is the process of producing offspring for the survival of the species,
and passing on hereditary traits from one generation to the next.
The male and female reproductive systems contribute to the events
leading to fertilization. Then, the female organs assume responsibility
for the developing human, birth, and nursing. The male and female
gonads (testes and ovaries) produce sex cells (ova and sperm) and
the hormones necessary for the proper development, maintenance,
and functioning of the organs of reproduction and other organs and
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system comprises the reproductive organs. In the male, the organs
include the testes, accessory ducts, accessory glands, and penis.
In the female, the organs include the uterus, uterine tubes, ovaries,
vagina, and vulva.
The testes are
paired reproductive organs in the scrotum, which hangs outside the
human body. Normal sperm production requires the cooler outside
temperature. Each testis contains coiled seminiferous tubules where
sperm (male reproductive cells) production occurs. Between the seminiferous
tubules are Leydig cells, clusters of endocrine (secretory) cells.
Leydig cells produce androgens (sex hormones), mostly testosterone.
Each sperm cell
has three parts: a head, middle piece, and tail. An acrosome at
the head tip produces enzymes that help penetrate the female ovum
(egg). During conception, chromosomes (genetic material) in the
nucleus (cell control center) join with chromosomes in the ovum.
The middle piece contains mitochondria, structures that provide
energy for the sperm. The mitochondria are tightly spiraled around
the axial filaments (contractile portion) of the flagellum (tail).
Centrioles form the tail, which moves the sperm toward the ovum.
An ejaculation (ejection of sperm from the penis) has 300 to 500
ducts store secretions from the testes and accessory glands and
deliver secretions to the penis. The epididymis, a coiled tube next
to each testis, receives sperm from the seminiferous tubules. The
epididymis has three parts: a head, body, and tail. The epididymis
stores sperm and propels it toward the penis. Smooth muscle contractions
in the epididymis walls move sperm through the duct. As sperm pass
through the epididymis, the sperm mature and receive nourishment.
The vas deferens
is the dilated continuation of the epididymis. The vas deferens
travels out of the scrotum and into the abdomen (gut cavity) through
the inguinal canal. Once in the abdomen, the vas deferens passes
behind the urinary bladder and expands to form an ampulla (expanded
end part). Each ampulla joins with a seminal vesicle (an accessory
gland) to form an ejaculatory duct. The vas deferens is the main
sperm carrier. Its walls contain three layers of smooth muscle innervated
by sympathetic nerves. Stimulation of these nerves propels sperm
into the ejaculatory ducts. Here, the ampulla of the vas deferens
and seminal vesicles meet and secretions from the seminal vesicles
and sperm are stored. From this junction, the ejaculatory ducts
pass through the prostate gland, where they receive more secretions,
then join with the single urethra (tube through which sperm and
urine pass out of body).
is the final section of the duct system. It passes from the urinary
bladder and the ends of the ejaculatory ducts through the prostate
gland and into the penis. The urethra receives secretions from the
ejaculatory ducts, the prostate gland, and the bulbourethral glands
(accessory glands). The urethra carries sperm through the penis
during intercourse; during urination, urine passes through it. The
urethra cannot execute both functions simultaneously. During ejaculation,
a muscular sphincter (ring of muscle) closes off the bladder.
glands produce fluids that nourish and energize the sperm for the
journey to the ovum. For example, during sexual excitement the seminal
vesicles add secretions to the sperm in the ejaculatory duct. These
secretions provide energy for the sperm and a neutralizing chemical
that reduces vaginal acidity.
gland lies under the urinary bladder and surrounds the first part
of the urethra. Its secretions also help neutralize vaginal acidity
and make sperm motile (able to move).
glands secrete a clear fluid that neutralizes the acidity of remaining
urine in the urethra. When secretions of these glands combine with
sperm, the result is seminal fluid, or semen. Only 1 percent of
semen is sperm. The remainder contains fructose to nourish the sperm,
an alkaline component to neutralize vaginal and urethral acidity,
and salts and phospholipids, substances that make sperm motile.
The penis (male
sexual organ) deposits semen into the vagina during sexual intercourse
and carries urine through the urethra during urination. It contains
erectile tissue that becomes engorged with blood during sexual excitement,
resulting in an erection. The penis includes the shaft (tubular
portion), glans (penis tip and sexual sensation center), and the
prepuce, or foreskin (loose skin fold over glans). In a circumcision
procedure, the prepuce is removed.
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The female reproductive
system is more complex than that of the male. It produces ova (egg
cells); nourishes, carries, and protects the developing embryo;
and nurses the newborn after birth. The system structures are the
ovary, uterine tubes, uterus, vagina, vulva, and mammary glands.
Ovaries, a pair
of female gonads (sex organs), reside in the pelvic part of the
abdomen on either side of the uterus. Ovaries produce ova and estrogen
(female sex hormone).
At puberty onset,
the menstrual (uterine) cycle, a series of cyclic changes to the
endometrium (uterine lining) begins. The ovarian cycle, fluctuating
levels of ovarian hormones in the blood, causes the menstrual cycle.
and menstrual cycles begin each month when a follicle (developing
ovum surrounded by a cluster of cells) develops in the ovary. The
hypothalamus in the brain produces hormones that cause these cycles.
The hypothalamus releases gonadotropin-releasing hormone (GnRH),
which acts on the anterior pituitary gland. GnRH causes the pituitary
to release two more hormones: follicle-stimulating hormone (FSH)
and luteinizing hormone (LH). FSH causes the primary oocyte within
the follicle to develop into a secondary oocyte. Development occurs
through meiosis (cell division that reduces the chromosome number
in the cell from 46 to 23). Each secondary oocyte completes this
division only when sperm fertilizes it.
follicle produces estrogen, which causes the endometrium to prepare
to nourish a fertilized egg. Estrogen also inhibits pituitary gland
production of FSH. The elevated estrogen level causes the anterior
pituitary to release LH. This action causes ovulation, a process
in which the follicle rapidly enlarges and releases the secondary
oocyte. LH also causes the collapsed follicle to become the corpus
luteum, an endocrine (secretory) body. The corpus luteum secretes
estrogen and progesterone (hormone that stimulates endometrium thickening).
