ing of the maturation of B and T cells, which increases sus-
ceptibility to infections as a person ages.
What general types of changes occur in the glands of the
endocrine system with aging?
How do the structures and functions of particular endocrine
glands change over a lifetime?
The daily fall and rise of melatonin levels may even out
somewhat with age, which may alter control of the sleep/
wake cycle. People usually require less sleep as they age.
Changes to the tempo of the body clock may, in turn, affect
secretion of other hormones.
The thymus gland begins to noticeably shrink before age
twenty, with accompanying declining levels of thymosins.
By age sixty, thymosin secretion is nil. The result is a slow-
a. Prostaglandins are paracrine substances that
have powerful hormonelike effects, even in small
b. Prostaglandins modulate hormones that regulate
formation of cyclic AMP.
CONTROL OF HORMONAL SECRETIONS
The concentration of each hormone in the body ﬂ
1. Control sources
a. Some endocrine glands secrete hormones in
response to releasing hormones the hypothalamus
b. Some endocrine glands secrete in response to
c. Some endocrine glands secrete in response to
changes in the plasma concentration of a substance.
2. Negative feedback systems
a. In a negative feedback system, a gland is sensitive
to the concentration of a substance it regulates.
b. When the concentration of the regulated substance
reaches a certain concentration, it inhibits the gland.
c. As the gland secretes less hormone, the controlled
substance also decreases.
PITUITARY GLAND (PAGE 492)
The pituitary gland, attached to the base of the brain, has
an anterior lobe and a posterior lobe. Releasing hormones
from the hypothalamus control most pituitary secretions.
1. Anterior pituitary hormones
a. The anterior pituitary consists largely of epithelial
cells, and it secretes GH, PRL, TSH, ACTH, FSH,
b. Growth hormone (GH)
(1) Growth hormone stimulates body cells to grow
(2) Growth hormone-releasing hormone and
somatostatin from the hypothalamus control
c. Prolactin (PRL)
(1) PRL promotes breast development and
stimulates milk production.
(2) In males, prolactin decreases secretion of LH
INTRODUCTION (PAGE 483)
The nervous sytem and the endocrine system work
together to control body functions. Endocrine glands
secrete their products into body ﬂ
uids (the internal
environment); exocrine glands secrete their products into
ducts that lead to the outside of the body.
GENERAL CHARACTERISTICS OF THE
ENDOCRINE SYSTEM (PAGE 483)
A hormone’s target cells have speci±
Hormones from endocrine glands regulate metabolic
HORMONE ACTION (PAGE 484)
Endocrine glands secrete hormones into the bloodstream
which carries them to all parts of the body.
1. Chemistry of hormones
a. Steroid hormones are lipids that include complex
rings of carbon and hydrogen atoms.
b. Nonsteroid hormones are amines, peptides, and
2. Actions of hormones
a. Steroid hormones and thyroid hormones
(1) Steroid hormones enter target cells and
combine with receptors to form complexes.
(2) These complexes activate speci± c genes in
the nucleus, which direct synthesis of speci± c
(3) The degree of cellular response is proportional
to the number of hormone-receptor complexes
b. Nonsteroid hormones
(1) Nonsteroid hormones combine with receptors
in the target cell membrane.
(2) A hormone-receptor complex stimulates
membrane proteins, such as adenylate cyclase,
to induce the formation of second messenger
(3) A second messenger, such as cAMP, activates
(4) Protein kinases activate certain protein
substrate molecules, which, in turn, change
(5) The cellular response to a nonsteroid hormone
is ampli± ed because the enzymes induced by a
small number of hormone-receptor complexes
can catalyze formation of a large number of
second messenger molecules.