The seminal vesicle (Japanese: 精嚢, seinō) is a paired tubular secretory gland situated within the male pelvis, behind the bladder and above the prostate. It produces the bulk of semen (roughly 50-65%) and supplies the nutrients and buffering agents that support sperm activity.

Anatomical position and structure

The seminal vesicles lie posteroinferior to the bladder and anterior to the rectum, as a left-right pair. Each is about 5-7 cm long, with a coiled tubular lumen, and secretes a viscous yellowish-white fluid. The duct of each seminal vesicle joins the vas deferens to form the ejaculatory duct, which passes through the prostate and opens into the urethra.

At ejaculation, the seminal vesicles, vasa deferentia, and prostate contract in coordination to expel semen through the urethra. This sequence of contractions constitutes the physical mechanism of ejaculation.

Components produced by the seminal vesicle

The secretion of the seminal vesicle contains several principal components.

Fructose is the primary energy source for sperm. Sperm derive their motile energy not from blood glucose but from seminal-vesicle fructose. Reduced seminal-vesicle function appears as lowered fructose concentration in semen, leading to reduced sperm motility.

Prostaglandins, hormone-like substances secreted by the seminal vesicles, are thought to promote peristalsis of the female reproductive tract and assist the upward transport of sperm into the uterus.

Semen-coagulating factors are involved in the brief gel-like coagulation of semen immediately after ejaculation. The semen liquefies a few minutes later through enzymes of prostatic origin.

Alkaline buffering agents protect sperm from the acidic environment of the vagina.

Interest in sexual science

The seminal vesicle attracts attention for its direct relationship to sexual pleasure. Seminal-vesicle contraction at ejaculation is a component of the sensation of orgasm, and the degree of fullness (accumulated semen volume) is held by some to influence the intensity of that sensation. Many people report that after a long period of abstinence the seminal vesicles fill and the pleasure of the next ejaculation is heightened.

Seminal vesiculitis (inflammation of the seminal vesicle) can cause pain at ejaculation and haematospermia (blood in the semen), making it a urological concern affecting sexual function.

The region between the prostate and seminal vesicles is also studied in connection with the male “prostate orgasm”, with reports that internal stimulation through the anus can produce an ejaculatory sensation mediated by the prostate and seminal vesicles.

Related Terms

• Testis (seisou)
• Penis
• Testicles (kougan)