What Is the Prostate? Location, Function, Size, Fluids, and Age Changes

The prostate is a small gland found in men. It sits deep in the pelvis, below the bladder and in front of the rectum. Its main job is to make fluid that mixes with sperm to form semen. This fluid helps protect sperm and helps them move, making the prostate important for reproduction.

What the Prostate Does in the Male Body

A side-by-side medical illustration showing where the prostate is located in the male body. The left panel shows a front view of the pelvis with the bladder and prostate labeled. The right panel shows a side view of the pelvic anatomy with clear labels including the bladder, pubic bone, prostate, urethra (urine tube), rectum, seminal vesicle (semen fluid gland), vas deferens (sperm tube), and testicle (sperm maker).
How the prostate gland fits into your body: The front view on the left highlights the prostate sitting deep in the lower pelvis directly underneath the bladder. The side view on the right shows how the gland surrounds the urethra (urine tube) and sits tightly packed between the bladder, pubic bone, and rectum.

The prostate has one main role in the male body. It helps make semen, the fluid that carries sperm out of the body.

Sperm cells are made in the testicles. However, sperm need fluid to travel and survive. The prostate makes part of this fluid. The fluid helps support sperm and helps them move.

The prostate also contains small muscle fibers. During ejaculation, these muscles squeeze. This helps push semen into the urethra and out through the penis. At the same time, muscles near the bladder neck close, helping stop semen from going backward into the bladder and helping prevent urine from mixing with semen.

Where the Prostate Sits in the Pelvis?

The location of the prostate explains why prostate changes can affect urination. The gland sits in a tight space in the lower pelvis, close to the bladder, urethra, rectum, and penis.

Prostate Between the Bladder and Penis

The prostate is located directly below the bladder neck. The bladder neck is the opening where urine leaves the bladder. From there, the prostate sits above the base of the penis.

A normal young adult prostate is often about the size of a walnut. It is usually about 3 centimeters (1.2 inches) long, 4 centimeters (1.6 inches) wide, and 2 centimeters (0.8 inches) thick. It weighs about 20 to 30 grams (0.7 to 1 ounce), though size can vary.

Because the prostate sits just below the bladder, changes in its size or shape can affect urine flow.

Urethra Passing Through the Prostate

The urethra is the tube that carries urine from the bladder out through the penis. It also carries semen during ejaculation.

The urethra passes through the center of the prostate. Because the prostate surrounds this tube, it can affect urine flow if it grows larger or becomes swollen. When the prostate is normal in size, urine can usually pass through easily. When the prostate enlarges, it can press inward on the urethra and narrow the passage.

Rectum Behind the Prostate and the Prostate Exam

The rectum is the final part of the large intestine. It sits directly behind the prostate, with only a thin layer of tissue between them.

Because of this close position, a doctor can feel part of the prostate through the rectum during a digital rectal exam. This exam can help check the size, shape, firmness, and tenderness of the prostate. It may also help find a hard area or lump. However, it cannot check every part of the prostate and cannot rule out all prostate problems by itself.

What the Prostate Adds to Semen?

Semen is not made by one organ alone. It is a mixture of fluids made by several parts of the male reproductive system. The prostate plays an important role in this mixture.

Prostate Fluid and Sperm Movement

The prostate makes about 20% to 30% of the fluid in semen. This fluid is thin and milky. It contains substances that help sperm move and survive.

Prostate fluid is slightly alkaline. This helps protect sperm from the acidic environment in the vagina. The fluid also contains prostate-specific antigen, or PSA. PSA helps thin semen after ejaculation so sperm can move more freely.

Seminal Vesicles Working with the Prostate

The prostate works closely with two small glands called the seminal vesicles. These glands sit behind the bladder and above the prostate.

The seminal vesicles make most of the fluid in semen, often about 60% to 70%. Their fluid is thicker and contains fructose, a sugar that helps provide energy for sperm.

During ejaculation, fluid from the seminal vesicles, prostate fluid, and sperm mix together in the urethra. This mixture forms semen.

Ejaculation and the Prostate Gland

Ejaculation is a coordinated process involving nerves, muscles, sperm, and reproductive fluids.

During ejaculation, the prostate muscles squeeze. This pushes prostate fluid into the urethra. Fluid from the seminal vesicles also enters the urethra, and sperm from the reproductive tubes join the mixture. The semen is then pushed out through the penis.

