The prostate is a small gland that produces most of the fluid in semen, helps control urine flow, and plays an active role during ejaculation. It sits just below the bladder, in front of the rectum, and wraps around the urethra, the tube that carries both urine and semen out of the body. In a healthy adult, it weighs about 25 grams, roughly the size of a walnut.
Producing the Fluid That Keeps Sperm Alive
The prostate’s primary job is manufacturing a nutrient-rich fluid that mixes with sperm during ejaculation. This fluid makes up a significant portion of semen and contains an unusual cocktail of substances found at far higher concentrations than anywhere else in the body: citrate, zinc, calcium, and magnesium. The prostate pumps out citrate at concentrations above 100 millimolar, hundreds of times higher than in typical human cells. That citrate serves as a fuel source, giving sperm the energy they need to survive after leaving the body.
Zinc is another standout. The prostate concentrates zinc at levels high enough to act as an antimicrobial agent, helping protect the reproductive tract from bacterial infections. Men with chronic prostatitis (ongoing prostate inflammation) consistently show depleted zinc in their prostatic fluid, which likely makes the gland more vulnerable to recurring infections. Interestingly, taking zinc supplements by mouth raises blood levels but fails to restore the depleted zinc inside the prostate itself, suggesting the gland has its own independent absorption system.
Controlling the Flow of Urine
Because the prostate wraps completely around the urethra, it has a direct mechanical effect on urination. The gland contains smooth muscle fibers that maintain a baseline level of tension, or “tone,” around the urethra. This helps regulate how urine passes from the bladder. When everything is working normally, you don’t notice the prostate’s role at all.
Problems show up when the prostate grows. An enlarged prostate squeezes the urethra from the outside, creating resistance that makes it harder to start urinating, weakens the stream, or causes the feeling that your bladder hasn’t fully emptied. This obstruction comes from two sources at once: the physical bulk of the growing tissue and increased contraction of the smooth muscle within it. That’s why one of the most common treatments for urinary symptoms works by relaxing those muscle fibers, easing the squeeze on the urethra without shrinking the gland.
Its Role During Ejaculation
The prostate isn’t passive during sex. It plays a coordinated, mechanical role in ejaculation that involves precise timing. During the first phase, called emission, semen collects in a small chamber near a structure called the verumontanum, a tiny mound inside the urethra at the level of the prostate. The prostate and surrounding muscles dilate to create space for this fluid to accumulate.
During the second phase, expulsion, the muscles around this chamber contract in a coordinated sequence. The verumontanum tilts forward, which directs semen outward rather than backward toward the bladder. Smooth muscle in the urethra closes off the rear of the chamber at the same time. This tilt-and-squeeze mechanism is what ensures semen travels in the right direction. Damage to this system, from surgery or nerve injury, can cause retrograde ejaculation, where semen flows backward into the bladder instead.
How Testosterone Drives the Prostate
The prostate depends on male hormones to function. Testosterone, produced mainly in the testes, travels to the prostate and gets converted into a more potent form called DHT (dihydrotestosterone). DHT binds to receptors inside prostate cells and drives both the gland’s secretory activity and its growth. Think of DHT as an amplifier: it takes testosterone’s signal and makes it stronger at the tissue level.
This hormonal dependence is a double-edged sword. DHT is essential for the prostate to develop normally during puberty, but in adulthood, it fuels ongoing growth that the body doesn’t need. The prostate never stops growing in response to DHT, which is the root cause of age-related enlargement. Medications that block the conversion of testosterone to DHT can shrink the prostate over time, confirming just how central this hormone is to the gland’s behavior.
How the Prostate Changes With Age
A walnut-sized gland at age 25 can grow to the size of a lemon by your 60s or 70s. This gradual enlargement, called benign prostatic hyperplasia (BPH), is nearly universal in men who live long enough. By age 60, half of all men have a measurably enlarged prostate. By 85, that number reaches 90%.
Not everyone with an enlarged prostate has symptoms. The gland can double in size without causing noticeable problems, depending on which direction it grows and how much it compresses the urethra. When symptoms do appear, they tend to develop slowly: getting up more often at night to urinate, a hesitant or weak stream, dribbling afterward, or a persistent sense that the bladder isn’t empty. These changes are driven by the combination of tissue growth and increased smooth muscle tone described above, both of which narrow the urethral channel.
Prostate inflammation can also affect the gland’s ability to do its job. When the prostate is inflamed, concentrations of citrate, zinc, calcium, and magnesium all drop significantly. This means the protective and nutritive qualities of prostatic fluid decline, which can affect both fertility and resistance to infection. The degree of decline varies widely between individuals, but the pattern is consistent: inflammation disrupts the prostate’s core secretory function.