These hormones complete the endometrium development and maintain
the endometrium for 10 to 14 days.
fertilize the secondary oocyte, the corpus luteum begins to degenerate,
dropping blood progesterone levels. Without progesterone to maintain
the endometrial lining, the lining is shed with the degenerated
oocyte approximately 14 days after ovulation.
estrogen and progesterone act in the bloodstream to inhibit anterior
pituitary production of LH and FSH. This negative feedback control
ensures that only one follicle develops each cycle. Each cycle lasts
approximately 28 days.
tubes (oviducts or fallopian tubes) are paired tubes that receive
the developing ovum from the ovary. The infundibulum end is beside
the ovary; its fimbria (feathery structures) "sweep" the developing
ovum into the tube. The ampulla, the middle part of the uterine
tube, contains smooth muscle to move the egg. Cilia (inner wall
little projections) also sweep the egg along the tube. The unfertilized
ovum degenerates in the ampulla; the fertilized ovum resumes its
journey to the uterus. The isthmus end of the uterine tube opens
into the uterus.
The uterus is
a hollow muscular organ in front of the rectum and behind the urinary
bladder. The fundus is the wide upper portion. The body is the tapered
middle part that ends at the cervix (junction between the vagina
and uterus). The isthmus is the constricted region between the body
and cervix. The round ligaments hold the uterus anteverted (inclined
forward) over the urinary bladder.
The uterus has
three layers. The outer serous layer forms ligaments that hold it
to the pelvic walls. The middle muscular layer has three muscle
layers used in labor to deliver a baby. The endometrium inner mucosal
lining has two layers, the stratum functionalis and stratum basalis.
the stratum functionalis is built up in response to estrogen secretion.
It contains blood vessels and glands to nourish the fertilized ovum.
Unless sperm fertilizes the secondary oocyte, the corpus luteum
disintegrates into corpus albicans, and estrogen and progesterone
secretion cease. Without these hormones, the endometrium breaks
down and menstruation (expulsion of endometrial lining from the
uterus through the vagina) occurs. After menstruation, progesterone
and LH levels decrease. The inhibition of LH causes the anterior
pituitary to secrete FSH, which stimulates development of another
ovum. The monthly cycle begins again.
The vagina is
a muscular tube from the uterus to outside the body. In some women,
the hymen (thin tissue) partially covers the vaginal orifice. Initial
sexual intercourse or other form of penetration ruptures the hymen.
The vagina receives sperm from sexual intercourse, channels menstrual
flow out of the body, and is a birth canal for the baby during childbirth.
Normally collapsed, it can enlarge to accommodate an erect penis
or a birth.
The vulva, external
genitalia, includes the mons pubis, labia majora, labia minora,
and clitoris. The mons pubis is a mound of fatty tissue at the junction
of the thighs and torso. During puberty, pubic hair covers it. The
labia majora are skin folds that form the vulva outer border. During
puberty, pubic hair covers the labia majora, too. The labia minora
are inner, smaller skin folds that surround the urethral and vaginal
openings. The labia minora merge anteriorly to form the prepuce
(foreskin) of the clitoris. This small erectile structure, comparable
to the male penis, becomes engorged with blood during sexual excitement
and is the female center of sexual sensation.
glands have 15 to 20 lobes of glandular tissue. The lobes contain
lactiferous ducts that converge toward the nipple. These ducts dilate
just before they reach the lactiferous sinus, then constrict again
before passing out of the nipple through 15 to 20 openings.
glands are in the breasts. These glands overlie the pectoral muscles
and are attached to them via fascia (connective tissue). The glands
are connected to the skin by the suspensory ligaments of the breast.
These glands are modified sweat glands that produce and secrete
milk during the lactation process to feed the newborn. During pregnancy,
high blood estrogen and progesterone levels stimulate lactation.
The corpus luteum produces these hormones during early pregnancy;
the placenta takes over later. The hormones stimulate the ducts
and glands in the breasts, enlarging the breasts.
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of sex cells
of sex cells begins before birth; spermatozoa form in males and
oocytes in females. Spermatogenesis (sperm cell production) occurs
in the seminiferous tubules. Spermatogonia (stem cells) line these
tubules at birth and contain 46 chromosomes (genetic material).
After birth, spermatogonia continue to divide during mitosis. This
cell division process produces two daughter cells with the same
chromosome number (46) as the parent.
At puberty onset,
some spermatozoa grow to become primary spermatocytes. These cells
undergo meiosis, the cell division process that cuts back the number
of chromosomes from 46 to 23. Each primary spermatocyte undergoes
the first meiotic division to produce two secondary spermatocytes.
Each secondary spermatocyte undergoes the second meiotic division
to produce two spermatids. Each spermatid develops into a mature
spermatozoon (sperm cell). In this way, meiosis produces millions
of sperm every day.
the formation of the ovum (female sex cells), which begin as hundreds
of thousands of oogonia (stem cells) in the fetal ovaries. During
prenatal development, the oogonia grow to become primary oocytes
that contain 46 chromosomes. Each oocyte undergoes meiosis; at birth,
oocytes are in prophase. During this first meiotic division, oocytes
enter a resting phase that lasts until the oocyte resumes development
during the ovarian cycle (puberty). The female is born with all
the oocytes she will ever have.
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