Human Male Reproductive Anatomy
The human male's external
reproductive organs are the scrotum and penis. The internal reproductive organs
consist of gonads that produce both sperm and reproductive hormones, accessory
glands that secrete products essential to sperm movement, and ducts that carry
the sperm and glandular secretions (Figure 1).
Figure 1
Testes
The male gonads, or testes
(singular, testis), produce sperm in highly coiled tubes called seminiferous
tubules. Most mammals produce sperm properly only when the testes are cooler
than the rest of the body. In humans and many other mammals, testis temperature
is maintained about 2°C below the core body temperature by the scrotum, a fold
of the body wall. The testes develop in the abdominal cavity and descend into
the scrotum just before birth (a testis within a scrotum is a testicle). In
many rodents, the testes are drawn back into the cavity between breeding
seasons, interrupting sperm maturation. Some mammals whose body temperature is
low enough to allow sperm maturation such as whales and elephants retain the
testes in the abdominal cavity at all times.
Ducts
From the seminiferous tubules of
a testis, the sperm pass into the coiled duct of an epididymis. In humans, it
takes 3 weeks for sperm to travel the 6-m length of this duct, during which
time the sperm complete maturation and become motile. During ejaculation, the
sperm are propelled from each epididymis through a muscular duct, the vas
deferens. Each vas deferens (one from each epididymis) extends around and
behind the urinary bladder, where it joins a duct from the seminal vesicle,
forming a short ejaculatory duct. The ejaculatory ducts open into the urethra,
the outlet tube for both the excretory system and the reproductive system. The
urethra runs through the penis and opens to the outside at the tip of the
penis.
Accessory Glands
Three sets of accessory glands-the
seminal vesicles, the prostate gland, and the bulbourethral glands-produce
cretions that combine with sperm to form semen, the fluid that is ejaculated.
Two seminal vesicles contribute about 60% of the volume of semen. The fluid
from the seminal vesicles is thick, yellowish, and alkaline. It contains mucus,
the sugar fructose (which provides most of the sperm's energy a coagulating
enzyme, ascorbic acid, and local regulator called prostaglandins.
The prostate gland secretes its
products directly into the urethra through small ducts. Thin and milky, the
fluid from this gland contains anticoagulant enzymes and citrate sperm
nutrient). The prostate undergoes benign (noncancerous) enlargement in more
than half of all men over age 40 and in almost all men over 70. In addition,
prostate cancer, which most often afflicts men 65 and older, is one of the most
common human cancers.
The bulbourethral glands are a
pair of small glands along the urethra below the prostate. Before ejaculation,
they secrete clear mucus that neutralizes any acidic urine remaining in the
urethra. There is evidence that bulbourethral fluid carries some sperm released
before ejaculation, which contribute to the high failure rate of the withdrawal
method of birth control (coitus interruptus).
Penis
The human penis contains the
urethra as well as three cylinders of spongy erectile tissue. During sexual
arousal, the erectile tissue fills with blood from the arteries. As this tissue
fills, the increasing pressure seals off the veins that drain the penis,
causing it to engorge with blood. The resulting erection enables the penis to
be inserted into the vagina. Alcohol consumption, certain drugs, emotional
issues, and aging all can cause an inability to achieve an erection (erectile
dysfunction). For individuals with long-term erectile dysfunction, drugs such
as Viagra promote the vasodilating action of the local regulator nitric oxide;
the resulting relaxation of smooth muscles in the blood vessels of the penis
enhances blood flow into the erectile tissues Although all mammals rely on
penile erection for mating, the penis of raccoons, walruses, whales, and
several other mammals also contains a bone, the baculum, which is thought to
further stiffen the penis for mating The main shaft of the penis is covered by
relatively thick skin. The head, or glans, of the penis has a much thinner
outer layer and is consequently more sensitive to stimulation. The human glans
is surrounded by a fold of skin called the prepuce or foreskin, which is
removed if a male is circumcised.
Function of Male Reproductive
System
The purpose of the organs of
the male
reproductive system is to perform the following functions:
- To produce, maintain, and
transport sperm (the
male reproductive cells) and protective fluid (semen).
- To discharge sperm within the
female reproductive tract during sex.
- To produce and secrete male sex hormones
responsible for maintaining the male reproductive system.
Human Female Reproductive Anatomy
The human female's external
reproductive structures are the clitoris and two sets of labia, which surround
the clitoris and vaginal opening. The internal organs consist of gonads which
produce eggs and reproductive hormones, and a sys- tem of ducts and chambers,
which receive and carry gametes and house the embryo and fetus (Figure 2).
Figure 2
Ovaries
The female gonads are a pair of
ovaries that flank the uterus and are held in place in the abdominal cavity by ligaments.
The outer layer of each ovary is packed with follicles, each consisting of an
oocyte, a partially developed egg, surrounded by support cells. The surrounding
cells nourish and protect the oocyte during much of its formation and
development.
