LESSON 14 THE ENDOCRINE SYSTEM
The endocrine system is composed of glandular tissue located in most areas of the body.
They are referred to as endocrine glands because they release their specific chemicals
directly into the bloodstream. These chemicals are called hormones because they can
regulate many different body functions. Hormones stimulate the growth of bones, cause
the maturation of sex organs and reproductive cells, and control the metabolic rate within
the individual cells of the body. A few endocrine glands secrete a wide variety of
different hormones that travel through the bloodstream and regulate the activities of other
Hormones produce their effects by connecting onto receptor sites. There are receptor
sites on the various target cells on which hormones act. The receptors initiate specific
physiological effects when the hormones connect to them. Each hormone has its own
receptor; the connecting of a receptor to a hormone is much like the function of a key
opening a lock.
All of the endocrine glands secrete their hormones directly into the bloodstream rather
than into ducts leading to the exterior of the body. Those glands that send their chemical
substances into ducts and out of the body are called exocrine glands. Examples of
exocrine glands are sweat, mammary, mucous, salivary and lacrimal glands.
The ductless, internally secreting, endocrine glands are listed below:
pancreas, islets of Langerhans
ovaries in female
testes in male
The pineal gland, located in the central portion of the brain, secretes a substance called
melatonin. Melatonin is believed to affect the brain and influence the rate of sex organ
maturation. Melatonin is thought to help in the ability to sleep at night by reducing the
amount of brain serotonin.
The thymus gland, located behind the sternum, resembles a lymph gland in structure. It
contains lympathic tissue and antibody-producing lymphocytes. The gland produces a
hormone called thymosin. The thymus gland is important in the development of immune
responses, especially in newborns. It is large in children but becomes smaller in most
adults. Removal of the thymus gland is used in treating a muscular-neurological disorder
called myasthenia gravis, and other autoimmune diseases, such as lupus and multiple
Some hormones are produced by organs. For example, the kidney secretes a hormone
called erythropoietin, which stimulates the production of red blood cells by the bone
marrow. The gastrointestinal tract secretes three hormones: gastuin, secretin, and
cholecystokinin. These hormones stimulate the secretion of gastric acid and enzymes, the
secretion of pancreatic enzymes, and the contraction of the gallbladder. The skin
produces vitamin D, which is considered a hormone, when sunlight is allowed to shine on
it for a length of time. Vitamin D stimulates the calcium absorption from the
gastrointestinal tract and is necessary for the maintenance of proper amounts of calcium
in the tissues, especially in the bones and in the bloodstream.
Prostaglandins are hormone-like substances that affect the body in many ways. They are
produced in cells throughout the body. Protaglandins stimulate the contraction of the
uterus, regulate body temperature, platelet aggregation, and acid secretion in the stomach,
and have the ability to lower blood pressure. They are released from cells and tissue to
Location and Structure
The thyroid gland is located on either side of the trachea just below the thyroid cartilage.
The thyroid cartilage covers the larynx and produces the prominence on the neck known
as the Adam’s apple. The isthmus of the thyroid gland is a narrow strip of glandular
tissue that connects the two lobes on the front surface of the trachea.
Two hormones are secreted by the thyroid gland, thyroxine or tetraiodothyronine (T4)
and triiodothyronine (T3). These hormones are synthesized in the thyroid gland from
iodine which is picked up from the blood stream and from an amino acid called tyrosine.
T4 contains four atoms of iodine and is much more concentrated in the blood. T3
contains three atoms of iodine and is far more potent in affecting the metabolic rate
within cells. Most thyroid hormones are bound to a protein molecule when they travel
through the bloodstream.
T3 and T4 are necessary in the body to maintain a normal metabolic rate in all cells.
Thyroid hormones help in the uptake of oxygen. Injections of thyroid hormone will
increase the metabolic rate. Removal of the thyroid gland lowers the thyroid hormone
concentration in the body which results in a lower metabolic rate, heat loss, and poor
mental and physical development.
Another hormone produced by the thyroid gland is called calcitonin. Calcitonin is
secreted when calcium levels in the blood are high. It stimulates calcium to leave the
blood and enter the bones, thus lowering blood calcium back to normal.
