Male Reproductive System
The male reproductive system includes external (penis,
scrotum, epididymis, and testes) and internal (accessory) organs.
The organs of the male reproductive system are
specialized for three primary functions:
·
To produce, maintain, transport, and nourish
sperm (the male reproductive cells), and protective fluid (semen).
·
To discharge sperm within the female
reproductive tract.
·
To produce and secrete male sex hormones.
Penis
In human males, the penis serves as both a reproductive
organ and as a urinal duct.
The human penis has no baculum or erectile bone.
Instead, it relies entirely on engorgement with blood to achieve an erection.
Parts of the Penis:
·
The human penis is made up of three columns of
tissue: two corpora cavernosa that lie next to each other on the dorsal side
and a corpus spongiosum that lies between the corpora cavernosa on the ventral
side.
·
The glans penis is the bulbous end of the penis
formed by the corpus spongiosum. It supports the foreskin (prepuce) that
retracts to expose the glans.
·
The area on the underside of the penis, where
the foreskin is attached, is called the frenum (or frenulum). The rounded base
of the glans is called the corona. The perineal raphe is the noticeable line
along the underside of the penis.
Scrotum
The scrotum is a dual-chambered suspended sack of skin
and smooth muscle that contains the testes and is homologous to the labia
majora in females. It is an extension of the perineum and is located between
the penis and anus.
The purpose of the scrotum is to provide the testes with
a chamber of appropriate temperature for optimal sperm production.
The function of the scrotum appears to be to keep the
temperature of the testes slightly lower than that of the rest of the body. For
human beings, the temperature should be one or two degrees Celsius below body
temperature (around 35 degrees Celsius or 95 degrees Fahrenheit); higher
temperatures may be damaging to sperm count.
Testes
The testes are homologous to the ovary in that it
produces the male gamete (sperm).
The testes are oval bodies that average about 1.5 to 3
inches (4 to 7 centimeters) in length and 2 to 3 teaspoons (20 to 25
milliliters) in volume. Usually the left testis hangs slightly lower than the
right one. The testes have two primary functions:
·
Producing sperm (which carry the man's genes)
·
Producing testosterone (the primary male sex
hormone)
The male testes produce spermatozoa (more commonly
referred to as sperm), which are released at sexual climax, or orgasm. This is
known as ejaculation. The sperm leaves the penis in a mixture of secretions
designed to nourish and transport the cells into the female reproductive system
for procreation.
There are up to 750 million sperm cells contained in a
single ejaculation, but it only takes one sperm cell to fertilize a woman’s
egg.
·
Sperm fertilizes the oocyte, donate the paternal
chromatin, and provide the centrosome that maintains the zygote’s microtubule
system.
·
Sperm have three parts: a head, which holds the
chromatin, a midpiece filled with mitochondria to provide energy, and a
flagellum or tail to move the sperm from the vagina to the oocyte.
·
Sperm with one tail, such as human sperm, are
referred to as spermatozoa.
·
Sperm quality and quantity decrease with age.
·
Sperm are produced within seminiferous tubules.
Accessory organs include the following:
Epididymis
The epididymis is a long, coiled tube that rests on the
backside of each testicle. It transports and stores sperm cells that are
produced in the testes. It also is the job of the epididymis to bring the sperm
to maturity, since the sperm that emerge from the testes are immature and
incapable of fertilization. During sexual arousal, contractions force the sperm
into the vas deferens.
Vas deferens
The vas deferens is a long, muscular tube that travels
from the epididymis into the pelvic cavity, to just behind the bladder. The vas
deferens transports mature sperm to the urethra, the tube that carries urine or
sperm to outside of the body, in preparation for ejaculation.
Ejaculatory ducts
These are formed by the fusion of the vas deferens and
the seminal vesicles (see below). The ejaculatory ducts empty into the urethra.
Urethra
The urethra is the tube that carries urine from the
bladder to outside of the body. In males, it has the additional function of
ejaculating semen when the man reaches orgasm. When the penis is erect during
sex, the flow of urine is blocked from the urethra, allowing only semen to be
ejaculated at orgasm.
Prostate gland
The prostate gland is a walnut-sized structure that is
located below the urinary bladder in front of the rectum. The prostate gland
contributes additional fluid to the ejaculate. Prostate fluids also help to
nourish the sperm. The urethra, which carries the ejaculate to be expelled
during orgasm, runs through the center of the prostate gland.
Bulbourethral gland
Also called Cowper's gland, these are pea-sized
structures located on the sides of the urethra just below the prostate gland.
