Semen is produced by several glands working together across the male reproductive system. Sperm cells themselves come from the testes, but they make up only a small fraction of the fluid. The bulk of semen, roughly 50% to 80% of its volume, comes from the seminal vesicles, with the prostate gland and other smaller glands contributing the rest. A normal ejaculation produces at least 1.5 milliliters of fluid, though the average is closer to 2 to 3 milliliters.
Where Sperm Cells Are Made
Sperm production begins in the testes at puberty and continues throughout life. Inside each testicle, tightly coiled tubes called seminiferous tubules serve as the production line. The process has three main phases: cells multiply, then divide in a way that halves their genetic material, and finally reshape into the streamlined form most people picture, complete with a head and tail.
This initial production takes about 30 to 40 days. Newly formed sperm then travel into the epididymis, a coiled tube sitting behind each testicle, where they finish maturing and gain the ability to swim. The full cycle from the earliest cell stage to ejaculation-ready sperm takes roughly 42 to 76 days, with older estimates placing it at 60 to 70 days. This is why lifestyle changes aimed at improving sperm quality take two to three months to show results.
The Seminal Vesicles: Most of the Volume
Two small glands tucked behind the bladder, the seminal vesicles, produce the majority of semen’s liquid. Their fluid is rich in fructose, a sugar that serves as the primary energy source for sperm. It also contains prostaglandins, compounds that help prevent the vaginal immune system from attacking sperm as a foreign invader, along with alkaline substances and clotting factors that extend sperm survival.
This fluid mixes with sperm during ejaculation as it passes through the ejaculatory ducts. Without it, sperm would lack both the fuel and the protective environment they need to reach an egg.
What the Prostate Adds
The prostate gland, a walnut-sized organ below the bladder, contributes additional fluid containing enzymes, zinc, and citric acid. These components nourish sperm and lubricate the urethra. The prostate also plays a mechanical role: muscles within the gland help push semen forward through the urethra during orgasm.
Pre-ejaculate and the Cowper’s Glands
Before ejaculation, two pea-sized glands below the prostate, called the Cowper’s glands (or bulbourethral glands), release a small amount of thick, clear mucus. This pre-ejaculate has a specific job: neutralizing traces of acidic urine left in the urethra so sperm have a safer passage. It also provides some lubrication.
The Route From Production to Ejaculation
When ejaculation begins, sperm are forcefully expelled from the tail of the epididymis into the vas deferens, a muscular tube on each side. These tubes travel upward through the spermatic cord, loop over the bladder, and join with the ducts from the seminal vesicles to form the ejaculatory ducts. Those ducts pass through the prostate and empty into the urethra. Rhythmic muscle contractions propel the fully mixed semen outward.
This means the final product is assembled on the spot. Sperm from the testes, fluid from the seminal vesicles, secretions from the prostate, and the lubricating mucus from the Cowper’s glands all converge in a matter of seconds.
What Normal Semen Looks Like by the Numbers
A healthy ejaculate has at least 15 million sperm per milliliter. Below that threshold, fertility drops because fewer sperm are available to reach and fertilize an egg. The fluid itself is slightly alkaline, with a normal pH between 7.2 and 8.0. That alkalinity is critical: the vaginal environment is acidic, and without the buffering provided by seminal fluid, most sperm would die before getting anywhere. Research shows that semen pH is directly correlated with both sperm count and motility, meaning the chemistry of the fluid matters just as much as the sperm it carries.
How Age Affects Production
Men continue producing sperm throughout their lives, but both the quantity and quality decline with age. Daily sperm production drops more than 30% in men over 50. After 45, semen volume gradually decreases as the seminal vesicles and prostate become less active. In one study, men in the 45 to 48 age group averaged about 2.8 milliliters per ejaculation, while those between 56 and 80 averaged 1.95 milliliters.
The sperm themselves change too. After 40, the percentage of normally shaped sperm begins to fall, with one study finding 40% fewer normal forms in men over 50 compared to men in their early twenties. Tail defects increase by about 27% in the older group. Motility, how well sperm swim, declines by roughly 0.8% per year, and progressive motility (forward movement) drops even faster. These changes don’t mean older men can’t father children, but they do explain why conception often takes longer with paternal age.