Internal Anatomy 2: Featuring the Testes and Associated Organs

In my last post with a focus on internal anatomy, I described organs and processes associated with bodies that produce eggs. Today I continue with the other main type of body out there: the sperm-producing body.

The biggest difference between these two types of bodies (besides the fact that one of them is equipped to grow a baby over the course of nine months) is that sperm-producing bodies are constantly in a state of, well, production. Bodies that produce eggs are born with every single sex cell that they will ever need (about seven million), while bodies that produce sperm only start the process of making their sex cells (in scientific terms this is called gametogenesis) at puberty. And once they start making sex cells, they produce hundreds of millions of sperm per day. This post describes the structures associated with this sperm production and storage, as well as the process of the expulsion of sperm from a body: that is, ejaculation.

Internal Anatomy: The Big Picture

The diagram below depicts the various tubules and organs of an internal reproductive system that produces sperm and semen. Although these structures come from the same embryological tissues that form the uterus and ovaries, the layout of these organs are fairly different. While the path of an egg from the ovaries to the uterus to the vagina is pretty straightforward, the path of a sperm from testes to urethral opening is winding and complicated. In the following descriptions, I begin at the testes and trace the path that a few million sperm might take to reach the outside world.

Internal Anatomy Penis

 Testes – the testes (testicle or testis, singular) are the organs primarily responsible for sperm production. Unlike the ovaries, which come equipped with all the sex cells they will ever expel into the uterus, the testes are constantly producing new sex cells that will eventually exit the body. The testes are filled with seminiferous tubules – miles and miles of small tubes that produce and mature new sperm cells. Every day, the testes produce a few hundred million tiny, functional sex cells that could possibly maybe one day fertilize an egg.

Epididymis – the epididymis is a storage center for the sperm that the testes produce. Once a sperm cell matures in the testes, it travels up to the epididymis. Although my drawing depicts the epididymis as a hat resting on top of a single testicle, it is actually a really long tube rolled over and over on itself (if you’re sensing a theme here, you’re right: sperm-producing bodies have a lot of tubes). Sperm generally stay in the epididymis for two to three months.

Vas Deferens – the vas deferens is a tube that connects the epididymis to the urethra. While sperm will enter and live in the testes and epididymis no matter what, they only enter the vas deferens if a person is about to ejaculate. If ejaculation is nigh, some sperm from the epididymis will be propelled into the vas deferens, which loops around the bladder and eventually intersects with the urethra. Note: sperm leaving the epididymis cannot swim – they only activate when they experience a dramatic pH change and/or a drop in zinc levels, which happens only after they leave a body.

Seminal Vesicles – in case you didn’t know, semen (also referred to as ejaculate – yes, it has the same spelling as, well, ejaculate, but is pronounced ejaculit and is thus recognized as a noun) is not simply composed of sperm, but a whole host of other liquids from various organs in the body. The first organs that produce these liquids are the seminal vesicles (there are two, one for each testicle). These vesicles secrete a thick, mucousy fluid that consists of mucus, vitamin C, fructose, proteins, enzymes, and prostaglandins (which promote muscle contraction). All together, these ingredients make up 65 to 80% of the total ejaculate exiting the body.

Prostate Gland – after passing the seminal vesicles, the vas deferens enters the prostate. If you haven’t heard about the prostate, I’m sorry – next to the penis and clitoris, it’s one of the organs in the body most likely to give you extreme feelings of pleasure. The prostate sits right underneath the bladder (fittingly, the word prostate roughly translates to “guardian of the bladder” in Greek) and contributes another 30% of the components of semen, which is why the vas deferens passes through it. Included in this prostatic fluid are zinc and a chemical called spermine. Spermine, according to the Guide to Getting it On, is the reason that sperm often smells like Clorox bleach and also why getting ejaculate in your eye hurts (spermine, it turns out, is highly corrosive in large quantities. In small quantities, it can cause minor discomforts. This might also be why swallowing semen sometimes causes a burning sensation in the throat).

The more interesting part of the prostate, of course, is its ability to cause intense pleasurable sensations. The prostate becomes active during sexual activity – it often engorges and grows as the glands inside it produce prostatic fluid – especially when a person nears orgasm. Hence, prostate stimulation can make a person feel like they are approaching orgasm, and can even cause orgasm (the ability to orgasm from prostate stimulation alone varies from person to person). Reportedly, prostate orgasms can be more full-bodied than orgasms resulting only from penile stimulation, seeming to travel all over the body rather than staying only in the genital area. Prostate massage can also help people achieve multiple orgasms. In their short episode on the “P-Spot,” the podcast The Heart (formerly known as Audio Smut) recorded different people with prostates describing the sensations they associated with prostate stimulation. One person described the sensations as “overwhelming…waves,” and another stated that they could “come for…ten or twenty minutes” and “feel it throughout [their] whole body.” I’ll discuss prostate massage and how exactly to stimulate the prostate in a later post.