These glands produce a clear, slippery fluid that empties directly into the
urethra. This fluid serves to lubricate the urethra and to neutralize any
acidity that may be present due to residual drops of urine in the urethra.
Female Reproductive System
The function of the external female reproductive structures
(the genital) is twofold: To enable sperm to enter the body and to protect the
internal genital organs from infectious organisms.
External reproductive organs
mons pubis: A fleshy protuberance over the pubic bones
that becomes covered with hair during puberty.
clitoris: A small, sensitive, and elongated erectile
organ at the anterior part of the vulva in female, homologous with the penis.
urethral orifice: The external opening of the urethra,
from which urine is ejected during urination. It is located about 2.5 cm behind
the clitoris and immediately in front of the vagina in the vulval vestibule.
vaginal orifice: A median slit below and behind the
opening of the urethra; its size varies inversely with that of the hymen.
labia majora: The two outer rounded folds of adipose
tissue that lie on either side of the opening of the vagina.
labia minora: The two inner folds of skin within the
cleft of the labia majora.
Internal reproductive organs
Vagina
The vagina is an elastic, muscular canal with a soft,
flexible lining that provides lubrication and sensation.
It connects the uterus to the outside world. The vulva
and labia form the entrance, and the cervix of the uterus protrudes into the
vagina, forming the interior end.
It receives the penis during sexual intercourse and
serves as a conduit for menstrual flow from the uterus.
During childbirth, the baby passes through the vagina
(birth canal).
Uterus
The uterus sits in the middle of the pelvis, behind the
bladder and in front of the rectum. The actual position of the uterus within
the pelvis varies from person to person.
Each position has its own name:
·
Anteverted uterus: An anteverted uterus tips
slightly forward.
·
Retroverted uterus: A retroverted uterus bends
slightly backward.
Both positions are normal, and the position of the uterus
can change throughout a woman’s life, most frequently after a pregnancy.
Broad Ligament of Uterus
The broad ligament is a flat sheet of the peritoneum,
associated with the uterus, fallopian tubes, and ovaries.
Extends from the lateral pelvic walls on both sides, and
folds over the internal female genitalia, covering their surface anteriorly and
posteriorly.
Subdivisions:
·
Mesometrium: Surrounds the uterus and is the
largest subsection of the broad ligament. It runs laterally to cover the
external iliac vessels, forming a distinct fold over them. The mesometrium also
encloses the proximal part of the round ligament of the uterus.
·
Mesovarium: Part of the broad ligament
associated with the ovaries. It projects from the posterior surface of the
broad ligament and attaches to the hilum of the ovary, enclosing its
neurovascular supply. It does not, however, cover the surface of the ovary
itself.
·
Mesosalpinx: Originates superiorly to the
mesovarium, enclosing the fallopian tubes.
Parts of the uterus:
Fundus
The fundus is the upper part of the uterus. It’s broad
and curved. The fallopian tubes attach to the uterus just below the fundus.
Corpus
The corpus is the main body of the uterus. It’s very
muscular and can stretch to accommodate a developing fetus. During labor, the
muscular walls of the corpus contract to help push the baby through the cervix
and vagina.
The corpus is lined by a mucous membrane called the
endometrium. This membrane responds to reproductive hormones by changing its
thickness during each menstrual cycle. If an egg is fertilized, it attaches to
the endometrium. If no fertilization occurs, the endometrium sheds its outer
layer of cells, which are released during menstruation.
Isthmus
The portion of the uterus between the corpus and the
cervix is called the isthmus. This is where the walls of the uterus begin to
narrow toward the cervix.
Cervix
The cervix is the lowest part of the uterus. It’s lined
with a smooth mucous membrane and connects the uterus to the vagina. Glands in
the cervical lining usually produce thick mucus. However, during ovulation,
this becomes thinner to allow sperm to easily pass into the uterus.
The cervix has three main parts:
·
Endocervix: This is the inner part of the cervix
that leads to the uterus.
·
Cervical canal: The cervical canal links the
uterus to the vagina.
·
Exocervix: The exocervix is the outer part of
the cervix that protrudes into the vagina.
During childbirth, the cervix dilates (widens) to allow
the baby to pass through the birth canal.
Ovary
The ovaries have two main reproductive functions in the
body. They produce oocytes (eggs) for fertilization and they produce the
reproductive hormones, estrogen, and progesterone.
