the pregnancy.
- Pineal Gland
- This gland is located near the center of the brain in humans, and is
stimulated by nerves from the eyes. In some other animals, the pineal gland
is closer to the skin and directly stimulated by light (some lizards even
have a third “eye”). The pineal gland secretes melatonin at night
when it’s dark, thus secretes more in winter when the nights are longer.
Melatonin promotes sleep (makes you feel sleepy). It also affects
reproductive functions by depressing the activity of the gonads. Additionally,
it affects thyroid and adrenal cortex functions. In some animals, melatonin
affects skin pigmentation. Because melatonin production is affected by the
amount of light to which a person is exposed, this is tied to
circadian rhythm
(having an activity cycle of about 24 hours), annual cycles, and biological
clock functions. SAD or seasonal affective disorder (syndrome)
is a disorder in which too much melatonin is produced, especially during
the long nights of winter, causing profound depression, oversleeping,
weight gain, tiredness, and sadness. Treatment consists of exposure to
bright lights for several hours each day to inhibit melatonin production.
It has also been found that melatonin levels drop 75% suddenly just before
puberty, suggesting the involvement of melatonin in the regulation of the
onset of puberty. Studies have been done on blind girls (with a form of
blindness in which no impulses can travel down the optic nerve and reach
the brain and pineal gland), which showed that these girls tended to have
higher levels of melatonin for a longer time, resulting in a delay in the
onset of puberty. While some older people, who don’t make very much
melatonin, thus don’t sleep well, might benefit from a melatonin supplement,
I’m skeptical of the recent melatonin craze in this country. When so many
people apparently are suffering from SAD, I question the wisdom of purposly
ingesting more melatonin, especially since the pineal gland is one
of the least-studied, least-understood of the endocrine glands.
Local regulators are hormones with target cells nearby
or adjacent to the endocrine gland in question. For example,
neurotransmitters are secreted in the synapses of our nervous system and
their target cells are in the same synapses.
Copyright ©; 1996 by J. Stein Carter. All rights reserved.
This page has been accessed times since 14 Mar 2001.