The prostate is a small gland that produces most of the fluid in semen, helping sperm survive and reach an egg. About the size of a walnut and weighing around 25 grams, it sits just below the bladder and wraps around the urethra, the tube that carries both urine and semen out of the body. That location gives it a second important job: acting as a traffic controller between the urinary and reproductive systems.
Producing Protective Fluid for Sperm
The prostate’s primary job is manufacturing a thin, milky fluid that makes up roughly 20 to 30 percent of semen volume. This fluid is rich in zinc, citrate, and specialized enzymes that protect sperm once they enter the vagina, where the environment is naturally acidic. Without prostatic fluid buffering that acidity, most sperm would die before they could reach an egg.
The fluid also contains PSA (prostate-specific antigen), an enzyme best known as a blood test marker but one that actually has a hands-on reproductive purpose. After ejaculation, semen initially forms a gel-like clot. PSA breaks down the proteins in that clot, liquefying the semen within minutes so sperm can swim freely. Men with lower PSA activity in their semen tend to have reduced sperm motility, which is one reason the prostate matters for fertility even though it doesn’t produce sperm itself.
Powering Ejaculation
The prostate isn’t just a fluid factory. It’s packed with smooth muscle tissue that actively contracts during ejaculation to push semen out. Electrical recordings of the prostate during ejaculation show rhythmic bursts of muscle activity, with each contraction squeezing prostatic secretions into the urethra. These contractions happen in quick pulses, roughly one per second, producing the spurts that make up an ejaculation (typically four or five per episode). The pressure inside the prostatic urethra rises significantly with each contraction, which is what propels semen forward and out of the body.
Separating Urine From Semen
Because the urethra runs directly through the center of the prostate, the gland plays a key role in making sure urine and semen never mix. During ejaculation, an internal sphincter at the base of the bladder clamps shut under signals from the sympathetic nervous system. This prevents semen from flowing backward into the bladder, a condition called retrograde ejaculation that can happen when the prostate or its nerves are damaged.
During urination, the process reverses. The internal sphincter relaxes, the bladder contracts, and urine flows through. The prostate essentially sits at the crossroads, and the muscular and nerve coordination around it determines which fluid passes through at any given moment. This is also why prostate problems so commonly cause urinary symptoms: when the gland swells, it squeezes the urethra and disrupts that flow.
Converting Testosterone to Its Active Form
The prostate is one of the body’s key sites for converting testosterone into a more potent form called DHT (dihydrotestosterone). An enzyme inside prostate cells handles this conversion, and DHT is roughly five to ten times more powerful than testosterone at activating certain receptors. Before birth, DHT drives the formation of male external genitalia. In adulthood, DHT maintains prostate tissue and influences its growth.
This conversion is a double-edged sword. DHT keeps the prostate functioning, but it also drives the gland’s gradual enlargement with age, a condition called benign prostatic hyperplasia (BPH) that affects most men over 50. It’s also why some treatments for an enlarged prostate work by blocking this testosterone-to-DHT conversion, shrinking the gland by cutting off the hormone signal that fuels its growth.
Defending Against Infection
The prostate’s high zinc concentration isn’t just for sperm protection. Zinc has direct antibacterial properties, and the prostate maintains some of the highest zinc levels of any organ in the body. This helps guard the urinary tract against bacterial infections. When zinc levels in the prostate drop, as they do during chronic prostatitis (long-term prostate inflammation), bacterial infections become harder to clear. Chronic bacterial prostatitis is actually the most common cause of recurring urinary tract infections in men, and depleted prostatic zinc appears to be part of the reason these infections keep coming back.
Animal studies have confirmed that restoring zinc levels in infected prostate tissue inhibits bacterial growth, reinforcing the idea that the prostate serves as an active immune barrier, not just a reproductive organ.