Everything You Wanted to Know About Semen

What exactly is the fluid that men’s little swimmers are, well, swimming in?

Remember this diagram of the male reproductive system you last saw in high school biology class? Those are going to come in handy. While we tend to think of the testicles as the main heroes of the male reproductive show (or at least the best sidekicks to the penis), seminal fluid—the technical term for the stuff his sperm is swimming in—actually doesn’t come from the testicles at all. That’s why guys can still ejaculate even if they get a vasectomy.

Actually, semen is produced in the seminal vesicles (hence the word “seminal”) and the prostate gland. The seminal vesicles are located behind the bladder. During sex, the seminal vesicles work hard to push their fluid out into the urethra. But this fluid is behind the sperm, which is produced by the testicles, in a kind of penis traffic jam. This is why, when men ejaculate, that first spurt usually contains most of the sperm.

Ejaculation itself is a powerful and well-choreographed dance. At a man’s climax, the seminal vesicles contract violently, sending the ejaculatory fluid into the urethra. At the same time, the bladder’s internal sphincter, the muscles responsible for holding in pee, clamps down to prevent semen from backing up into the bladder, and to prevent pee from leaking out. Meanwhile, the external sphincter, the muscles that control the doorway to the penis, opens up to allow the seminal fluid to enter the bulbous urethra, which is a kind of reservoir at the base of the penis.

In a final burst, the super powerful muscles around the bulbous urethra contract. Like squeezing water out of a baster, this contraction makes the semen squirt out of the urethra. Amazingly, semen leaves the penis at a speed of around 30 miles per hour—that’s faster than Usain Bolt! Men feel this complex series of muscle contractions as orgasm. After all that work, it’s no wonder men tend to doze off after sex!