Cowper’s Glands – these glands, also sometimes referred to as the bulbourethral glands, contribute a mixture of galactose and mucus to semen. The mucus is especially important because it creates a thinner (when compared with the mucus in the vagina) fluid for the sperm to swim in, and also helps them stick together while they swim toward the uterus. Cowper’s glands are also responsible for the small bit of pre-ejaculate (pre-cum) that appears on the tip of the penis when a person first becomes aroused.

Urethra – here’s where things get a little confusing: the urethra is the tube that allows both pee and semen to exit the body. While egg-producing bodies have one tube related to reproduction and one tube for peeing, sperm-producing bodies just have one tube for both of those things. The handy thing is that it’s exceedingly difficult for both liquids to leave the body at the same time. In fact, most people can’t pee when they have an erection. This is because the swelling of the prostate causes the internal urethral sphincter to close, essentially blocking the exit of the bladder (looking at the diagram, note how the part of the urethra connected to the bladder goes through the prostate).

Root of Penis – if you remember my post on the anatomy of the penis, you might remember that the average length of a penis is between 4.6 and 6.2 inches. Of course, there’s always the smart-ass who likes to point out that this isn’t entirely accurate, because about 1/3 of the penis exists inside a person’s body. This invisible part of the penis is its root, which, as the name suggests, keeps the penis attached to the body. When compared to the clitoris, it is analogous to the smaller organ’s two legs, or crura. It is also made of the same erectile tissue as the rest of the penis and also fills with blood during an erection.

Anus/Rectum – unlike most of the other structures described here, the anus remains the same in both egg-producing and sperm-producing bodies. They are both dense with sensitive nerve clusters and can bring great pleasure when stimulated. In sperm-producing bodies, however, the anus also offers access to the sensitive prostate. As it’s difficult to reach up into a penis all the way to the prostate, people access this organ by pressing against the thin wall of the rectum. This slight pressure, as mentioned above, can feel very, very good.


Ejaculation is the process by which sperm exit a person’s body, assumedly in a quest to go find an egg that they might fertilize. Innate responses in the body cause the seminal vesicles, prostate, and Cowper’s glands to produce their various liquids while at the same time releasing a few million sperm hanging around in the epididymis. Eventually, smooth muscle contractions propel the sperm and these other fluids (semen/ejaculate) out of a person’s body via the penis. Depending on where said penis is at the time, the sperm might fly into the air, onto that person’s body or another person’s, into a sock/condom/tissue, or perhaps into a vagina or rectum. (Obviously, the only scenario in which these sperm might find an egg to fertilize is if they’re deposited into the vagina of a person whose body is equipped to produce eggs).

Unsurprisingly, there’s a fair amount of cultural anxiety surrounding ejaculation, especially when the sperm leaving a penis are not entering a vagina. You might remember the essay I mentioned in my last post by Emily Martin that detailed cultural anxieties surrounding menstruation and the scientific language that implies that this natural and renewing bodily process is somehow wasteful or a failure of reproduction. This idea of wastefulness extends to sperm-producing bodies, perhaps even more aggressively, and definitely more overtly, than egg-producing bodies. This is probably due to the fact that ejaculation is often understood as a choice, whereas menstruation usually happens whether a person wants it to or not. According to Mary Roach, the first printed condemnation of masturbation/non-copulatory ejaculation (excluding religious works) arrived in 1760. Titled Onanism; or, A Treatise upon the Disorders Produced by Masturbation, it listed a variety of terrible consequences linked with masturbation, including (surprise surprise) impotence. The idea behind this text was that a body contains a limited amount of sperm, and thus wasting it would eventually cause a person to become incapable of reproduction and their ability to achieve an erection would vanish.

Despite the fact that this is clearly (at least in hindsight) bullshit, this book and others like it led to some horrific and hysterical inventions, including a “Penile Pricking Ring,” which expanded when its wearer got an erection while sleeping (because while you can presumably control your urge to masturbate while awake, you might accidentally ejaculate while sleeping), exposing sharp pins that would jolt the ring wearer awake and (perhaps literally) kill their boner. Another contraption: a cap for the glans of the penis to wear with clips meant to attach to pubic hair (when the person wearing the cap got an erection, the clips would tug on the hair. Ouch). Some parents went as far as to tie their children’s hands to their headboard while they slept or sew them pants without pockets; some luckless individuals were escorted to the doctor to have a hot wire applied to their genitals (double ouch). While, hopefully, these practices are extinct today, there is still a general stigma around sexual activity that does not result in conception (e.g. sex between individuals with the same genitalia/reproductive system, anal sex, solo sex/masturbation). This is really too bad, because the people who are most concerned with baby production should be encouraging ejaculation as much as possible: sperm that stay inside a body for too long can become deformed and thus incapable or fertilizing anything. If you’re trying to conceive, it turns out, the optimal amount of time to wait between ejaculations is five days. That way, the semen exiting a person’s body will have optimal amount of healthy, well-developed sperm in it.

