The prostate is a small gland that produces roughly 25% to 30% of the fluid in semen, and its muscles help push that fluid out during ejaculation. 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 the prostate an outsized influence on both reproductive and urinary function, which is why problems with this walnut-sized gland can affect so many aspects of daily life.
Where the Prostate Sits
The prostate is positioned directly below the bladder and in front of the rectum. It surrounds a section of the urethra like a donut, which is why any swelling of the prostate can squeeze the urethra and make urination difficult. In younger men, the gland weighs about 28 grams. By the time a man reaches his 60s, it typically grows to around 35 grams, and in men over 75, it can reach 46 grams or more. That steady growth is the root cause of many prostate-related symptoms later in life.
Making Semen
The prostate’s primary job is producing a portion of seminal fluid. While the seminal vesicles (two small glands behind the bladder) contribute the majority, the prostate adds its own fluid rich in zinc, citrate, and enzymes that sperm need to function. This fluid mixes with sperm from the testes and secretions from other glands to form semen.
One of the most important enzymes the prostate produces is PSA (prostate-specific antigen). When semen first leaves the body, it forms a thick gel. PSA breaks down the proteins responsible for that gel-like consistency, liquefying the semen within minutes so sperm can swim freely. Without this step, sperm would remain trapped and unable to reach an egg.
Protecting Against Infection
Prostate fluid contains unusually high concentrations of zinc, roughly 10 times higher than levels found in most other body fluids. That zinc acts as a natural antibacterial agent. In healthy men, prostatic fluid zinc levels average around 448 micrograms per milliliter. Men with chronic bacterial prostate infections, by contrast, show levels that drop to an average of just 50 micrograms per milliliter. This zinc appears to serve as an in-body defense against bacteria that could otherwise travel up the urethra and cause urinary tract infections.
Pumping Fluid During Ejaculation
The prostate isn’t just a factory for fluid. It’s also a pump. The gland contains smooth muscle fibers that contract rhythmically during orgasm, squeezing prostatic secretions into the urethra in coordinated bursts. Electrophysiology studies have measured these contractions happening at intervals of roughly one second, averaging about four to five bursts per ejaculation. Each burst coincides with a measurable spike in urethral pressure, which is what propels semen outward. These contractions also help ensure that semen moves forward through the urethra rather than backward into the bladder.
Hormones That Drive the Prostate
The prostate depends on testosterone to grow and function. But the hormone doing most of the heavy lifting inside the gland isn’t testosterone itself. An enzyme in prostate tissue converts testosterone into a more potent form called DHT, which reaches concentrations inside the prostate roughly 10 times higher than what’s found in the bloodstream. DHT is what drives the prostate to mature during puberty and maintain its secretory activity throughout adulthood.
This same hormonal mechanism is also behind the prostate’s tendency to enlarge with age. Because the prostate keeps responding to DHT over a lifetime, it gradually grows. When it gets large enough to squeeze the urethra, men experience symptoms like a weak urine stream, frequent urination, or trouble fully emptying the bladder. Medications that block the enzyme converting testosterone to DHT can shrink the prostate and relieve these symptoms by lowering the hormone levels inside the gland.
Why Prostate Size Matters With Age
Prostate growth is nearly universal in aging men. A gland that weighs about 28 grams in a man’s 40s can nearly double to 46 grams by his late 70s. This condition, called benign prostatic hyperplasia (BPH), isn’t cancer, but it can significantly affect quality of life. Because the prostate encircles the urethra, even modest enlargement can create noticeable urinary symptoms: getting up multiple times at night, straining to start urination, or feeling like the bladder never fully empties.
PSA and Prostate Screening
PSA, the same enzyme the prostate makes to liquefy semen, also leaks into the bloodstream in small amounts. A blood test can measure those levels. Elevated PSA can signal several things: an enlarged prostate, an infection, or potentially prostate cancer. The U.S. Preventive Services Task Force recommends that men aged 55 to 69 make an individual decision about PSA screening after discussing the trade-offs with a clinician. For men 70 and older, routine screening is generally not recommended because the harms of overdiagnosis and unnecessary treatment tend to outweigh the benefits at that age.
Black men and men with a family history of prostate cancer face higher risk and may want to begin these conversations earlier than 55, though no specific earlier screening age has been formally established. The key factor is understanding personal risk well enough to make an informed choice.