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The Endocrine System CHAPTER
SYNOPSIS This chapter discusses the endocrine system, a communication and control system that exists in the body. It distinguishes endocrine glands from exocrine glands and differentiates between hormones and prostaglandins. The principal components of the endocrine system are identified according to their location, structure, chemical secretion, effect on body tissues, and disorders associated with malfunction. Action of hormones is discussed in conjunction with target organs involved. Regulation of secretions is described as a function of negative feedback control mechanisms LEARNING
OBJECTIVES WITH RATIONALE The student will be able to: 1. Distinguish between endocrine and exocrine glands, and define
the terms hormone and prostaglandin. Glands that discharge their secretions into ducts are
called exocrine glands. Endocrine glands are ductless glands. They secrete
chemicals called hormones directly into the bloodstream. Prostaglandins, or tissue hormones, are important and
extremely powerful substances found in a wide variety of body tissues. They
play an important role in communication and control of many body functions but
do not meet the definition of a typical hormone. A prostaglandin is produced in
a tissue and diffuses only a short distance to act on cells within that tissue.
Typical hormones influence and control activities of widely separated organs;
typical prostaglandins influence activities of neighboring cells. |
MENU Anatomy and Physiology Mechanisms of Disease Integuementary System and Body Membranes The Lymphatic System and Immunity
State of Texas Information |
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2. Identify and locate the primary endocrine glands and list the
major hormones produced by each gland. The following is a list of the primary endocrine glands,
their location, and their hormonal secretions. Pituitary gland—lies deep in the cranial cavity, in the
small depression of the sphenoid bone called the sella turcica. Secretions of
the anterior lobe include growth hormone, thyroid stimulating hormone,
adrenocorticotropic hormone, follicle-stimulating hormone, luteinizing hormone,
melanocyte-stimulating hormone, and prolactin. Secretions of the posterior lobe
include antidiuretic hormone and oxytocin Thyroid gland—lies in the neck, just below the larynx;
secretes thyroxine, triiodothyronine, and calcitonin Parathyroid glands—four small glands found on the back of
the thyroid; secrete parathyroid hormone Adrenal glands—located over the top of each kidney. The
adrenal cortex secretes mineralocorticoids, glucocorticoids, and small amounts
of sex hormones. The adrenal medulla secretes epinephrine and norepinephrine Islets of Langerhans—clumps of cells scattered among
pancreatic cells; secrete glucagon and insulin Sex glands—ovaries of the female located toward the back of
the pelvic cavity; secrete estrogen and progesterone. Testes of the male
located in the scrotum; secrete testosterone Thymus—located in the mediastinum; secretes thymosin Placenta—temporary
endocrine gland formed during pregnancy; secretes chorionic gonadotropin Pineal gland—small, cone-shaped gland that lies near
the roof of the third ventricle of the brain; secretes melatonin 3. Describe the mechanisms of steroid and protein hormone
action. Steroid hormones are called anabolic steroids because
they stimulate the production of protein in muscles and bones. Protein hormones
serve as first messenger, delivering the messages via the bloodstream and
providing communication between endocrine glands and target organs. Steroid
hormones can pass intact directly through the cell membrane of the target organ
cell. Once inside the cell they combine with specific receptors and then enter the
nucleus to influence cell activity by acting on specific genes. 4. Explain how negative and positive feedback mechanisms
regulate the secretion of endocrine hormones. Negative feedback regulates the secretion of
endocrine glands by decreasing their function in response to a stimulus. An
example is when the pancreas releases insulin in response to an elevation of
sugar in the blood. Blood sugar levels then decrease, which causes the
endocrine cells in the pancreas to cease the production and release of insulin.
Most feedback systems in the body are negative. Positive feedback usually
intensifies a change rather than reversing a change. Some of the positive
feedback mechanisms are destructive and disrupt the homeostasis of the body.
There are times that positive feedback can be helpful, such as during labor and
delivery. When labor begins, the cervix becomes distended and stimulates the
synthesis of oxytocin, which stimulates the uterus to contract. As the
contractions increase, the levels of oxytocin increase; thus a positive
feedback cycle is established. The cycle is broken by the birth of the baby. 5. Identify the principal functions of each major endocrine
hormone, and describe the conditions that may result from hyposecretion or
hypersecretion. Hormones play a vital part in our survival and
quality of life. When this mechanism is disturbed, the homeostasis of the body
is affected. The conditions listed in the table below are those mentioned in
the textbook. They develop as a result of too much or too little hormone
secretion. It should be understood that other, perhaps not so obvious,
conditions may exist when hormone levels are unbalanced.