The function of the ovaries is controlled by a gonadotrophin-releasing hormone released from nerve cells in the hypothalamus
which sends their messages to the pituitary gland to produce luteinizing hormone
and follicle stimulating hormone. These are carried in the bloodstream to
control the menstrual cycle.
The major hormones secreted by the ovaries are estrogen
and progesterone, both important hormones in the menstrual cycle. Estrogen
production dominates in the first half of the menstrual cycle before ovulation,
and progesterone production dominates during the second half of the menstrual
cycle when the corpus luteum has formed. Both hormones are important in
preparing the lining of the womb for pregnancy and the implantation of a
fertilized egg, or embryo.
Fallopian Tube (uterine tube)
Fallopian tubes are the female structures that transport
the ova from the ovary to the uterus each month. In the presence of sperm and
fertilization, the uterine tubes transport the fertilized egg to the uterus for
implantation.
Each uterine tube is approximately 10 cm in length and 1
cm in diameter and is situated within the mesosalpinx.
A uterine tube contains 3 parts:
·
The first segment, closest to the uterus, is
called the isthmus.
·
The second segment is the ampulla, which becomes
more dilated in diameter and is the most the common site for fertilization.
·
The final segment, located farthest from the
uterus is the infundibulum. The infundibulum gives rise to the fimbriae,
fingerlike projections that are responsible for picking up the egg released by
the ovary.
Menstrual Cycle
Females of reproductive age (beginning anywhere from
11-16 years of age) experience cycles of hormonal activity that repeat at about
one-month intervals. Menstru means "monthly"; hence the term
menstrual cycle. With every cycle, a woman’s body prepares for a potential
pregnancy, whether that is the woman’s intention. The term menstruation refers
to the periodic shedding of the uterine lining.
The average menstrual cycle takes about 28 days and
occurs in phases: the follicular phase (development of the egg), the ovulatory
phase (release of the egg), and the luteal phase (hormone levels decrease if
the egg is not fertilized).
There are four major hormones (chemicals that stimulate
or regulate the activity of cells or organs) involved in the menstrual cycle:
follicle-stimulating hormone, luteinizing hormone, estrogen, and progesterone.
Follicular phase
This phase starts on the first day of your period. During
the follicular phase of the menstrual cycle, the following events occur:
·
Two hormones, follicle-stimulating hormone (FSH)
and luteinizing hormone (LH) is released from the brain and travel in the
blood to the ovaries.
·
The hormones stimulate the growth of about 15-20
eggs in the ovaries, each in its own "shell," called a follicle.
These hormones (FSH and LH) also trigger an increase in the production of the
female hormone estrogen.
·
As estrogen levels rise, like a switch, it turns
off the production of follicle-stimulating hormone. This careful balance of
hormones allows the body to limit the number of follicles that will prepare
eggs to be released.
·
As the follicular phase progresses, one follicle
in one ovary becomes dominant and continues to mature. This dominant follicle
suppresses all the other follicles in the group. As a result, they stop growing
and die. The dominant follicle continues to produce estrogen.
Ovulatory phase
·
The ovulatory phase, or ovulation, starts about
14 days after the follicular phase started. The ovulatory phase is the midpoint
of the menstrual cycle, with the next menstrual period starting about 2 weeks
later. During this phase, the following events occur:
·
The rise in estrogen from the dominant follicle
triggers a surge in the amount of luteinizing hormone that is produced by the
brain. This causes the dominant follicle to release its egg from the ovary.
·
As the egg is released (a process called
ovulation) it is captured by finger-like projections on the end of the
fallopian tubes (fimbriae). The fimbriae sweep the egg into the tube.
·
Also, during this phase, there is an increase in
the amount and thickness of mucus produced by the cervix (lower part of the
uterus.) If a woman were to have intercourse during this time, the thick mucus
captures the man's sperm, nourishes it, and helps it to move towards the egg
for fertilization.
Luteal phase
·
The luteal phase begins right after ovulation
and involves the following processes:
Once it releases its egg, the
empty ovarian follicle develops into a new structure called the corpus luteum.
·
The corpus luteum secretes the hormones estrogen
and progesterone. Progesterone prepares the uterus for a fertilized egg to
implant.
·
If intercourse has taken place and a man's sperm
has fertilized the egg (a process called conception), the fertilized egg
(embryo) will travel through the fallopian tube to implant in the uterus. The
woman is now considered pregnant.
·
If the egg is not fertilized, it passes through
the uterus. Not needed to support a pregnancy, the lining of the uterus breaks
down and sheds, and the next menstrual period begins.
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