The prostate is a small gland, roughly the size of a walnut and weighing about 25 grams, that sits just below the bladder in men. Its primary job is producing a fluid that nourishes and protects sperm, making up a significant portion of semen. But the prostate does more than that. It also plays a mechanical role during ejaculation and helps regulate urine flow.
Producing the Fluid That Keeps Sperm Alive
The prostate’s most important function is secreting a slightly alkaline fluid that mixes with sperm and fluid from the seminal vesicles to form semen. This prostatic fluid contains a cocktail of components that sperm need to survive and reach an egg. One of the most notable is citric acid, which can reach concentrations up to about 180 millimolar in the prostate. That citric acid serves as an energy source for sperm cells, essentially fuel for their long journey through the reproductive tract.
Prostatic fluid also contains zinc, which is released alongside citrate and influences sperm motility. The enzyme acid phosphatase is another key ingredient, helping to break down certain compounds in the ejaculate. And the fluid is rich in a family of enzymes called kallikreins, the most well-known of which is prostate-specific antigen, or PSA, the same protein measured in PSA blood tests.
How PSA Helps Sperm Move
When semen first leaves the body, it’s in a gel-like state. Sperm are essentially trapped in this thick consistency and can’t swim effectively. PSA solves this problem. Within minutes, PSA breaks down specific gel-forming proteins in semen, a process called liquefaction. As those proteins dissolve, the semen becomes more fluid and sperm are freed to move.
Without this liquefaction step, sperm would remain stuck and unable to travel through the cervix. It’s one of the reasons PSA exists in such high concentrations in prostatic fluid, far higher than the trace amounts that leak into the bloodstream and show up on screening tests.
Protecting Sperm From Vaginal Acidity
The vaginal environment is naturally acidic, with a low pH maintained by bacteria that produce lactic acid through fermentation. This acidity is part of the vagina’s natural defense system against infections. But it’s also hostile to sperm, which don’t survive well in acidic conditions.
Prostatic fluid helps counteract this. Its slightly alkaline pH (semen overall ranges from about 7.2 to 7.8) partially neutralizes the vaginal acidity, creating a window of time in which sperm can survive and swim toward the egg. The prostate contributes alkaline compounds like spermine that raise the pH of the surrounding environment just enough to keep sperm viable.
Its Role During Ejaculation
The prostate isn’t just a fluid factory. It also contains smooth muscle tissue that contracts during ejaculation. These contractions are triggered by the sympathetic nervous system and help propel prostatic fluid into the urethra, where it mixes with sperm arriving from the vas deferens. This is part of the “emission” phase of ejaculation, the moment when the different components of semen are assembled in the urethra just before being expelled.
The prostate’s position surrounding the urethra makes it perfectly placed for this job. The urethra runs directly through the center of the gland, so when the prostate’s smooth muscle squeezes, it pushes fluid directly into the channel where it needs to go.
Controlling Urine Flow
Because the prostate sits right where the bladder meets the urethra, it also helps regulate urination. A small valve exists at the junction of those two structures, and the muscles around it keep the valve closed when urination isn’t appropriate and open it when it is. You can’t consciously control this valve; it operates automatically. The prostate’s position essentially means it acts as a gatekeeper between the bladder and the outside world, and during ejaculation it helps prevent semen from flowing backward into the bladder.
How Hormones Drive the Prostate
The prostate depends on hormones to function. Specifically, it relies on dihydrotestosterone (DHT), a potent form of testosterone. The prostate itself can produce large amounts of DHT locally, and this local production is what stimulates the gland to grow during puberty and to maintain its normal secretory activity throughout adulthood.
DHT is responsible for prostate development from before birth onward. During puberty, rising DHT levels drive the prostate to its adult size and activate its fluid-producing capabilities. In adulthood, continued DHT stimulation keeps the gland functioning, but this same hormonal dependency is also what makes the prostate vulnerable to problems later in life.
How the Prostate Changes With Age
The prostate doesn’t stay walnut-sized forever. In many men, ongoing DHT stimulation causes the gland to gradually enlarge with age, a condition called benign prostatic hyperplasia (BPH). Because the urethra passes through the center of the prostate, even modest growth can start squeezing that tube and restricting urine flow. This is why older men commonly experience symptoms like a weak urine stream, difficulty starting urination, or feeling like the bladder hasn’t fully emptied.
BPH is extremely common. It doesn’t mean anything is wrong with the prostate’s secretory function, but the mechanical consequence of growth in such a tight space creates real problems. An enlarged prostate can grow from its normal walnut size to something closer to a golf ball, progressively narrowing the urethra and making urination increasingly difficult. The gland’s location, so perfectly designed for mixing and propelling semen, becomes a liability when the tissue expands.