CHAPTER 16-THE ENDOCRINE SYSTEM
I. THE ENDOCRINE SYSTEM
A. This system, along with the nervous system, aids in maintaining homeostasis in the
human body.
B. The endocrine system is composed of glands that secrete chemical messengers known
as hormones.
C. Endocrine glands secrete their hormones into the bloodstream. These chemical agents
then travel all over the body to stimulate specific cells or organs known as
Target Cells or Target Organs.
1. How are endocrine glands different from exocrine gland?
II. FUNCTIONS OF THE ENDOCRINE SYSTEM
A. Maintaining homeostasis
B. Regulating: metabolism, reproduction, growth and development
III. THE ENDOCRINE SYSTEM VS. THE NERVOUS SYSTEM
A. Recall that the nervous system produces impulses which regulate muscle and
gland activities. Impulses are typically quick responses to some stimulus.
B. The endocrine system responds to a stimulus by releasing hormones. Hormones
usually have widespread effects on the body but they are slower responding than
impulses created in the nervous system.
IV. HORMONES-as noted earlier, these are chemical substances secreted by glands that regulate
the metabolic functioning of cells and organs.
A. Hormones are classified based on their chemical structure. The major types of hormones
include:
1. Amino acid based hormones-these vary in size from small amino acids to large
protein based hormones. Most human hormones are composed of amino acids.
2. Steroid based hormones-are composed of compounds that are synthesized from
cholesterol. The gonadal and adrenocortical hormones are steroid based.
3. Eicosanoids-these are typically released by cell membranes and these are often
involved in inflammation and allergic reactions. Examples of eicosanoids
include leukotrienes and prostaglandins.
B. Hormones are specific for certain target cells or target organs in the body.
C. Hormonal secretion is regulated by the nervous system. The Autonomic Nervous
System plays a key role in governing hormonal secretions. The release of most
hormones is regulated by a Negative Feedback System.
D. Hormones influence activities in the body by:
1. Altering cell membrane permeability to certain molecules.
2. Stimulating the production of proteins or enzymes.
3. Activating or deactivating enzymes.
4. Inducing secretions of other materials in the body.
5. Stimulating mitosis
V. MAJOR ENDOCRINE GLANDS
A. The Pituitary Gland (Hypophysis)-sits in the sella turcica of the sphenoid bone at the
base of the brain.
1. This gland secretes a number of important hormones.
2. The pituitary gland is composed of two major lobes: The Adenohypophysis
(Anterior lobe) and the Neurohypophysis (Posterior lobe).
3. The Adenohypophysis-is often called the Master Gland. It secretes the
following hormones:
a. Somatotrophic Hormone (STH)-regulates the growth of the skeleton. It
is regulated by the hypothalamus via Growth Hormone Releasing.
Factor and Growth Hormone Inhibiting Factor. This is also known as
Human Growth Hormone.
1) What is gigantism?
2) What is acromegaly?
3) What is pituitary dwarfism?
b. Thyrotropin Hormone (TSH)-stimulates secretions from the thyroid
gland. It is also regulated by the hypothalamus.
c. Adrenocorticotrophic Hormone (ACTH)-stimulates hormonal secretions
of the adrenal glands.
d. The Gonadotrophins-includes Follicle-Stimulating Hormone (FSH) and
Lutenizing Hormone (LH). These hormones regulate the growth and
development of the sex organs and they can act to regulate the female
menstrual cycle.
e. Prolactin (PRL)-a protein hormone that stimulates breast development
and milk secretion from the mammary glands. Estrogen stimulates the
release of prolactin in women.
1) In pregnant women PRL levels increase dramatically and the
infant’s suckling stimulates PRL release in the mother which
encourages additional milk production.
2) Males do secrete prolactin but the function of this hormone in
men is not understood.
4. The Neurohypophysis-this portion of the pituitary gland only stores hormones
that have been synthesized by neurons in the hypothalamus of the brain. The
two Hormones Stored by The Neurohypophysis are:
a. Oxytocin (OT)-regulates blood pressure and stimulates contractions of
the uterine muscles during childbirth. This hormone also plays a role in
regulating the release of prolactin during milk ejection.
b. Antidiuretic hormone (ADH)-regulates water balance in the kidney by
preventing dehydration and water overload.
1) Drinking alcoholic beverages inhibits ADH secretion and leads
to the formation of large supplies of urine.
2) What is diabetes insipidus and what causes this problem?
B. The Thyroid Gland-located in the neck, just below the larynx on either side of the
trachea. The two lobes of the thyroid gland are connected by a mass of tissue known
as the isthmus.
1. Internally, the thyroid gland is composed of hollow follicles which contain cells
that are capable of secreting hormones.
