The seminal vesicles, also known as seminal glands, are part of the male reproductive system and are located behind the bladder but in front of the rectum. They produce most of the fluid that makes up semen, which is ejaculated during the sexual response. 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 fluids carry fructose, which provides energy, prostaglandins, which keep the vagina from seeing sperm as an invader, alkaline fluid, which helps keep the sperm alive in the vagina, which is acidic, and clotting factors, which also help to keep sperm alive longer. The seminal vesicles contain muscle tissue that contracts during ejaculation to push the fluid through the reproductive tract, where it mixes with sperm from the testicle and additional fluids from the prostate. The seminal vesicles work together with other parts of the reproductive system, such as the vas deferens and its ampulla, to push out ejaculate into and from the penis.