Chapter 25
The Endocrine System
I. The Endocrine System (pp. 753–755, Figs. 25.1–25.2)
A. The endocrine system is composed of several ductless glands that release hormones directly into the blood or
lymph; hormones trigger physiological changes in target cells throughout the body. (p. 753)
B. The endocrine system controls and integrates the functions of other systems in the body and closely interacts
with the nervous system. (p. 753)
C. Endocrine organs are small and widely separated from each other in the body; mostly endocrine cells are
epithelial in origin, but others are hormone-secreting neurons, muscle cells, and fibroblast-like cells. (p. 753,
Fig. 25.1)
1. The primary (“pure”) endocrine glands are the pituitary, thyroid, parathyroid, adrenal, and pineal glands. (p.
754)
2. Endocrine cells also are located in the gonads, pancreas, kidney, alimentary canal, heart, thymus, and skin.
(p. 753)
D. Hormones are messenger molecules that signal changes in target cells. (pp. 753–755)
1. Classes of hormones are either amino acid derivatives (amines, peptides, or proteins) or lipid-based steroids.
(p. 754)
2. Basic hormone action incorporates circulation throughout the entire body and contact with specific target
cells. (p. 754)
3. Control of hormone secretion is regulated by three major types of stimuli: humoral, neural, or hormonal
feedback. (pp. 754–755, Fig. 25.2)
II. The Major Endocrine Organs (pp. 755–756, Figs. 25.3–25.10, Table 25.1)
A. The major endocrine glands of the body are the pituitary gland, thyroid gland, parathyroid glands, adrenal
glands, pineal gland, pancreas, thymus, and gonads. (p. 755)
B. The pituitary gland is suspended from the diencephalon of the brain and secretes at least nine major hormones;
the pituitary has two basic divisions: the adenohypophysis and the neurohypophysis. (pp. 755–759, Fig. 25.3,
Table 25.1)
1. The adenohypophysis has three divisions; the largest is the pars distalis with five cell classes that secrete
seven different protein hormones. (pp. 755–756, Figs. 25.3–25.4, Table 25.1)
2. Hypothalamic control of hormone secretion from the adenohypophysis is exerted by peptide hormones
called releasing hormones (factors). (pp. 757–758)
3. The neurohypophysis secretes two hormones and is structurally part of the brain. (pp. 758–759, Fig. 25.5)
C. The thyroid gland is located in the anterior neck; it secretes thyroid hormone and calcitonin. (pp. 759–760, Fig.
25.6)
D. The parathyroid glands usually lie on the posterior aspect of the thyroid gland and secrete a hormone
(parathormone) which is essential for life. (p. 761, Fig. 25.7)
E. The adrenal (suprarenal) glands are located on the superior surface of the kidneys; each adrenal gland is two
glands in one: the adrenal medulla and the adrenal cortex. (pp. 760–763, Fig. 25.8)
1. The adrenal medulla is part of the autonomic nervous system (Chapter 15) and secretes amine proteins that
enhance the “fight, flight, or fright” response. (pp. 762 and 471, Fig. 25.8)
2. The adrenal cortex secretes a variety of hormones, all of which are lipid-based steroid hormones. (pp. 762–
763, Fig. 25.8)
3. Structure of steroid-secreting cells incorporates many distinctive ultrastructural features including abundant
smooth ER and no secretory granules at all. (p. 763, Fig. 25.9)
F. The pineal gland lies on the roof of the diencephalon and secretes the hormone melatonin, which helps regulate
circadian rhythms. (pp. 763–764, Figs. 25.1 and 13.15)
G. The pancreas is located in the posterior wall of the abdominal cavity; it functions as a dual organ in that it
secretes digestive enzymes as well as hormones. (p. 764, Fig. 25.10)
H. The thymus, an important organ of the immune system, secretes thymic hormones that are essential for the
production of T lymphocytes. (pp. 764–765, Fig. 25.1)
I. The gonads (testes and ovaries) are the main source of the steroid sex hormones. (p. 765)
III. Other Endocrine Structures (p. 765)
A. Endocrine cells occur within various organs of the body including the heart, the gastrointestinal tract and its
derivatives, the placenta, the kidneys, and the skin. (p. 765)
IV. Disorders of the Endocrine System (pp. 765–768, Figs. 25.11–25.13)
A. Most disorders of the endocrine glands involve either a hypersecretion (oversecretion) or a hyposecretion
(undersecretion) of a given hormone. (p. 765)