Location and Structure:
The parathyroid glands are four small oval bodies located on the back side of the thyroid
Parathyroid hormone is secreted by the parathyroid glands. This hormone mobilizes
calcium from bones into the bloodstream where calcium is necessary for proper
functioning. Calcium in food is absorbed from the intestine and carried by the blood to
the bones, where it is stored.
The level of calcium in the blood is a good example of the way hormones control the
homeostasis of the body. A decrease in blood calcium causes the parathyroid hormone to
be secreted in larger amounts, which in turn causes calcium to leave the bones and enter
the bloodstream bringing blood calcium back to normal. In a situation of increased blood
calcium, the parathyroid hormone secretion decreases, and blood calcium is again
brought back to normal.
Location and Structure:
The adrenal glands are two small glands situated one on top of each kidney. Each gland
consists of two parts, an outer portion called the adrenal cortex and an inner portion
called the adrenal medulla. The cortex and medulla are two glands in one, each secreting
its own different endocrine hormones. The cortex secretes hormones called
coriticosteroids, and the medulla secretes hormones called catecholarnines.
The adrenal cortex secretes three types of steroid hormones known as corticosteroids:
Mineralocorticoids. These hormones are essential to life because they regulate the
amounts of electrolytes that are retained in the body. A proper balance of water
and salt in the blood and tissues is essential to the normal function of the body.
The most important mineralocorticoid hormone is called aldosterone. The
secretion of aldosterone by the adrenal cortex increases the reabsorption of
sodium into the bloodstream by the kidney tubules. At the same time,
aldosterone stimulates the excretion of the electrolyte potassium.
Glucocorticoids. These steroid hormones have an important influence on the
metabolism of fats, sugars, and protein within cells and have an anti-inflammatory
effect. Cortisol, also called hydrocortisone, is the most important glucocorticoid
hormone. Cortisol increases the ability of cells to make new sugars out of fats and
proteins and regulates the quantity of fats, sugars, and proteins in the blood and
Cortisone is a hormone very similar to cortisol and can be prepared
synthetically. Cortisone is useful in treating inflammatory conditions such
as multiple sclerosis and arthritis.
Androgens, Estrogens, and Progesterone. These are male and female hormones
that maintain the secondary sex characteristics such as beard and breast
development, and are necessary for reproduction. These hormones are also
produced in the ovaries and testes. Excess adrenal androgen secretion in females
leads to the development of male characteristics; for example, facial hair. Excess
adrenal estrogen and progesterone secretion in males develops feminine
characteristics such as soft skin and higher pitched voices.
The adrenal medulla secretes two types of catecholamine hormones:
Epinephrine. Its trade name is adrenaline, by which it is mostly known. This
hormone increases cardiac rate, dilates bronchial tubes, and stimulates the
production of glucose from glycogen when glucose is needed by the body.
Norepinephrine. Its common or trade name is noradrenaline. This hormone
constricts vessels and raises blood pressure.
Both epinephrine and norepinephrine are called sympathomimetic and
parasympathomimetic agents because they mimic the actions of the sympathetic nervous
system or the parasympathic nervous system. During stress, these hormones are secreted
by the adrenal medulla in response to stimulation. They help the body respond to
stressful situations by raising blood pressure, increasing heartbeat and respiration, and
bringing sugar out of storage and into the cell for more energy.
Location and Structure
The pancreas is located behind the stomach in the region lumbar region. The specialized
cells in the pancreas that produce hormones are called the islets of Langerhans.
The islets of Langerhans produce two important hormones, insulin and glucagon. Both
of these hormones play an important role in the proper metabolism of carbohydrates in
the body. Insulin is necessary in the bloodstream so that sugars can cross from the blood
into the cells of the body where they are burned and then released as energy. When
blood sugars are high, insulin is released by the islet of Langerhans cells. The insulin
causes glucose either to enter body cells to be used as energy or converts it to glycogen to
be stored in muscle tissue or in the liver. Sugar can leave the blood to be stored as
glycogen or used to release energy. Glucagon, the opposite “twin” of insulin, is released
into the blood when sugar levels are below normal. It causes the breakdown of stored
liver glycogen into glucose so that there is a rise in the sugar content of blood leaving the
The islets of Langerhans carry on an endocrine function of releasing digestive enzymes
and juices. These cells secrete their digestive enzymes and juices into the gastrointestinal
Location and Structure
The pituitary gland, also called the hypophysis, is a small, pea-sized gland located at the
base of the brain in a small, pocket-like depression of the skull called the sella turcica.
