10 Facts About Testicles Everyone Needs to Know

Have you ever wondered why it hurts to get kicked in the balls? Why does one testicle hang lower than the other? What does it mean if your balls feel full? And are "blue balls" actually real?

If you're curious about these testicle mysteries, there's a medical answer for most of them. We spoke to a healthcare provider specializing in sexual health to give us the facts on all things testicles. These testicle facts will surprise you — even if you don't have a pair. 

Testicles Are Sperm Factories

The testicles, also called the testes, produce about 1,500 sperm every second, Michael Reitano, MD, a physician-in-residence at the men's health company Roman, told Health. That's about 90,000 sperm every minute, 5.4 million every hour, and 130 million every day. The testicles constantly create all these sperm through a process called spermatogenesis. Your testes also help produce the hormone testosterone, which is needed to make sperm in the first place. Additionally, this sex hormone is responsible for muscle development, body hair, and deepening the voice.

But each person is different, and sperm count does vary. The average ejaculation (which is about one-half to one teaspoon of semen) contains anywhere from 40 to 130 million sperm, said Dr. Reitano. Although it only takes one sperm to fertilize an egg, having plenty of extra sperm helps increase the chances of conception. And if sperm isn't released through ejaculation, it's reabsorbed back into the body.

They're 5 Degrees Colder Than the Rest of the Body

Why are the testicles outside of the body and so vulnerable? Having the testicles hang outside of the body helps ensure a lower temperature. Testicular temperature must be about 5 degrees Fahrenheit (3 degrees Celsius) lower than the rest of the body to properly create and preserve sperm. The scrotum — the wrinkled pouch of skin that holds the testicles — is about 93.4 degrees Fahrenheit (34 degrees Celsius), while the body is typically 98.6 degrees Fahrenheit (37 degrees Celsius).

The scrotal muscles act as temperature control for the testicles by moving them closer to or farther away from the body as the air gets warmer or cooler. For example, when the testicles get too chilly, the scrotum will contract and pull them closer to the body for warmth. And if they can't take the heat, the scrotum will relax and let the testes hang low.

One Testicle Hangs Lower for a Reason

The left testicle usually hangs a little lower than the right one, and it's no accident. "This allows the temperature of one testis to change without that energy being sent to the other testis, as it would if they were adjacent or touching," said Dr. Reitano. This way, the body can increase or decrease one testis's temperature without affecting the other's temperature. 

It's also normal to have one slightly bigger testicle. The right tends to be larger than the left, added Dr. Reitano.

Getting Kicked in the Balls Hurts for a Biological Reason

The testicles are crucial to reproduction if you have a penis, but they're vulnerable because they hang outside of the body. Dr. Reitano explained that the testicles evolved to have a ton of nerves that are crazy sensitive to pain, so people are more likely to protect them.

The muscles in the scrotum will also pull the testes closer to the body if they sense any danger, even if it's not always actual danger. For example, just stroking the inner thigh can trigger this reflex, and emotions related to fight or flight can also create this response. "If a man feels threatened or afraid, the muscles will [contract] without him even being aware of it," said Reitano. 

The Scrotum Is Very Sexually Sensitive

We know the testicles are incredibly sensitive to pain, but all those nerve endings in the testicles can also lead to pleasure. The scrotal sack around the testes is full of nerve endings and is considered an erogenous zone.

In terms of just how sexually sensitive the scrotum is, Dr. Reitano said scrotum sensitivity is comparable to the vulva and labia, the outer folds of skin around the vagina. "We can estimate that the sensations are fairly similar in that we know the nerve distribution is almost identical," added Dr. Reitano. These nerves make the vulva area sensitive to touch and arousal, but they're not an orgasm hot spot like the clitoris. So scrotum play alone probably won't make you orgasm either, but it'll still feel pleasurable.

Tight-Fitting Underwear Is Bad for Sperm

A study found that people with testes who primarily wore boxer shorts averaged 25% higher sperm concentration and 17% higher total sperm count compared to men who wore other types of underwear, like jockeys or briefs.

