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SCIENCE
CHEMICAL REGULATION
Ch. 18 - CLASSWORK
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CHEMICAL REGULATION - HORMONES
In Chapter 17 we saw the many ways in which the nervous system controls
the activities of the body. In this chapter we are going to discuss the endocrine
system, which regulates many other body functions.
The endocrine system consists of a set of glands, which are located at various
sites in the body (see Figure 18-1). Each of these glands secretes one or more
substances called hormones, which affect many metabolic processes. In general, the
hormones are involved in the regulation of growth and development and
maintenance of a constant internal environment.
The secretions of the endocrine glands are released directly into the
bloodstream. These secretions do not pass through ducts like the secretions of
digestive glands. Thus the endocrine glands are sometimes referred to as the
ductless glands.
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The endocrine and nervous systems do not function independently of each
other. Instead, the proper functioning of the body depends on the balanced and
coordinated interaction of these two systems. The nervous system makes rapid
adjustments in response to various types of stimuli. The endocrine system controls
processes that require prolonged stimulation.
Hormones are found in many different groups of organisms besides man and
other vertebrates. They are found in plants and in many invertebrates.
HORMONES. Hormones include a variety of types of chemical compounds.
These substances are released from the endocrine glands directly into the
bloodstream, which carries them to distant organs where they exert their effect.
The tissue acted on by a given hormone is called its target tissue.
In the nervous system, impulses from a given fiber can affect only a limited
number of other cells. The hormones, on the other hand, are carried to many parts
of the body in the blood. The action of some hormones is very specific, affecting
only one type of tissue. Other hormones act on many different types of tissues.
Hormones, like enzymes, regulate the rates of metabolic reactions. Only very
small quantities of hormone are necessary to affect a reaction. However, these
substances are rapidly inactivated in the body, so that the level of secretion requires
constant adjustment. Exactly how hormones work is not known. They may act by
controlling the enzyme systems involved in given reactions. Another possibility is
that hormones may alter the permeability of cell membranes. Still a third
possibility is that hormones may act on DNA, which in turn controls the synthesis of
enzymes and other proteins.
NEGATIVE FEEDBACK. Before we discuss the functioning of the
endocrine system, there is a concept you should understand--this is negative
feedback. It is a basic principle involved in control systems.
An automatic thermostat is a common type of control system using negative
feedback. You set the thermostat at a desired temperature, say 70 degrees. When
the room temperature drops below 70 degrees, the thermostat turns on the furnace.
When the temperature rises to 70 degrees, the thermostat automatically turns off
the furnace. The condition regulated by the thermostat is the temperature, and
information about the temperature is continually fed back to the thermostat. The
system is called negative feedback because the final conditions and the response to
them were opposite to the initial conditions and initial response. That is, the high
temperature turned off the furnace, while low temperatures turned it on.
Negative feedback is also involved in nervous control. We have already seen
an example of this in Chapter 15 when we discussed the effects of increased carbon
dioxide levels in the blood. In this case, increases in carbon dioxide concentrations
stimulate the breathing center of the brain, which in turn increases respiratory rate
and depth. This reduces carbon dioxide levels and increases the oxygen
concentration. With the drop in blood carbon dioxide, the activity of the breathing
center returns to normal and so does breathing.
The secretion of hormones is generally controlled by negative feedback. As a
simple example, we will use two hormones, A and B, secreted by organs A and B.
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Release of hormone A into the bloodstream stimulates production and secretion of
hormone B. When the level of hormone B in the blood reaches a certain point, it
inhibits the further production of hormone A. Without hormone A, secretion of
hormone B stops. When the blood level of hormone B drops below a certain point,
production of hormone A resumes. In this simple example we showed negative
feedback using two hormones. However, in the body endocrine activity is also
affected by substances other than hormones.
Through this cycle, the proper concentrations of the hormones are
maintained. It also allows the body to manufacture these substances only when they
are actually needed.
ENDOCRINE GLANDS. We are only going to discuss the major endocrine
organs of the body here. However, you should know that hormones are produced
by the stomach and intestines. Also, the thymus gland in the chest and the pineal
body in the head may also have endocrine functions, but relatively little is known
about them.
1. PITUITARY GLAND. The pituitary gland, located under the brain,
secretes many different hormones. It had often been called the "master
gland" of the body because its hormones regulate such a wide variety of
activities.
Some pituitary hormones, including ACTH, gonadotropic hormones,
and thyrotropic hormone, regulate the activity of other endocrine glands.
Others, such as growth hormone, exert numerous effects on many
different body tissues.
If too much or too little of a hormone is secreted for any length of
time, an abnormal condition will develop. For example, if too much
growth hormone is present during childhood, the growth of bones and
muscles is overstimulated. A person suffering from this condition may
develop into a giant, reaching a height of 9 feet. If too little growth
hormone is present, normal growth does not occur, and the person will be
a dwarf.
The secretion of thyrotropic, or thyroid-stimulating hormone (TSH),
provides a good example of the regulation of endocrine activity by
negative feedback. TSH secreted by the pituitary is carried in the blood
to the thyroid gland, where it stimulates the secretion of the thyroid
hormone, thyroxin. When the concentration of thyroxin in the blood
reaches a certain level, it affects the pituitary in such a way that further
secretion of TSH is inhibited. When the concentration of thyroid
hormone in the blood decreases below a certain level, the pituitary once
again secretes TSH.
