The vas deferens is the set of tubes that's responsible for transporting sperm cells from the epididymis, where sperm is matured and stored, to the ejaculatory duct, so it can exit the body. This is the tube that's cut during a vasectomy, as a form of birth control. Each tube is about 11–17 inches long.
Also known as the ductus deferens, the vas deferens is a fibromuscular tube in the male reproductive system that mounts the posterior border of each testis. Measuring around 30–40 cm in length, it carries mature sperm from the epididymis, which stores sperm cells, to the urethra in preparation for ejaculation by way of muscular contractions. There are three layers to the vas deferens, with the outermost layer being made of connective tissue, the second layer made of muscle fibers, and the innermost mucus layer.
Without the vas deferens, sperm would not be able to make their way from the testicles to the urethra on their own. When the penis becomes engorged with blood and erect, sperm cells are prompted to evacuate the epididymis and move into the vas deferens, which acts as a canal.
The vas deferens then propels sperm toward the ampulla, an enlarged reservoir for sperm, where it mixes with secretions, such as proteins and prostaglandins, from the seminal vesicle. The sperm, now part of the semen, is then projected through the ejaculatory ducts and out through the urethra.
During a vasectomy, each vas deferens tube is blocked or cut to keep sperm from joining with semen, rendering a patient infertile. While the testes will still produce sperm, they will be reabsorbed by the body, instead of expelled.
In some individuals, the vas deferens does not develop properly, resulting in a condition called congenital absence of the vas deferens. Though the testes may develop and function normally, sperm cannot travel through the vas deferens to join with semen. This condition causes infertility unless the individual uses assisted reproductive technologies like testicular sperm extraction combined with intracytoplasmic sperm injection. Though congenital bilateral absence of the vas deferens may occur without a known cause, it’s also a symptom of cystic fibrosis or being a CF carrier.
Another issue affecting the vas deferens is obstructive azoospermia, in which either the vas deferens, the epididymis, or the ejaculatory ducts are blocked. This obstruction prevents sperm from leaving the body, and can be caused by trauma or injury.
In some cases, inflammation or scar tissue from sexually transmitted infections or severe urinary tract infections can cause damage to the vas deferens, also blocking sperm from leaving the body.