Dr. Reitano said he recommends loose-fitting underwear (e.g., boxers) because it gives the testicles enough space to regulate temperature. If the testicles get too warm, they will decrease sperm count and quality. Having more room in boxers allows the scrotum to move away from the body to regulate temperature, rather than trapping the scrotum against the body in tight-fitting briefs. 

But tight undies aren't the only thing that can heat up the testes and harm sperm. Anything that turns up the heat, like regularly sitting in a hot tub for long periods of time, may also decrease sperm count and quality.

Testicular Cancer Is Often Caught Late

Signs of testicular cancer include achiness or pain in your back, groin, belly, or scrotum; a heavy feeling in the scrotum; a change in the size of one or both testicles; or a lump on the testicles. It's also the most common cancer in 20- to 35-year-old men.

Unfortunately, people often catch testicular tumors late because they don't notice symptoms. This is because testicular tumors can grow without compressing any other organs or bones nearby, making them relatively painless, said Dr. Reitano. The skin of the scrotum is also so loose that a testicular tumor can grow without constraint. 

Luckily, testicular cancer is highly curable, but self-examinations are still important. To complete a testicular self-exam, examine one testicle at a time after you jump out of the shower — this is when the scrotum's skin is more relaxed. Use both hands to gently roll one testicle between the thumb and fingers. Normal testicles should feel symmetrical and round, and you may notice a thin, coiled tube at the back of each testicle. If you notice any hard lumps or bumps — or changes in the size or shape of your testicles — contact your healthcare provider to get checked out.

One Testicle Sometimes Hides in the Body

When you're developing in the womb, the testicles typically drop from the abdomen to the scrotum. But sometimes, a baby is born with one or both testicles still in their abdomen, known as undescended testicles. Risk factors linked to a hidden testicle include premature birth, low birth weight, family history of undescended testicles, and alcohol or cigarette use by the birth parent during pregnancy.

Sometimes undescended testicles will drop on their own in the first few months of life. But surgery is usually needed if the testicles haven't dropped by the six-month mark. Though surgery can resolve the problem early, having an undescended testicle at birth still increases your risk of health complications later in life. Adults born with undescended testicles are 40 times more likely to develop testicular cancer, said Dr. Reitano. They're also more likely to deal with low sperm count and poor sperm quality.

The Testicles Can Twist and Cause a Medical Emergency

Severe scrotal pain, scrotal swelling, nausea, vomiting, a hard testis, and a high-riding testis are symptoms of an emergency medical issue called testicular torsion. If you have these symptoms, you'll want to head to the emergency room ASAP.

Testicular torsion is when a testicle twists and rotates the spermatic cord that moves blood to your testicle. This twisting action cuts off blood flow to the testis, resulting in pain and dying testicle tissue. If the testicle isn't untwisted with surgery, usually within 8 hours of symptoms, you risk losing your testicle. Other complications can include infection, infertility, a cosmetically deformed testicle, and testicular atrophy. Testicular torsion is rare and is more common in people under 25. It is usually caused by an abnormality in the pouch that covers the testis, and testicular torsion may happen randomly, after a lot of exertion, or trauma to the testicle.

Balls Don't Actually Turn Blue — But They Can Double in Size

If your balls feel full and appear larger, it's usually because you're aroused. But if you're aroused and don't get any release via an orgasm, you may also experience an uncomfortable aching feeling in the testicles, known as "blue balls." However, despite the name, your testicles don't actually turn blue. At most, you might notice a slightly blue tint because there's more blood volume.

Blue balls, known medically as epididymal hypertension (EH), happen when the blood vessels to the penis and testicles expand to make room for increased blood flow, said Dr. Reitano. This is how the penis becomes erect for sex, and you may also notice the testicles almost double in size. This increased blood flow subsides after an orgasm. But if an orgasm doesn't happen, you may experience some mild discomfort. Blue balls isn't dangerous, and you don't have to have partnered sex to relieve the symptoms. Waiting it out or masturbating can also make the discomfort go away.