The vas deferens are an important part of the male anatomy. Each male has two. The vas deferens are thick and muscular tubes that carry the sperm from the epididymis (another tube that transports sperm) to the urethra, which transports the sperm out of the penis. During ejaculation, the vas deferens contract, pushing sperm into the urethra where it mixes with other fluids to form semen.

The severing of the vas deferens is referred to as a vasectomy. This is a technique of permanent male sterilization. To understand a vasectomy, you must first understand some basic male anatomy. The sperm are made in the testes (testicles). They then travel through the testis and are collected in a structure outside the testis called the epididymis. The epididymis is a thin-walled tube that runs down the center of the back of the testis. As it gets closer to the bottom of the testis, the epididymis becomes thicker and straighter. Eventually, when it reaches the scrotum, it straightens out completely and becomes quite thick and muscular. This muscular tube is called the vas deferens, and it carries the sperm in a very small amount of fluid.

When a physician performs a vasectomy, he is interrupting the flow of sperm by cutting the vas deferens, the tube that carries the sperm. The inside of the tube is sealed and the two ends are blocked off. A few doctors may specifically damage the epididymis, but this is unnecessary and inadvisable as it increases the chances of inflammation and makes surgical reversal more difficult.

When a man ejaculates, he pushes out fluid containing mature sperm, but only 5% of the ejaculated fluid actually comes from the testicles. Most of the fluid (semen) comes from organs other than the testicles (the prostate, seminal vesicles, and bulbourethral glands). These organs do not make sperm. This is why men who have had a vasectomy still have normal ejaculation with significant fluid: only the 5% of the fluid made in the testicles, and the sperm, are missing.