Oviducts and Uterus
An oviduct, or fallopian tube,
extends from the uterus to The dimensions of ward a funnel-like opening at each
ovary this tube vary along its length, with the inside diameter near the uterus
being as narrow as a human hair. Upon ovulation cilia on the epithelial lining
of the oviduct help collect the egg by drawing fluid from the body cavity into
the oviduct. Together with wavelike contractions of the oviduct, the cilia
convey the egg down the duct to the uterus, also known as the womb. The uterus
is a thick, muscular organ that can expand during pregnancy to accommodate a
4-kg fetus The inner lining of the uterus, the endometrium, is richly sup-
plied with blood vessels. The neck of the uterus, called the cervix, opens into
the vagina.
Vagina and Vulva
The vagina is a muscular but
elastic chamber that is the site for insertion of the penis and deposition of
sperm during copulation. The vagina, which also serves as the birth canal
through which a baby is born, opens to the outside at the vulva, the collective
term for the external female genitalia. A pair of thick, fatty ridges, the
labia maj encloses and protects the rest of the vulva. The vaginal opening and
the separate opening of the urethra are located within a cavity bordered by a
pair of slender skin folds, the labia minora. A thin piece of tissue called the
hymen partly covers the vaginal opening in humans at birth and usually until
sexual intercourse or vigorous physical activity ruptures it. Located at the
top of the labia minora, the clitoris consists of erectile tissue supporting a
rounded glans, or head, covered by a small hood of skin, the prepuce. During
sexual arousal, the clitoris, vagina, and labia minora all engorge with blood
and enlarge. Richly supplied with nerve endings, the clitoris is one of the
most sensitive points of sexual stimulation. Sexual arousal also induces the
vestibular glands near the vaginal opening to secrete lubricating mucus,
thereby facilitating intercourse.
Mammary Glands
The mammary glands are present in
both sexes, but they normally produce milk only in females. Though not part of
the reproductive system, the female mammary glands are important to
reproduction. Within the glands, small sacs of epithelial tissue secrete milk,
which drains into a series of ducts that open at the nipple. The breasts
contain connective and fatty (adipose) tissue in addition to the mammary
glands. Because the low level of estradiol in males limits the development of
the fat deposits, male breasts usually remain small.
Function of Female Reproductive
System
The female reproductive system
(or female genital system) serves multiple functions. Firstly, it is
responsible for producing the eggs necessary for reproduction, thus
facilitating the occurrence of reproduction. In addition, it produces the
female sex hormones that maintain the reproductive cycle and that play a direct
or indirect role elsewhere in the body.
Menstrual Cycle
Phases of the Menstrual Cycle
The four main phases of the menstrual cycle are:
- menstruation
- the follicular phase
- ovulation
- the luteal phase.
- Menstruation
Menstruation is the elimination of the thickened lining of the uterus
(endometrium) from the body through the vagina. Menstrual fluid contains blood,
cells from the lining of the uterus (endometrial cells) and mucus. The average
length of a period is between three days and one week.
Sanitary pads or tampons are used to absorb the menstrual flow. Both pads and
tampons need to be changed regularly (at least every four hours). Using tampons
has been associated with an increased risk of a rare illness called toxic shock
syndrome (TSS).
Follicular Phase
The follicular phase starts on the first day of menstruation and ends with
ovulation. Prompted by the hypothalamus, the pituitary gland releases follicle
stimulating hormone (FSH). This hormone stimulates the ovary to produce around
five to 20 follicles (tiny nodules or cysts), which bead on the surface.
Each follicle houses an immature egg. Usually, only one follicle will mature
into an egg, while the others die. This can occur around day 10 of a 28-day
cycle. The growth of the follicles stimulates the lining of the uterus to
thicken in preparation for possible pregnancy.
Ovulation
Ovulation is the release of a
mature egg from the surface of the ovary. This generally occurs mid-cycle,
around two weeks or so before menstruation starts. During the follicular phase,
the developing follicle causes a rise in the level of oestrogen. The
hypothalamus in the brain recognises these rising levels and releases a
chemical called gonadotrophin-releasing hormone (GnRH). This hormone prompts
the pituitary gland to produce raised levels of luteinising hormone (LH) and
FSH.
Within two days, ovulation is triggered by the high levels of LH. The egg is
funnelled into the fallopian tube and towards the uterus by waves of small,
hair-like projections. The life span of the typical egg is only around 24
hours. Unless it meets a sperm during this time, it will die.
Luteal Phase
During ovulation, the egg bursts
from its follicle, but the ruptured follicle stays on the surface of the ovary.
For the next two weeks or so, the follicle transforms into a structure known as
the corpus luteum. This structure starts releasing progesterone, along with
small amounts of oestrogen. This combination of hormones maintains the
thickened lining of the uterus, waiting for a fertilised egg to stick
(implant).
If a fertilised egg implants in the lining of the uterus, it produces the
hormones that are necessary to maintain the corpus luteum. This includes human
chorionic gonadotrophin (HCG), the hormone that is detected in a urine test for
pregnancy. The corpus luteum keeps producing the raised levels of progesterone
that are needed to maintain the thickened lining of the uterus.
If pregnancy does not occur, the corpus luteum withers and dies, usually around
day 22 in a 28-day cycle. The drop in progesterone levels causes the lining of
the uterus to fall away. This is known as menstruation. The cycle then repeats.
References:
Campbell, Reece, Urry, Cain,
Wasserman, Minorsky, Jackson. Biology A Global Approach. 10th ed. Pearson:
Pearson Education Limited, 2015.
2018.