6. Define diabetes insipidus, diabetes mellitus, gigantism, goiter,
cretinism, and glycosuria. Diabetes insipidus—This is a disease caused by hyposecretion of
antidiuretic hormone produced by the posterior lobe of the pituitary. It causes
an individual to eliminate extremely large volumes of water each day. Symptoms
include severe thirst, dehydration, and electrolyte imbalances. Diabetes mellitus—When the islets of Langerhans secrete too little
insulin, a condition called diabetes mellitus results. Glucose cannot enter
into cells, causing blood glucose to increase greatly. Excess sugar is filtered
out of the blood by the kidneys to be lost along with urine. Gigantism—This condition is produced when the anterior lobe of the
pituitary gland secretes excessive growth hormone during the early years of
life. The child grows to giant size. Goiter—Low dietary intake of iodine causes a painless enlargement of the
thyroid gland called simple goiter. This condition was once common in areas of
the United States where the iodine content of soil and water was low. The use
of iodized salt has dramatically reduced the incidence of simple goiter caused
by low iodine intake. In simple goiter the gland enlarges in an attempt to
compensate for the lack of iodine in the diet necessary for the synthesis of
thyroid hormones. Cretinism—Hyposecretion of thyroid hormones during the formative years
causes cretinism. It is characterized by a low metabolic rate, retarded growth
and sexual development, and frequently mental retardation. Glycosuria—This condition
is defined as excess sugar in the urine. In the case of diabetes mellitus, it
results when excess blood sugar is filtered out of the body by the kidneys to
be transferred to urine. LECTURE OUTLINE I. MECHANISMS OF HORMONE ACTION A. Endocrine glands secrete chemicals (hormones) into the blood (Figure
10-1) B. Hormones perform general functions of communication and control but a
slower, longer-lasting type of control than that provided by nerve impulses C. Cells acted on by hormones are called target organ cells D. Protein hormones (first messengers) bind to receptors on the target
cell membrane, triggering second messengers to affect the cell’s activities
(Figure 10-2) E. Steroid hormones bind to receptors within the target cell nucleus and
influence cell activity by acting on DNA (Figure 10-3) II. REGULATION OF HORMONE SECRETION A. Hormone secretion is controlled by homeostatic feedback B. Negative feedback—mechanisms that reverse the direction of a change
in a physiological system (Figure 10-4) C. Positive feedback—(uncommon) mechanisms that amplify physiological
changes III. PROSTAGLANDINS A. Prostaglandins (PGs) are powerful substances found in a wide variety
of body tissues B. PGs are often produced in a tissue and diffuse only a short distance
to act on cells in that tissue C. Several classes of PGs include prostaglandin A (PGA), prostaglandin E
(PGE), and prostaglandin F (PGF) D. PGs influence many body functions, including respiration, blood
pressure, gastrointestinal secretions, and reproduction IV. PITUITARY GLAND (Figure 10-5) A. Anterior pituitary gland (adenohypophysis) 1. Names of major hormones a. Thyroid-stimulating hormone (TSH) b. Adrenocorticotropic hormone (ACTH) c. Follicle-stimulating hormone (FSH) d. Luteinizing hormone (LH) e. Melanocyte-stimulating hormone (MSH) f. Growth hormone (GH) g. Prolactin (lactogenic hormone) 2. Functions of major hormones a. TSH—stimulates growth of the thyroid gland; also stimulates it to
secrete thyroid hormone b. ACTH—stimulates growth of the adrenal cortex and stimulates it to secrete
glucocorticoids (mainly cortisol) c. FSH—initiates growth of ovarian follicles each month in the ovary and
stimulates one or more follicles to develop to the stage of maturity and
ovulation; FSH also stimulates estrogen secretion by developing follicles;
stimulates sperm production in the male d. LH—acts with FSH to stimulate estrogen secretion and follicle growth
to maturity; causes ovulation; causes luteinization of the ruptured follicle
and stimulates progesterone secretion by corpus luteum; causes interstitial
cells in the testes to secrete testosterone in the male e. MSH—causes a rapid increase in the synthesis and spread of melanin
(pigment) in the skin f. GH—stimulates growth by accelerating protein anabolism; also
accelerates fat catabolism and slows glucose catabolism; by slowing glucose
catabolism, tends to increase blood glucose to higher than normal level
(hyperglycemia) g. Prolactin or lactogenic hormone—stimulates breast development during
pregnancy and secretion of milk after the delivery of the baby B. Posterior pituitary
gland (neurohypophysis) 1. Names of hormones a. Antidiuretic hormone (ADH) b. Oxytocin 2. Functions of hormones a. ADH—accelerates water reabsorption from urine in the kidney tubules
into the blood, thereby decreasing urine secretion b. Oxytocin—stimulates the pregnant uterus to contract; may initiate