Among the other pituitary hormones, adrenocorticotropic hormone,
ACTH, stimulates the production of hormones by the adrenal gland; the
gonadotropic hormones are involved in reproduction; and antiduretic
hormone, ADH, controls the reabsorption of water by the kidneys and
affects blood pressure.
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2. THYROID GLAND. The secretions of the thyroid gland control the rate
of metabolism of the body. More specifically, thyroid hormone controls
the rate of cellular respiration--the process by which the body
metabolizes glucose for the release of energy. Because this hormone
affects almost all the tissues in the body, any pronounced increase or
decrease in its secretion has serious consequences. Both physical and
mental health are affected.
In children, whose tissues are rapidly growing and developing, too
little thyroid hormone can result in cretinism. This condition is marked
by mental retardation, a dwarflike appearance, a protruding abdomen,
wrinkled skin, and deformed bones. If it is diagnosed before the damage
becomes permanent, cretinism can be successfully treated by the
administration of thyroid hormone.
One of the basic ingredients in thyroid hormone is iodine. If iodine is
lacking in the diet, the thyroid cannot produce adequate amounts of
thyroid hormone. With continued stimulation, the thyroid becomes
enlarged. The resulting condition, which is marked by a swelling of the
front of the neck, is called a goiter. Although enlargement of the thyroid
can also result from other conditions, simple goiter caused by an iodine
deficiency is treated by increasing the amount of iodine in the diet.
In the past there were certain parts of the world that were known as
"goiter belts" because so many people in those regions suffered from
goiter. It was found that soils in these areas, which were generally inland
or in the mountains, had been depleted of iodine. When iodine was added
to the diet, the goiters disappeared. Iodine is now routinely added to
table salt, so that iodine deficiency has become relatively rare. Seafood is
also an excellent source of iodine.
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Embedded in the thyroid gland are two pairs of small endocrine glands
called the parathyroids. They secrete the hormone parathormone, which
is involved in calcium and phosphorus metabolism.
3. PANCREAS. Scattered throughout the pancreas are small masses of
cells called the islands of Langerhans. These cells produce two hormones,
insulin and glucagon. The effect of insulin is to lower the concentration
of sugar in the blood, while the effect of glucagon is just the opposite. It
raises the blood sugar level. Along with other hormones, insulin and
glucagon regulate carbohydrate metabolism.
Insulin stimulates the breakdown of glucose to carbon dioxide and
water, the conversion of glucose to glycogen, and the conversion of
glucose to fat. Thus, it works in three ways to keep the blood sugar level
within a normal range.
A deficiency of insulin results in a condition called diabetes. In
diabetes the body cannot metabolize glucose normally. After a meal, the
blood sugar level rises abnormally. The sugar level may be so high that
some sugar is excreted in the urine. This does not occur under normal
conditions, so that the presence of sugar in the urine is often a symptom
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of diabetes. Without adequate insulin the body cannot obtain all the
energy that it requires from the breakdown of glucose. Thus, body fat is
broken down for energy. This disturbance of normal fat metabolism
results in high levels of fats in the blood. The products of fat breakdown
under these circumstances seriously upset the blood biochemistry, and
are dangerous.
Diabetes can be controlled by observing a strict diet and by the
administration of insulin, if it is needed. The tendency to develop
diabetes seems to be hereditary. It is also more common among
overweight people that among those of normal weight.
4. ADRENAL GLANDS. There are two adrenal glands in the body, one on
top of each kidney. Each gland is made up of two distinct layers--an
outer layer called the cortex, and an inner layer called the medulla. Each
layer secretes its own hormones.
The two hormones of the adrenal medulla are epinephrine, or adrenalin,
and norepinephrine, or noradrenalin. Although epinephrine has powerful
effects when introduced into the body as a drug, its role in normal
metabolism is not essential for life. Epinephrine increases the heart rate,
raises blood pressure, increases the level of blood sugar, and produces
gooseflesh. These effects often occur in times of sudden stress, such as
fear or surprise. However, exactly the same effects can be caused by
stimulation of part of the nervous system. Because of its stimulatory
effects on the circulatory system, epinephrine is useful in treating shock,
heart stoppage, and similar conditions.
Unlike the adrenal medulla, the adrenal cortex is essential for life. It
produces about thirty different hormones, which belong to a class of
compounds called steroids. As a group, these hormones are known as the
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adrenocortical steroids, and they have important metabolic effects in the
body. Their production is stimulated by ACTH from the pituitary. A
deficiency of these hormones affects the metabolism of carbohydrates,
proteins, salts, and water. Skin, bone, cartilage, lymphatic structures,
bone marrow, and many other tissues are affected.
5. GONADS. As part of their role in reproduction, the gonads---the testes
of the male and the ovaries of the female---also function as endocrine
organs. The testes secrete the hormone testosterone. Testosterone is
necessary for normal reproductive functions, and it also produces and
maintains the male secondary sex characteristics. These are the traits that
we normally associate with maleness, such as
the low voice, facial beard and body hair, and heavier muscular
development.
The ovaries produce two hormones---estrogen and progesterone. Both are
involved in the regulation of the female reproductive cycle Estrogen
regulates the female secondary sex characteristics---high voice,
development of breasts, and so on.
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