Notes

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Chapter # TitleChapter 39 Endocrine and Reproductive Systems
A gland is an organ that makes and releases a secretion. Exocrine
glands release their secretions through ducts directly to the organs
that use them. Endocrine glands release hormones into the blood-
stream. The endocrine system is made up of endocrine glands.
Hormones are chemicals made in one part of the body that
affect cells elsewhere in the body. Hormones bind to target cells.
Target cells are specific chemical receptors on cells.
There are two types of hormones: steroid hormones and
nonsteroid hormones. Steroid hormones can cross cell membranes
of target cells, enter the nucleus, and turn genes on or off.
Nonsteroid hormones cannot cross cell membranes. Compounds
called secondary messengers carry the messages of nonsteroid
hormones inside target cells.
All cells, except for red blood cells, produce hormonelike sub-
stances called prostaglandins. Prostaglandins usually affect only
nearby cells and tissues. They are known as “local hormones.”
The endocrine system is controlled by feedback mechanisms
that help maintain homeostasis. For example, the level of a
hormone in the blood may be the feedback that signals a gland to
make more or less of the hormone. Two hormones with opposite
effects may work together to maintain homeostasis. This is called
complementary hormone action.
39–2 Human Endocrine Glands
There are several endocrine glands scattered throughout the body.
• The pituitary gland secretes nine hormones that regulate body
functions and control the actions of other endocrine glands.
• Hormones from the hypothalamus control the secretions of
the pituitary gland.
• Hormones from the thyroid gland regulate metabolism.
• Hormones from the thyroid gland and parathyroid glands
maintain blood calcium levels.
• The adrenal glands make hormones that help the body
prepare for and deal with stress.
• The pancreas is both an exocrine gland and an endocrine
gland. Hormones produced in the pancreas help keep levels
of glucose in the blood stable.
• Reproductive glands, or gonads, make gametes and secrete
sex hormones. The female gonads, ovaries, produce eggs.
The male gonads, testes, produce sperm.
Summary
39–1 The Endocrine System

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385
39–3 The Reproductive System
Hormones released by the ovaries and testes cause puberty.
Puberty is a period of rapid growth and sexual maturation. It
usually starts between the ages of 9 and 15. At the end of puberty,
the male and female reproductive organs are fully developed and
become fully functionable.
• The main role of the male reproductive system is to make
and deliver sperm. The testes are the main organs of this
system. The testes are held in the scrotum. In the testes,
sperm are made in tiny tubes called seminiferous tubules.
The mature sperm move through a tube and leave the body
through the urethra. The urethra is the tube in the penis that
leads to the outside. Contractions eject sperm from the penis
in a process called ejaculation.
• The main roles of the female reproductive system are to
make eggs and prepare the female body to nourish an
embryo. The ovaries are the main organs of this system.
Each ovary has thousands of follicles. A follicle is a cluster of
cells that surround an egg. A mature egg moves through the
Fallopian tube to the uterus. The uterus is connected to the
outside of the body by the vagina.
Each month, females go through the menstrual cycle. This
series of events prepares the body to care for a fertilized egg. The
endocrine system and reproductive system are both involved in
the menstrual cycle. The menstrual cycle has four phases:
• During the follicular phase, an egg matures in its follicle.
• Ovulation occurs when the mature egg is released from the
ovary. If sperm are present in the Fallopian tube, the egg may
be fertilized.
• During the luteal phase, the follicle develops into a structure
called the corpus luteum. If the egg was fertilized, it
implants in the lining of the uterus. If the egg was not
fertilized, it moves through the uterus without implanting.
• During menstruation, the lining of the uterus falls away
and leaves the body through the vagina.
Diseases spread during sexual contact are called sexually
transmitted diseases (STDs). Bacteria and viruses can cause STDs.
Chlamydia, syphilis, gonorrhea, and AIDS are STDs. Abstinence is
the only sure way to prevent infection with STDs.

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39–4 Fertilization and Development
Fertilization is the joining of a sperm and an egg. A fertilized
egg is a zygote.
• The zygote divides and undergoes repeated mitosis and
develops into a hollow ball of cells called a blastocyst. About
a week after fertilization, the blastocyst implants in the
lining of the uterus.
• Cells of the blastocyst start to specialize through
differentiation. Some cells migrate to form two cell layers.
A third layer is produced by a process of cell migration
called gastrulation. The three layers eventually develop into
the different organs of the embryo.
• Gastrulation is followed by neurulation, or the development
of the nervous system.
As the embryo develops, membranes form to protect and nour-
ish it. One membrane forms the placenta. The mother and embryo
exchange gases, food, and waste products across the placenta. It is
the embryo’s organ of respiration, nourishment, and excretion.
After eight weeks of development, the embryo is a fetus.
By the end of three months, most organs are fully formed. During
this time, the umbilical cord forms. The umbilical cord connects
the fetus to the placenta.
During the next six months before birth, the organ systems
mature. The fetus grows in size and mass.
Childbirth occurs when hormones cause contractions in the
mother’s uterus. The contractions push the baby out through
the vagina.
Growth and development continue throughout childhood.
Adolescence begins with puberty and ends with adulthood.
Development continues during adulthood. The first signs of
aging often appear in the thirties.
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