labor; causes glandular cells of the breasts to release milk into ducts V. HYPOTHALAMUS A. Actual production of ADH and oxytocin occurs in the hypothalamus B. After production in the hypothalamus, hormones pass along axons into
the pituitary gland C. The secretion and release of posterior pituitary hormones is
controlled by nervous stimulation D. The hypothalamus controls many body functions related to homeostasis
(temperature, appetite, and thirst) VI. THYROID GLAND (Figure 10-6) A. Names of hormones 1. Thyroid hormone—thyroxine (T4) and triiodothyronine (T3) 2. Calcitonin B. Functions of hormones 1. Thyroid hormones—accelerate catabolism (increase the body’s metabolic
rate) 2. Calcitonin—decreases the blood calcium concentration by inhibiting
breakdown of bone, which would release calcium into the blood VII. PARATHYROID GLANDS (Figure 10-6) A. Name of hormone—parathyroid hormone (PTH) B. Function of hormone—increases blood calcium concentration by
increasing the breakdown of bone with the release of calcium into the blood VIII. ADRENAL GLANDS (Figure 10-9) A. Adrenal cortex 1. Names of hormones (corticoids) a. Glucocorticoids (GCs)—chiefly cortisol (hydrocortisone) b. Mineralocorticoids (MCs)—chiefly aldosterone c. Sex hormones—small amounts of male hormones (androgens) secreted by
adrenal cortex of both sexes 2. Cell layers (zones) a. Outer layer—secretes mineralocorticoids b. Middle layer—secretes glucocorticoids c. Inner layer—secretes sex hormones 3. Mineralocorticoids—increase blood sodium and decrease body potassium
concentrations by accelerating kidney tubule reabsorption of sodium and
excretion of potassium 4. Functions of glucocorticoids a. Help maintain normal blood glucose concentration by increasing
gluconeogenesis—the formation of “new” glucose from amino acids produced by the
breakdown of proteins, mainly those in muscle tissue cells; also the conversion
to glucose of fatty acids produced by the breakdown of fats stored in adipose
tissue cells b. Play an essential part in maintaining normal blood pressure—make it
possible for epinephrine and norepinephrine to maintain a normal degree of
vasoconstriction, a condition necessary for maintaining normal blood pressure c. Act with epinephrine and norepinephrine to produce an
anti-inflammatory effect, to bring about normal recovery from inflammations of
various kinds d. Produce anti-immunity, antiallergy effect; bring about a decrease in the number of lymphocytes and plasma cells and therefore a decrease in the amount of antibodies formed e. Secretion of glucocorticoid quickly increases when the body is thrown
into a condition of stress; high blood concentration of glucocorticoids, in
turn, brings about many other stress responses (Figure 9-10) B. Adrenal medulla 1. Names of hormones—epinephrine (adrenaline) and norepinephrine 2. Functions of hormones—help the body resist stress by intensifying and
prolonging the effects of sympathetic stimulation; increased epinephrine
secretion is the first endocrine response to stress IX. PANCREATIC ISLETS (Figure 10-11) A. Names of hormones 1. Glucagon—secreted by alpha cells 2. Insulin—secreted by beta cells B. Functions of hormones 1. Glucagon increases the blood glucose level by accelerating liver
glycogenolysis (conversion of glycogen to glucose) 2. Insulin decreases the blood glucose by accelerating the movement of
glucose out of the blood into cells, which increases glucose metabolism by
cells X. FEMALE SEX GLANDS The ovaries contain two structures that secrete hormones—the ovarian
follicles and the corpus luteum A. Effects of estrogen (feminizing hormone) 1. Development and maturation of breasts and external genitals 2. Development of adult female body contours 3. Initiation of menstrual cycle XI. MALE SEX GLANDS The interstitial cells of testes secrete the male hormone testosterone A. Effects of testosterone (masculinizing hormone) 1. Maturation of external genitals 2. Beard growth 3. Voice changes at puberty 4. Development of musculature and body contours typical of the male XII. THYMUS A. Name of hormone—thymosin B. Function of hormone—plays an important role in the development and
function of the body’s immune system XIII. PLACENTA A. Name of hormones—chorionic gonadotropins, estrogens, and progesterone
B. Functions of hormones—maintain the corpus luteum during pregnancy XIV. PINEAL GLAND A. A cone-shaped gland near the roof of the third ventricle of the brain
1. Glandular tissue predominates in children and young adults 2. Becomes fibrous and calcified with age B. Called third eye because its influence on secretory activity is
related to the amount of light entering the eyes C. Secretes melatonin, which: 1. Inhibits ovarian activity 2. Regulates the body’s internal clock XV. OTHER ENDOCRINE STRUCTURES A. Many organs (for example, the stomach, intestines, and kidney) produce endocrine hormones B. The atrial wall of the heart secretes atrial natriuretic hormone
(ANH), which stimulates sodium loss from the kidneys C. Fat-storing cells secrete leptin, which controls how full or hungry
we feel
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