The seminal vesicle is a pair of sac-like glands that are part of the male reproductive system. They are located behind the bladder but in front of the rectum. The seminal vesicles produce and store the majority of the fluid that makes up semen. The fluid that they secrete has a number of properties and components that are important for semen function and sperm survival. The fluids that develop in the seminal vesicles make up an estimated 50% to 80% of semen’s volume. The substances in these fluids are designed to help sperm stay alive once they enter the vagina. The seminal vesicles are involved in fertility and play an important role in the reproductive process. During ejaculation, the smooth muscle layer of the seminal vesicles contracts, releasing the seminal vesicle fluid into the ejaculatory duct. The muscular tissue contracts to move seminal fluid and sperm into the urethra and out through the penis. The seminal vesicles work together with other parts of the reproductive system, such as the vas deferens and its ampulla (sperm repository), to push out ejaculate into and from the penis.