At the same time, muscles near the bladder neck close. This helps keep semen moving forward and helps stop semen from entering the bladder.

How Does the Prostate Change with Age?

The prostate changes throughout life. It is very small in childhood, grows during puberty, and may enlarge again later in life.

Small Prostate in Young Adult Men

During childhood, the prostate is very small. At puberty, testosterone rises and the prostate grows.

By early adulthood, the prostate usually reaches its adult size. In many men, it weighs about 20 to 30 grams (0.7 to 1 ounce), about the size of a walnut. During the 20s and 30s, the prostate often stays fairly stable.

Gradual Prostate Growth in Middle Age

In many men, the prostate begins to grow again around middle age. This growth usually happens in the inner part of the prostate, close to the urethra.

This age-related growth is usually non-cancerous. It is called benign prostatic hyperplasia, or BPH. The prostate may slowly grow from walnut size to a larger size, such as 40 grams (1.4 ounces), 80 grams (2.8 ounces), or more.

Why Age-Related Growth Can Affect Urination

As the inner part of the prostate grows, it can press against the urethra. This narrows the urine passage.

When the urethra becomes narrower, the bladder has to work harder to push urine out. Over time, this can lead to a weak stream, slower flow, frequent urination, or waking at night to urinate.

Life Stage Average Weight & Size What Is Happening Urinary Impact
Young Adults (Ages 18-35) 20 grams (0.7 ounces) / Walnut size Gland reaches its stable adult size. None. Urine flows freely.
Middle Age (Ages 40-60) 30 to 40 grams (1.1 to 1.4 ounces) / Golf ball size Inner tissue slowly begins a second growth phase. Mild changes, like waking up once at night.
Older Adults (Ages 70+) 60 to 80+ grams (2.1 to 2.8+ ounces) / Lemon size Gland keeps growing larger due to normal aging. Weak stream, frequent urination, slower flow.

Common Non-Cancer Prostate Conditions

Because the prostate surrounds the urethra and changes with age, several non-cancer prostate conditions are common. These conditions can affect urination, pain, and comfort.

Benign Enlargement as an Age-Related Prostate Problem

The medical term for a non-cancerous enlarged prostate is benign prostatic hyperplasia, or BPH. Benign means it is not cancer. BPH does not spread to other parts of the body.

BPH is common as men get older. It can cause a weak urine stream, trouble starting urination, a stop-start flow, and frequent urination, especially at night.

Prostate Inflammation as a Pain and Urination Problem

Prostate inflammation is called prostatitis. It can make the prostate swollen, tender, and painful.

Unlike BPH, which is mainly linked with aging, prostatitis can affect men of different ages, including younger adults. Symptoms may include pelvic pain, groin pain, lower back pain, burning during urination, painful ejaculation, frequent urination, or trouble passing urine.

Prostate Infection as a Treatable Medical Condition

Some cases of prostatitis are caused by bacteria. This is called bacterial prostatitis.

A bacterial prostate infection can appear suddenly. It may cause fever, chills, body aches, burning during urination, pelvic pain, and trouble passing urine. Doctors usually treat bacterial prostatitis with prescription antibiotics. The full course of medicine is important because bacteria can be difficult to clear from prostate tissue.

Written by Chris Morais, MSc, MPhil, PhD — Making complex health information simple.

Sources & Further Reading

  1. National Institute of Diabetes and Digestive and Kidney Diseases. The Prostate and Prostate Problems. Bethesda, MD: National Institutes of Health; 2024. https://www.niddk.nih.gov/health-information/urologic-diseases/prostate-problems
  2. Mayo Clinic. Benign prostatic hyperplasia (BPH): Symptoms & causes. Rochester, MN: Mayo Foundation for Medical Education and Research; 2025. https://www.mayoclinic.org/diseases-conditions/benign-prostatic-hyperplasia/symptoms-causes/syc-20370087
  3. Harvard Health Publishing. Understanding the prostate gland. Boston, MA: Harvard Medical School; 2024. https://www.health.harvard.edu/a-to-z/prostate-cancer-a-to-z

Note: External links were verified when published but may change over time, which is beyond our control.

Disclaimer: This educational content does not constitute medical advice. Always consult a qualified physician or urologist for personal health concerns or diagnostic decisions.

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