B. Pituitary disorders include gigantism, acromegaly, pituitary dwarfs, and diabetes insipidus. (pp. 765–767)
C. A disorder of the pancreas, diabetes mellitus, is caused by either insufficient insulin or resistance of body cells
to the effects of insulin. (p. 767)
D. Disorders of the thyroid gland include Graves’ disease, adult hypothroidism, and endemic goiter. (pp. 767–768,
Fig. 25.11)
E. Disorders of the adrenal cortex include Cushing’s disease and Addison’s disease. (p. 768, Fig. 25.12)
V. The Endocrine System Throughout Life (pp. 768–769, Fig. 25.13)
A. The endocrine glands have diverse developmental origins from all three germ layers. (pp. 768–769, Fig. 25.13)
B. Some hormonal secretions change little with aging, but others, such as GH, DHEA, and the sex hormones,
show marked drops in secretion with age. (p. 769)
SUPPLEMENTAL STUDENT MATERIALS to Human Anatomy,
Fifth Edition
Chapter 25: The Endocrine System
To the Student
It’s Saturday night, and your friends want to go out and eat pizza and drink beer. During the course of the evening, a
friend asks the question, “Why do you urinate more frequently after a couple beers?” You confidently answer,
“Because of the alcohol in beer, you have very successfully suppressed your ADH!” You are able to answer this
question, as well as celebrate, because you understand the basics of how the endocrine system works. This chapter
explores anatomical and functional relationships between the endocrine and nervous systems, explaining how both
work to maintain homeostasis and coordinate physiological functions throughout your body. There are several organs
that are purely endocrine in function, as well as several other organs that have endocrine cells but belong to other organ
systems. All these organs produce hormones that function as chemical messengers, going to specific target cells and
causing specific effects.
Step 1: Examine an overview of the endocrine system, describing glands and
hormones.
- Define an endocrine gland in terms of its basic structure and function, listing all endocrine glands covered by your
textbook.
- Define hormone.
- List several hormonally regulated processes.
- Explain similarities between the endocrine system and the nervous system.
- Describe how hormones are classified based on chemical structure.
- Explain the basics of hormonal action, defining target cells.
- Describe the mechanisms that control hormone secretion: humoral stimuli, neural stimuli, and hormonal stimuli.
Step 2: List and describe the major organs of the endocrine system, including
hormones produced and hormonal effects.
- Using a chart format, list the location and structure (region/area) of the pituitary gland, including all the hormones
secreted, their targets, and hormonal effects.
- Diagram a simplified feedback loop representing the endocrine interactions between the adenohypophysis and a
target endocrine gland.
- Explain how the hypothalamus controls the secretion of adenohypophyseal hormones.
- Define hypophyseal portal system, explaining its significance.
- Define and distinguish between releasing hormones and inhibiting hormones.
- Trace the path of releasing hormones through the pituitary gland.
- Describe the location and structure of the thyroid gland and the parathyroid glands, including hormones secreted,
their targets, and hormonal effects.
- Describe the location and name the two divisions of the adrenal gland, explaining their structure (region/zone) and
including hormones secreted, their targets, and hormonal effects.
- Describe the ultrastructure of a cell that secretes steroid hormones.
- Describe the location and structure of the pineal gland, pancreas, thymus, and gonads, including hormones
secreted, their targets, and hormonal effects.
Step 3: Describe disorders of the endocrine system and describe changes
associated with aging.
- Distinguish between hyposecretion and hypersecretion of hormones and name disorders associated with each.
- Describe diabetes mellitus.
- Describe the embryologic development of major endocrine glands.
- Describe the effects of aging on some endocrine glands.
Step 4: Solve short answer and essay questions in the textbook for an
excellent overall understanding of the endocrine system.
- Answer textbook questions 15–19, using photocopies of respective textbook diagrams.