The prostate gland secretes fluid that provides nutrients for sperm. WebMD describes the gland as walnut-size and resting directly in front of the colon. The urethra passes straight through the prostate, which allows it to discharge fluid directly into the passing rush of sperm.
Roughly 20 to 30 percent of the fluid in an ejaculation is secreted by the prostate, according to HowStuffWorks. This fluid provides nutrition for the sperm cells, improving their motility. Seminal fluid also serves to lubricate the urethra and ease the passage of sperm through the penis. The environment inside the vagina is generally hostile to sperm. Vaginal secretions are acidic, which can be lethal to sperm and inhibit fertilization. Seminal fluid creates a buffer between sperm and the acidic secretions present inside the vagina and temporarily alters the chemical balance to reduce the potential harm to sperm.
Sometimes, the prostate enlarges and restricts the flow of urine through the urethra. According to HowStuffWorks, when this condition is not caused by prostate cancer, it is known as benign prostatic hyperplasia, or BPH. It is estimated that more than half of all men over age 50 have some form of enlarged prostate, and that the incidence rises to over 80 percent past age 80.