Now that we’ve told you just about everything you need to know about the vagina, we bet you're probably a little curious about the penis and how it works. You might be hoping for a peep at a penis diagram to get answers to questions like: What exactly causes an erection? Are some parts of the penis more sensitive than others? And what is the function of testes, other than just sitting there?
Luckily for you, we’ve got diagrams and penis anatomy facts aplenty to help you demystify the dick. Because as with vagina anatomy, there’s a lot happening under the surface when it comes to the health and sexual functioning of people who have a penis. We’ll start by taking a look at all the different penis parts before getting into the nitty-gritty of how the disco stick works.
In this article, you’ll find:
What Is a Penis, Really?
If you look up penis definitions, you’ll find that most are pretty flaccid. (More on what that particular penis pun means later.) Few definitions tend to say more than the penis is the “male reproductive organ.”
If we’re talking about the penis purely in terms of sex characteristics — and not gender — then, sure, you’re not wrong to call the penis a major piece of the male genital anatomy. But that’s not the same thing as saying that all people with a penis are men or that having a penis is what makes you a man. There are plenty of transgender and intersex people who will, rightfully so, tell you different.
Remember: Sex is assigned to a person at birth based on their primary sex characteristics (or genitalia) and reproductive functions. Gender is often assumed at birth based on sex, but is, in reality, a distinct and socially constructed concept.
- The penis is an organ, not a muscle.
- The penis is made of two different types of sponge-like tissue, the corpus cavernosum and the corpus spongiosum, as well as nerves and blood vessels.
- It’s used for sex by many (but not all) people with a penis and for urination.
In the next section, let’s dig a little deeper into what the various aspects of the penis structure are and how they work.
Penis Diagram and Anatomy Facts
Almost every ding-dong out there will contain the parts shown in this penis diagram, but it’s important to remember that, as with vagina anatomy, the penis comes in different shapes and sizes.
Rainey Horwitz, MD, MS, a urology resident physician at Emory University and a sex educator, points out that even studies meant to determine average penis size — 5.1-5.5 inches, according to a 2020 review¹ of several studies — shouldn’t be taken for undisputed truth because many studies feature participants who voluntarily chose to have their penis size measured. That means they may not actually be representative of all penis sizes; the true average, Dr. Horwitz says, is probably “on the lower end of that range.”
As for penis shapes, some are wider, others are narrower. Some people might also notice a “slight curve or lean” to their penis, Dr. Horwitz says, which is often totally normal and more visible when the penis is erect (or hard). That said, if the curve of a penis worsens or if erections become painful, says Dr. Horwitz, a doctor should be consulted.
Here's another penis fun fact: When a baby is a fetus, all genitals start off looking female. As testosterone starts pumping through male babies, the labia connect; this is the line you see on the bottom of the penis shaft (or the long part of a penis). So, what ultimately becomes a clitoris in female anatomy becomes a penis in male anatomy; they start out the same.
The glans is the bulbous part at the tip of the penis that looks like a hat, called the head. Generally, it’s more sensitive than the penis shaft. In the glans is the opening (or meatus) to the urethra, the tube through which urine and semen come out. Semen contains sperm and can fertilize an egg in a person with a uterus; that’s what makes the penis a reproductive organ.
All people born with a penis are born with foreskin, says Dr. Horwitz, explaining further that it’s “basically extra skin on the shaft of the penis that can cover or protect the glans, or head, when pulled over it.”
When a penis is circumcised, the foreskin is removed, which is done on as many as 80%² of male babies in the United States. Globally, however, just 37-39%³ of penises are circumcised; this is because circumcision is done “for social, cultural, and religious reasons” and “is not medically necessary,” notes Dr. Horwitz.
For people who do have foreskin, they tend to have greater sensitivity in their penis, says Ashley Manta, a sex educator. And having foreskin is not inherently unhygienic: “A lot of people think foreskin is dirty, but that’s not true,” Manta says. “You can keep it clean the same way you would any other part of your body.”