2. Hormones secreted by the Thyroid Gland:
a. Thyroxine, Triiodothyronine-collectively are referred to as Thyroid
Hormone (TH).
1) These hormones stimulate the release of enzymes that regulate
glucose oxidation. Therefore, these hormones play a critical role
in human metabolism and in regulating heat production in the
body.
b. Calcitonin (CT)-functions by removing calcium from the blood and
placing it in the bones of the body.
3. Thyroid Disorders
a. What is Myxedema and what are some of the symptoms of the illness?
b. What is goiter?
c. What is cretinism?
d. What is Grave’s Disease? What are some symptoms of this illness?
What is exophthalmos?
C. The Parathyroid Glands-located on the posterior surface of the thyroid gland. There
are usually four of these glands, but the numbers do vary.
1. Parathyroid Hormone (PTH)-only hormone secreted by these glands. This
hormone has the opposite effect of calcitonin. PTH increases calcium levels
in the bloodstream during times of calcium need. It also regulates the use
of calcium by body tissues.
D. The Adrenal (Suprarenal) Glands-pyramid-shaped glands sitting on the kidneys.
1. Each of these glands has an outer region known as the cortex and an inner
portion known as the medulla.
2. Hormones secreted by the Adrenal Cortex-all are steroids:
a. Mineralocorticoids-collectively, these hormones regulate electrolyte
concentrations in extracellular fluid. These hormones especially
regulate sodium and potassium ion levels in body fluids.
1) Aldosterone-most abundant mineralocorticoid. It regulates
sodium levels in the body.
2) What is aldosteronism? What is Addison’s Disease?
b. Glucocorticoids-regulate the energy metabolism of most body cells and
they help us resist stressors. Their secretion is regulated by ACTH from
they Pituitary Gland.
1) What is Cushing’s Syndrome?
c. Gonadocorticoids-sex hormones, it is thought that these may
supplement the true sex hormones produced by the gonads.
3. Hormones secreted by the Adrenal Medulla-these are amines.
a. Epinephrine and Norepinephrine (NE)-these are sometimes referred to
as Adrenaline.
1) These hormones are involved in “Fight or Flight” responses that
are associated with the Autonomic Nervous System.
2) Epinephrine typically accounts for about 80% of the total
secretion when these two hormones are released into the blood.
3) Both hormones typically excite organs; thus increasing the
activity of the organ.
E. The Pancreas-located below and behind the stomach.
1. The pancreas is a dual organ-it has both digestive and glandular functions.
2. It is also composed of cells that can act as exocrine glands (Acini) and cells that
can act as endocrine cells (Islets of Langerhans-which contain alpha and beta
cells).
3. Hormones Secreted by the Pancreas
a. Glucagon-an amino acid hormone produced in the alpha cells of the
Islets of Langerhans. This hormone stimulates the liver to convert
glycogen into glucose (Glycogenolysis).
b. Insulin-a protein hormone produced in the beta cells of the Islets of
Langerhans. This hormone causes a decrease in blood sugar level by
stimulating the conversion of glucose into glycogen (Glycogenesis).
1) What are the following disorders relating to Insulin?
a) Diabetes mellitus-know the following symptoms/signs
of this disorder:
1) Ketoacidosis
2) Polyuria
3) Polydipsia
b) Hyperinsulinism or Hypoglycemia
F. The Gonads-Ovaries and Testes-these glands produce steroid sex hormones.
1. The Ovaries-located in the pelvis of the female. This is also a dual organserving as an endocrine organ and a reproductive organ. Hormones secreted
by the ovaries include:
a. Estrogen-stimulates the development of secondary sex characteristics of
females and it regulates the growth of the endometrium.
b. Progesterone-maintains the growth of the endometrium of the uterus.
2. The Testes-located below the pelvis inside the male scrotum. This is a dual
organ-serving as a reproductive structure and as an endocrine gland. The testes
produce only one hormone: Testosterone. This hormone stimulates the
development of the male secondary sex characteristics.
G. The Pineal Gland-located in the brain, attached to the thalamus. The pineal gland
secretes the hormone Melatonin. This hormone plays a role in regulating our
diurnal rhythms and sleep patterns.
H. The Thymus Gland-located in the upper mediastinum. Recall that it decreases with
age. The thymus gland secretes thymosin which regulates the production of
lymphocytes.
I. Other Structures that Serve as Endocrine Glands:
1. The GI Tract contains digestive glands that regulate the digestive process.
2. The Placenta-secretes hormones that influence the course of pregnancy.
3. The Kidneys-release erythropoietin which stimulates erythrocyte production
in red bone marrow.
4. The Skin-secretes hormones that are involved in vitamin D production.
VI. RELATED CLINICAL TERMS-at the end of the chapter.