Regardless of fertility or one’s desire to conceive or have a child, no one should feel ashamed about ejaculating, just as no one should feel ashamed about menstruating. The two are bodily processes, each related to the reproductive system, but not necessarily to reproduction itself.

Having covered all the stigma surrounding ejaculation, here’s how it generally happens: when a person first becomes aroused, they get an erection. Around the same time, the Cowper’s glands send a tiny bit of fluid through the urethra, cleaning out any debris there, including urine or leftover semen from a previous ejaculation (this is why it’s possible to get pregnant from pre-cum, although it’s highly unlikely). If a person’s erect penis is stimulated enough, the first stage of ejaculation, emission, begins. In this stage, the ingredients of semen gather together in a part of the urethra at the very beginning of the root of the penis, called the bulbous urethra. You might remember that these ingredients include fluid from the Cowper’s glands (about 5% of total ejaculate), the prostate gland (15-30%), the seminal vesicles (65-80%), and sperm (less than 1% of total ejaculate). All together, these ingredients form about a teaspoon of semen. Of course, this volume varies from person to person, especially if the person ejaculating received a prostate massage, which will often cause the prostate to produce more fluid than usual. The pressure from all of these liquids hanging out in the bulbous urethra causes the second stage of ejaculation, expulsion. Smooth muscle tissue around the penis contracts, sending the semen flying down the urethra and out of the urethral opening. Although this description suggests that ejaculation happens in one powerful cannon shot, it actually happens through a series of muscle contractions, usually between ten and fifteen. About half of these contractions actually expel fluid from the body, meaning that ejaculation happens in small squirts and not an elongated stream.

Semen usually exits a body as a gelatinous substance, but it quickly becomes watery thanks to a chemical in prostate fluid called prostate-specific antigen (PSA). Semen starts out mucousy thanks to the seminal vesicle’s contribution being quite thick, but once it comes into contact with PSA, it thins out. This allows the sperm to more easily swim in the vagina, but it also means that semen can be super tricky to clean up because it can spread everywhere if not contained and cleaned up fast. It also results in semen trickling out of bodily orifices long after intercourse.

Other Semen Facts

Semen is a really weird substance with over 100 ingredients in it. I’m not going to bother to list all of them, because that would be really boring and also take way too much time, but the all-knowing Guide to Getting it On has opened my eyes to some of the more interesting chemicals it contains. For example: prostaglandins. Prostaglandins cause smooth-muscle contractions. They’re the compounds that help out with labor and birth, cause the pain associated with headaches, and also the smooth-muscle contractions associated with orgasm (side note: acetaminophen is a prostaglandin inhibitor, which is why it helps with headaches, but it could also potentially make it difficult to achieve orgasm. Avoid taking painkillers before having sex). Their presence in semen might help sperm exit the body, or help push them along inside the vagina. But, if swallowed, prostaglandins might also cause stomach contractions. Which might be why some people report getting stomach aches after swallowing semen.

Other cool things: sperm survive best in a alkaline (basic) pH, so semen usually has a pH between 7 and 8. However, vaginas have a pH of about 4.5 or lower (this helps deter unwanted bacteria from living in them). Thus, semen has a fair amount of citrate ions in it, which act as a buffer (a buffer helps a liquid maintain its natural pH). As it turns out, citrate ions can form calcium citrate, which tastes salty and sour. Hence the salty-sour taste of semen. Furthermore, semen has a lot of zinc in it. Zinc has a very strong and unpleasant taste, unless you have a zinc deficiency, at which point it has no taste at all. So you might be able to tell whether or not you (or your partner(s)) has a zinc deficiency depending on how their semen tastes to you.

And with that last fun fact, we’re suddenly done with human anatomy, both internal and external. From here on out, I’ll be leaving the comforting world of facts and science for the less stable fields of philosophy and sociology. It’s going to be fun! As always, please email me with questions or comments at, and don’t forget to comment below if you have things to add!


Ashford, Molika. “What Does the Prostate Gland Do?Live Science. August 9, 2010.

Glickman, Charlie, PhD and Aislinn Emirzian. The Ultimate Guide to Prostate Pleasure. Berkeley: Cleis Press, 2013.

Harmon, Katherine. “Sperm Cells’ Swimming Secrets Revealed.” Scientific American Blog. February 4, 2010.

Joannides, Paul. Guide to Getting It On. Oregon: Goofy Foot Press, 2009.

Roach, Mary. Bonk: The Curious Coupling of Science and Sex. New York: W. W. Norton & Co., 2008.

Wikipedia: Prostate, Cowper’s Gland, Seminal Vesicle, SemenEjaculation


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