Speaking of sensitivity, Manta says, the frenulum, on the underside of the glans (on a circumcised penis, it’s where the foreskin would have met the head), is the most sensitive part of the penis.
Dr. Horwitz describes it as a “thin bridge of skin” that’s there to “help pull back the foreskin over the head of the penis for those who aren’t circumcised.” Regardless of whether a person is circumcised, though, she agrees that it’s an “especially sensitive erogenous region.”
On a person with a penis, the perineum, or taint, is the area of skin between the scrotum and the anus; in people with a vagina, it’s the area between the vagina and the anus. Says Dr. Horwitz, this is the part of the body where “lots of muscles and fascia making up the pelvic floor come together at this point under the skin,” helping to “provide a framework and keep the right position for a lot of structures that help you pee, poop, and have sex.” It’s also a nerve-filled area that’s sensitive to touch and sexual arousal.
The scrotum is essentially a “loose bag of skin” that holds many other penis parts, Dr. Horwitz says. In the scrotum, you can find the testicles (more on those below); the spermatic cord (which holds the testicles’ blood supply and nerves); and the epididymis, which Dr. Horwitz calls a “small, spaghetti-sized tube” taking sperm from the testicles to the urethra, where it’s then expelled from the body during ejaculation.
Though the scrotum may seem to hang behind the penis for no reason, says Dr. Horwitz, its location within penis anatomy is super intentional. “Sperm requires a temperature less than normal human body temperature to mature correctly,” she explains, “which is why the scrotum holds the testicles outside of the body, to make it easier to keep them at a lower temperature.”
The scrotum also contains a thin muscle that, when a person is cold, contracts in order to pull the testicles closer to the body and warm them, Dr. Horwitz adds. That — combined with the constriction of blood vessels throughout the penis — is why the penis literally shrinks when it’s cold.
As mentioned above, the testicles/testes (or more casually, the balls) are located inside the scrotum. In addition to creating sperm cells — on its own a hugely important job — testes produce hormones, like testosterone, which are released to the rest of the body through the bloodstream.
This makes the function of testes pretty darn essential, which is partly why they’re so jam-packed with sensitive, protective nerve endings. And because the scrotum lacks fat, bones, and significant muscle, the testes — and all of the nerve endings — are especially sensitive to touch and vulnerable to pain. So, no, people with a penis aren’t just “being dramatic” when they complain about the pain of being hit in the balls.
We’ve already covered the different external penis parts. With the prostate — a “walnut-shaped gland that sits under the bladder,” as Dr. Horwitz describes it — we move into the territory of internal penis parts.
The prostate gland, along with the seminal vesicles attached to its back, create a fluid that mixes with sperm that's made in the testicles. Together, that adds up to semen, the “white-gray, thicker fluid” that comes out of the penis tip during ejaculation and is composed of sugar, mucous, vitamins, and protein. The prostate, Dr. Horwitz explains, adds “enzymes, zinc, citric acid, and other nutrients” to semen’s formula, which is designed to “support sperm cells on their long journey” out of the penis through the urethra. (The urethra, by the way, is entirely surrounded by the prostate; think of it “like a fleshy straw running through a small orange,” Dr. Horwitz says.)
The prostate itself is also very sensitive and can be stimulated through the rectum or the internal tube connected to the anus by fingers or sex toys.
How Does the Penis Work?
We’ve talked a lot about how different parts of penis anatomy function, working together to make the penis the reproductive organ it is. But you may still have some questions, such as…
Remember those two sponge-like tissues, or corpora, that make up the interior of the penis? When someone with a penis is sexually stimulated, blood will flow to the penis and fill up those erectile tissues, causing them to expand and become firm — in other words, causing the penis to go from flaccid (soft) to erect (hard). “In a penis, when a person becomes aroused, the corpora fill with blood, filling up most of the shaft,” Dr. Horwitz says, adding that this helps the penis stay firm enough for penetrative sex.
