Reproductive System

The vas deferens carries concentrated sperm from the scrotum into the abdominal cavity to the ejaculatory duct. Sperm that remain in the ejaculatory duct longer than a couple of weeks degenerate and are disposed of. The prostate surrounds the ejaculatory duct and contains a sphincter that closes off the bladder during ejaculation. Seminal fluid from the seminal vesicles, the prostate, and the bulbo-urethral glands (or Cowper's glands) is added to the sperm. The seminal fluid plus the sperm is called semen.

Seminal fluid is designed to carry and nourish sperm. Seminal vesicles are located on either side of the bladder and contribute about 60% of the fluid. Seminal vesicle fluid is rich in essential sperm nutrients such as fructose that sustains sperm for up to 72 hours after ejaculation. Seminal vesicle fluid also supplies prostaglandins that cause uterine contractions in the female reproductive tract to facilitate sperm movement to an egg. The prostate gland provides an alkaline mixture of calcium, enzymes, and other components that make up about 30% of the seminal fluid. The alkaline fluid functions to neutralize the acidic vaginal environment which can kill sperm. Additional fluid is provided by the Cowper's glands (below the prostate) which secrete a pre-ejaculatory urethral lubricant that may contain some sperm. For this reason, withdrawal is not a fool-proof contraceptive method. At ejaculation, additional Cowper secretions combine with the remaining seminal fluid and sperm. This semen is sent through the urethra in the penis.