Professor Dave here, let’s reproduce.
At this point, we’ve learned about all of the major systems in the human body, and we
hopefully have a much clearer picture about how the body functions as a whole.
But one thing we haven’t talked about yet is how humans make more humans.
Thousands of people are born every day, so how does that work on the anatomical level?
Of course most of us have had a pretty solid idea about this process since adolescence,
as this activity is intimately tied to the sex drive that begins to take hold at that time.
But let’s take a look at the reproductive system the same way that we’ve looked at
all the others, so that we can better understand these incredible organs and their functions.
When examining the reproductive system, the first things to mention are the primary sex organs.
These are also known as gonads, and the interesting thing with these is that unlike the lungs,
or the heart, or the liver, or the stomach, which are all found in every human, these
are organs which depend on the sex of the person.
The male gonads are called the testes, and the female gonads are called the ovaries.
The purpose of the gonads is to secrete several different sex hormones, and also to produce
sex cells, which are called gametes.
We may recall from the biology series, that gametes are haploid cells that are the products
of meiosis, rather than the diploid cells all around the rest of the body, which are
the products of mitosis.
We talked about these processes at great length, so if you’d like to review them now, check
out those tutorials for a refresher.
Otherwise, as we recall, the gametes produced in males are sperm cells, and the gametes
produced in females are egg cells, and when a sperm and an egg meet as the result of sexual
intercourse, fertilization can result, which leads to a zygote, embryonic development,
and eventually the birth of a new human being.
But the primary sex organs are not the only components of the reproductive system, so
there is lots more to discuss.
There are also accessory reproductive organs.
These include a number of ducts and glands, as well as the external genitalia.
Since the reproductive system is the only system that varies dramatically between males
and females, it is best to examine each separately, so let’s start with the male reproductive system.
As we said, the primary sex organ in males is the testes, which is where testosterone
is produced, and these are housed in a sac of skin called the scrotum, which is divided
by a midline called a septum.
It is here that sperm are produced, which may seem like a vulnerable place to store
this critical asset, but the lower temperature external to the body is ideal for sperm production.
Each testis is surrounded by the tunica albuginea, followed by the tunica vaginalis.
Inside, the testis is divided into lobules, which contain coiled seminiferous tubules,
and these are where sperm is made, from spermatogenic cells, a process called spermatogenesis.
These tubules converge to form a straight tubule that leads to the rete testis.
From here, sperm will then move through a system of ducts, starting with the epididymis,
where they are stored until ejaculation, during a sexual act.
When this happens, sperm cells move through the vas deferens, the ejaculatory duct, and
eventually moving into the urethra, just like urine at the end of the urinary system.
In doing so, it leaves the penis, which is a copulatory organ whose purpose is to deliver
sperm into the female reproductive tract, and together with the scrotum, these make
up the external genitalia of a male.
The penis contains three cylindrical shafts of erectile tissue, each covered by dense
fibrous connective tissue.
The erectile tissue is spongy, containing smooth muscle and vascular spaces, and when
these spaces fill up with blood during sexual arousal, the penis will enlarge and harden,
a status called erection, which enables penetration.
Beyond the gonads and external genitalia, there are also accessory glands.
These are the seminal glands, prostate, and bulbo-urethral glands.
Starting with the seminal glands, these sit on the surface of the bladder and produce
a fluid called semen.
This is not to be confused with sperm, as there are no sperm cells produced here, but
during ejaculation, sperm cells will mix with the semen in the ejaculatory duct, and the
semen will enhance sperm motility, or fertilizing ability.
It does this through a mixture of hormones and other compounds that suppress the immune
response in the female’s reproductive tract, destroy bacteria, and help sperm stick to
the walls of the vagina and prevent drainage.
Semen actually makes up the majority of the ejaculate by volume.
The prostate is a small gland that encircles the urethra near the bladder.
The purpose of the prostate is to contract during ejaculation in order to allow prostatic
secretion to enter the urethra and join the ejaculate and activate the sperm.
Lastly the bulbo-urethral glands sit near the prostate and produce a mucus that lubricates
the glans penis, or head of the penis, during sexual arousal.
Now let’s move on to the female reproductive system, which is quite a bit more involved.
As we mentioned, the female gonads are called ovaries.
Here, the female gametes are produced, which are egg cells, or ova, as well as the sex
hormones estrogen and progesterone.
In addition, there are accessory ducts, which are the uterine tubes and the uterus.
There is also the vagina, and the external genitalia of the female.
The ovaries are a bit like testes, though they are inside the body, surrounded by a
fibrous tunica albuginea followed by a germinal epithelium, held in place by various ligaments,
and supplied by ovarian arteries.
An ovary has an outer layer called the cortex, and an inner layer called the medulla.
In the cortex we see sacs called follicles, each of which houses an immature egg called
an oocyte, and the production of mature egg cells is called oogenesis.
When a primordial follicle matures, it becomes a vesicular follicle, with a cavity called
an antrum.
This follicle will then bulge out of the surface of the ovary, allowing it to eject the oocyte,
and this event is called ovulation.
This will then enter the uterine tubes, or fallopian tubes, which have sections called
the isthmus, ampulla, and infundibulum, which bears projections called fimbriae.
Fertilization will occur at some point along the fallopian tubes, which end at the uterus,
also known as the womb.
This receives the fertilized egg, retains it in the uterine wall, and nourishes it through
fetal development.
The uterus has a rounded top region called the fundus, and narrow neck at the bottom
called the cervix, with its cervical canal.
The wall of the uterus is quite thick, comprised of three layers.
First the perimetrium, then the bulky myometrium, made of smooth muscle, and then the endometrium,
which lines the uterine cavity, or lumen.
Going through the cervical canal, we reach the vagina, the copulatory organ for the female,
which receives the penis during intercourse.
Further out, we arrive at the external genitalia, collectively known as the vulva, which consists
of the mons pubis, skin folds called labia majora and labia minora, the clitoris, and
the vestibule.
Also qualifying as part of the female reproductive system are the mammary glands found in the
breasts, which are similar to sweat glands, but they start to produce milk during pregnancy
before a baby is born.
Each breast has a ring of pigmented skin called an areola, from which a nipple protrudes.
Each mammary gland contains lobes separated by fibrous connective tissue and fat, which
themselves contain smaller units called lobules, which contain structures called alveoli, which
produce milk.
This milk is transferred to a lactiferous duct, which leads to a lactiferous sinus,
and this is where milk collects when a mother is nursing.
There is a lot more to discuss here regarding the ovarian cycle and its hormonal regulation,
the menstrual cycle, and other things, but let’s take a break here and reflect.
In this series, we have examined the basic structure and function of every major system
in the human body.
If the basics are all you are interested in, feel free to move on to other topics that
require only rudimentary knowledge of human anatomy and physiology.
If you are interested going deeper, in order to enter a medical profession, or simply because
you are just that inquisitive, then stick around.
From this point forward, we will go through a variety of topics that involve revisiting
the systems we now know, but we will look at things in a lot more detail in order to
better understand highly specific bodily functions.
Either way, let’s learn some more science!