The pituitary consists of two parts: an anterior lobe called the adenohypophysis; and a
posterior lobe called the neurohypophysis. The hypothalamus is an area of the brain that
is located next to the pituitary gland. Signals transmitted from the hypothalamus control
the secretions by the pituitary gland. Secretion from the neurohypophysis is controlled
by nerve fibers originating in the hypothalamus and ending in the neurohypophysis.
Secretion by the adenohypophysis is controlled by special hormones called releasing and
inhibiting factors. These hormones are secreted by the hypothalamus and passed to the
adenohypophysis via the bloodstream.
The hormones of the anterior pituitary gland are:
Growth hormone, also called somatotropin. This hormone affects bone tissue to
accelerate its growth in the body.
Thyroid-stimulating hormone, also called thyrotropin. This hormone stimulates
the growth of the thyroid gland and its secretion of thyroxine.
Adrenocorticotropic hormone (ACTH)). This hormone stimulates the growth of
the adrenal cortex and increases its secretion of steroid hormones.
Gonadotropic hormones. There are several gonadotropic hormones that affect the
growth and hormone secretion of the ovaries in females and testes in males. In the
female, the follicle-stimulating hormone (FSH), and the luteinizing hormone
(LH), stimulate the growth of eggs in the ovaries, production of hormones, and
ovulation. The lutotrophic hormone (LTH) stimulates the development of
endometrial tissue inside the uterus. In the male, FSH influences the production of
sperm and LH stimulates the testes to produce testosterone.
Prolactin. This hormone stimulates and sustains milk production after birth.
Melanocyte-stimulating hormone. This hormone influences the formation of
melanin and causes increased pigmentation of the skin. This effect occurs only
when hypersecretion of the hormone occurs.
The posterior pituitary gland secretes two important hormones. These hormones are
formed in the hypothalamus but secreted through the posterior pituitary gland.
Antidiuretic hormone (ADH). This hormone, also known as vasopressin,
stimulates the reabsorption of water by the kidney tubules. ADH can increase
blood pressure by constricting arterioles.
Oxytocin. This hormone stimulates the uterus to contract during childbirth and
maintains labor during childbirth. Oxytocin is secreted during suckling and helps
in the production of milk from the mammary glands.
LESSON 14 GRAPHICS
TERMS FOR LESSON 14
THE ENDOCRINE SYSTEM
Endocrine Glands to Know
pituitary or hypophysis
ASSIGNMENT FOR LESSON 14
Medical Terminology, HS 280
The Endocrine System
---- 1 ex- a poison
---- 2 -one b cortex
---- 3 kal/i c excessive
---- 4 toxic/o d deficient
---- 5 adren/o e adrenal gland
---- 6 cortic/o f potassium
---- 7 thry/o g hormone
---- 8 acro/o h extremities
---- 9 hyper- i thyroid gland
---- 10 hypo- j outside
---- 11 hypokelemia a enlargement of legs and arms
---- 12 acromegaly b muscle in spasm
---- 13 hypercalcemia c poisons produced by thyroid
---- 14 acidosis d enlargement of all bones
---- 15 ketosis e blood Ph to acidic
---- 16 tetany f blood high in ketone
---- 17 thyrotoxicosis g excessive production of thyroxin
---- 18 gigantism h low production of thyroxin
---- 19 hypothyroidism i low potassium blood levels
---- 20 hyperthyroidism j high calcium blood levels
---- 21 somatropin a causing maturation of egg
---- 22 follicle-stimulating hormone b build up endometrium
---- 23 luteinizing hormone c growth hormone
---- 24 antidiuretic hormone d female hormone
---- 25 oxytocin e male hormone
---- 26 epinephrine f causing deliver of baby
---- 27 estrogen g stress hormones
---- 28 testosterone h retain water
---- 29 insulin i fights infections
---- 30 cortisol j moves sugar