But just because erections happen when you’re turned on doesn’t mean they can’t happen for other reasons too. Erections are “also triggered to occur by the parasympathetic nervous system, or our ‘rest and digest’ nerves,” Dr. Horwitz says. This is why some people have erections, or wet dreams, in their sleep.
Also, importantly, just because someone’s penis is hard doesn’t mean they’re consenting to sexual activity: “It’s also possible to become erect or for a vagina to produce lubrication or ‘wetness’ as an automated response from physically touching genitals, which may or may not be wanted,” Dr. Horwitz explains. “That’s why it’s always so important to get consent before engaging in any sort of touching — the body can respond in ways that don’t agree with someone’s thoughts or desires.”
Finally, erections don’t only happen for people with a penis. “The clitoris also becomes erect, but since most of the organ is hidden inside the body, not many people are aware of this,” Dr. Horwitz says. In people with a vagina, blood similarly rushes to the vagina when that person is turned on, which causes the clitoris to fill and firm up. It may be less noticeable than it is in penises, but it's still an erection!
When someone with a penis has an orgasm, semen comes out of the urethra at the penis tip with an explosive, pumping motion. Although ejaculation most often occurs during sexual climax or orgasm, Dr. Horwitz notes, these are, in reality, “separate but related” things.
“Ejaculation has two parts, emission and expulsion,” she says. “During emission, sperm is carried in the vas deferens from the testicles to the prostate where the fluids mix and are stored at the base of the penis. During the expulsion phase, the muscles at the base of the penis contract about every 0.8 seconds to force semen out of the penis through the meatus, or pee hole, in multiple bursts.”
If semen is a yellowish green color, rather than white-gray, or if it has a putrid smell, that could indicate an infection. People with a penis should do regular checks for STIs, including those that cause sores or lesions on the penis. Many STIs are transferred by skin-to-skin contact as well as by bodily fluids, so if you notice anything off, you or the person you’re with should get checked by a doctor before any contact happens.
Before full ejaculation, there’s also pre-ejaculate, or pre-cum, that can come out well before a person with a penis orgasms (or even without an orgasm at all). Pre-ejaculate can carry STIs as well as get a person with a vagina pregnant⁴. Do protect yourself any time you come in contact with another person’s fluids. Yes, that means all of the condoms, all of the time.
The answer to this one is a little tricky. The penis does not have bones and can’t technically “break” in that sense, but it can sustain some pretty serious injuries.
“Sometimes with especially rough sex or possibly missing the target with a thrust, the thick fascia or membrane that normally keeps all the blood in the erect penis contained in the corpora, or erectile part, can tear,” Dr. Horwitz explains. “That allows blood to leave the corpora and erectile bodies where it belongs and move into the space closer to the skin, which causes really bad bruising and can be very painful.” If this happens, she adds, it’s a medical emergency, and you or the person you’re with should go to the ER.
Penis anatomy is a wonderfully intricate thing. Below, you can find more sexual health tips and advice for people who have a penis.
Keep reading:
Do I have a “normal” penis? Eight myths to stop believing about your penis
How to masturbate if you have a penis: 12 tips and techniques
Sources:
- Average-Size Erect Penis: Fiction, Fact, and the Need for Counseling. Bruce M. King. https://pubmed.ncbi.nlm.nih.gov/32666897/
- Incidence of circumcision among insured adults in the United States. Nabavizadeh et al. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9576047/#:~:text=Circumcision%2C%20the%20surgical%20removal%20of,men%20are%20circumcised%20%5B2%5D.
- Estimation of country-specific and global prevalence of male circumcision - Population Health Metrics. Morris et al. https://pophealthmetrics.biomedcentral.com/articles/10.1186/s12963-016-0073-5
- Can you get pregnant from precum? Likelihood and important factors. https://www.medicalnewstoday.com/articles/325356
This article was first published in 2015 and has been updated to include the most accurate information.
