The prostate gland produces a nutrient-rich fluid that makes up roughly 20 to 30 percent of semen, nourishing and protecting sperm on their journey to fertilize an egg. About the size of a walnut and weighing around 25 grams in a healthy adult, the prostate sits just below the bladder and wraps around the urethra, giving it a secondary role in both urinary flow and ejaculation.
Producing Fluid That Keeps Sperm Alive
The prostate’s primary job is to secrete a thin, milky fluid that mixes with sperm from the testes and fluid from the seminal vesicles to form semen. This prostatic fluid contains a cocktail of ingredients designed to help sperm survive: citric acid (which sperm use as fuel), zinc, calcium, magnesium, cholesterol, and several enzymes including one called fibrinolysin that keeps semen from clotting too quickly.
One of the most important things prostatic fluid does is help make semen slightly alkaline overall. The vagina is naturally acidic, which would damage or kill sperm within minutes. By contributing alkaline components to semen, the prostate gives sperm a chemical buffer that extends their survival long enough to reach an egg. Without this protection, fertility would drop dramatically.
Pumping Semen During Ejaculation
The prostate is not just a gland. It contains smooth muscle tissue that actively contracts during ejaculation to push its secretions into the urethra, where they combine with sperm. Electrophysiology research has measured these contractions in detail: the prostate squeezes in rhythmic bursts roughly every 1.1 seconds, producing an average of about 4 to 5 ejaculatory spurts per event. Each contraction coincides with a measurable spike in pressure inside the prostatic urethra, which helps propel semen forward and out of the body.
At rest, the prostate generates low-level electrical waves that maintain baseline muscle tone. During ejaculation, those waves spike significantly in both frequency and strength, coordinating the muscular squeeze that makes expulsion possible. This means the prostate plays a direct mechanical role in ejaculation, not just a chemical one.
Fighting Bacteria in the Urinary Tract
The prostate also serves as a frontline defense against infection. Prostatic fluid in healthy men contains remarkably high concentrations of zinc, averaging around 448 micrograms per milliliter, with levels as high as 1,000 in some individuals. Research has identified zinc as the prostate’s primary antibacterial agent. It acts as an in vivo defense mechanism against bacterial invasion of both the prostate itself and the broader urinary tract.
When zinc levels drop, the protection drops with them. Men with chronic bacterial prostatitis (a recurring prostate infection) have prostatic zinc levels averaging just 50 micrograms per milliliter, roughly one-ninth the normal concentration. This strong correlation suggests zinc isn’t just incidentally present; it’s actively keeping bacteria in check.
How the Prostate Affects Urinary Flow
Because the urethra runs directly through the center of the prostate, the gland’s size and muscle tone directly influence how easily urine passes from the bladder. In a healthy, normal-sized prostate, this arrangement causes no problems. But as the prostate grows, which it tends to do with age, it can squeeze the urethra and restrict flow. This is why difficulty urinating, weak stream, and frequent nighttime trips to the bathroom are such common complaints in older men. The prostate’s smooth muscle can also constrict around the urethra, which is why some treatments work by relaxing that muscle tone rather than shrinking the gland itself.
Hormones That Control the Prostate
The prostate depends on hormones called androgens to function and grow. Testosterone is the main circulating androgen, but the prostate converts about 10% of the body’s daily testosterone into a more potent form called DHT using an enzyme in prostate tissue. DHT is the primary driver of prostate growth, starting during fetal development and ramping up again at puberty. Males born with a genetic deficiency that prevents DHT production typically have a small or absent prostate.
In adulthood, the prostate continues producing large amounts of DHT locally. This sustains normal secretory activity but also fuels the gradual enlargement that most men experience after age 50. The same hormonal pathway that makes the prostate functional in youth is what makes it problematic later in life.
PSA: A Protein the Prostate Makes
Prostate-specific antigen, or PSA, is a protein produced almost exclusively by prostate cells. Its biological job is to liquefy semen after ejaculation, allowing sperm to swim freely. But PSA also leaks into the bloodstream in small amounts, which is what makes it useful as a screening marker.
Normal PSA levels in the blood rise gradually with age as the prostate grows:
- Ages 40 to 50: up to 2.5 ng/mL
- Ages 50 to 60: up to 3.5 ng/mL
- Ages 60 to 70: up to 4.5 ng/mL
- Ages 70 to 80: up to 5.5 ng/mL
Levels above these thresholds don’t automatically signal cancer. An enlarged prostate, an infection, or even recent physical activity can push PSA higher. But persistently elevated or rapidly rising PSA often prompts further evaluation.
Current Screening Recommendations
The U.S. Preventive Services Task Force recommends that men aged 55 to 69 make an individual decision about PSA screening after discussing the tradeoffs with a clinician. There is no blanket recommendation to screen everyone in this age group, and screening is specifically recommended against for men 70 and older because the risks of overdiagnosis and unnecessary treatment outweigh the potential benefits at that age.
For Black men and those with a family history of prostate cancer, particularly men with multiple first-degree relatives who had the disease, the task force acknowledges a higher baseline risk and earlier potential onset. While no separate formal recommendation exists for these groups, the guidance encourages them to discuss screening earlier so they can make an informed choice. Digital rectal exams are no longer recommended as a screening tool due to insufficient evidence that they improve outcomes.