The prostate is a small gland about the size of a walnut. The prostate gets bigger as men get older. It is divided into 2 lobes and has an outer layer called the capsule.
The prostate is below the bladder surrounding the first part of a tube called the urethra. The urethra carries pee (urine) from the bladder to the penis. The same tube also carries semen, which is the fluid containing sperm. Just behind the prostate is the back passage (rectum). There are also some lymph nodes (sometimes called glands) near the prostate.
The prostate contains muscle tissue and glandular tissue. Glandular tissue is tissue that releases (secretes) certain substances.
The prostate produces a fluid that mixes with sperm (from the testicles) to make semen. The fluid is kept in a tube-shaped gland that sits behind the bladder. This gland is called the seminal vesicle. During sex, the muscle tissue helps force (ejaculate) prostate fluid and sperm into the urethra.
The male sex hormone testosterone (made in the testicles) controls how the prostate works. Testosterone is responsible for things like your sex drive, getting an erection, and muscle development.
The prostate also produces a protein called prostate-specific antigen (PSA). This helps to make semen more watery. A blood test can measure PSA. This is called a PSA test. Doctors use it to help diagnose different prostate problems, including cancer.