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Each seminal gland, forms as an outpocketing of the wall of ampulla of each vas deferens and the excretory duct of seminal gland opens into the vas deferens as it enters the prostate gland.
Since the prostate secretes the fluid that carries the sperm in the ejaculate, the ability to successfully reproduce relies on a having a healthy prostate. Just as women have regular cervical exams done to maintain sexual health, men need to have a digital rectal exam done regularly to maintain their sexual health.
Each seminal gland forms as an outpocketing of the wall of ampulla of each vas deferens. They can spread out to approximately five cm, but can contain the full length of seminal vesicle of approximately 10 cm because the vesicle is curled up inside the gland. The excretory duct of the seminal gland opens into the vas deferens as it enters the prostate gland. The seminal vesicles secrete a significant proportion of the fluid that ultimately becomes semen. Lipofuscin granules from dead epithelial cells give the secretion its yellowish color. About 50-70% of the seminal fluid in humans originates from the seminal vesicles, but is not expelled in the first ejaculate fractions which are dominated by spermatozoa and zinc-rich prostatic fluid. Seminal vesicle fluid is alkaline, resulting in human semen having a mildly alkaline pH. The alkalinity of semen helps neutralize the acidity of the vaginal tract, prolonging the lifespan of sperm. Acidic ejaculate (pH <7.2) may be associated with ejaculatory duct obstruction. The vesicle produces a substance that causes the semen to become sticky/jelly-like after ejaculation. This is thought to be useful in keeping the semen near the cervix.
The thick secretions from the seminal vesicles contain proteins, enzymes, fructose, mucus, vitamin C, flavins, phosphorylcholine, and prostaglandins. The high fructose concentrations provide nutrient energy for the spermatozoa when stored in semen in the laboratory. Spermatozoa ejaculated into the vagina are not likely to have contact with the seminal vesicular fluid, but transfer directly from the prostatic fluid into the cervical mucus as the first step on their travel through the female reproductive system. The fluid is expelled under sympathetic contraction of the muscularis muscle coat. In vitro studies have shown that sperm expelled together with seminal vesicular fluid show poor motility and survival, and the sperm chromatin is less protected. Therefore, the exact physiological importance of seminal vesicular fluid is not clear. It has been speculated that it is a developmental rest still seen among some rodents where the last part of the ejaculate form a spermicidal plug which reduces the chances for sperm from a later-arriving male to proceed to the oocyte.
The function of the prostate is to secrete a slightly acidic fluid, milky or white in appearance, that usually constitutes 20–30% of the volume of the semen along with spermatozoa and seminal vesicle fluid. The prostatic fluid is expelled in the first ejaculate fractions, together with most of the spermatozoa. In comparison with the few spermatozoa expelled together with mainly seminal vesicular fluid, those expelled in prostatic fluid have better motility, longer survival and better protection of the genetic material (DNA). The prostate also contains some smooth muscles that help expel semen during ejaculation.
To work properly, the prostate needs male hormones (testosterones), which are produced mainly by the testicles. Some male hormones are produced in small amounts by the adrenal glands. However, it is dihydrotestosterone that regulates the prostate. A healthy human prostate is classically said to be slightly larger than a walnut. The mean weight of the "normal" prostate in adult males is about 11 grams, usually ranging between seven and 16 grams. It surrounds the urethra just below the urinary bladder and can be felt during a rectal exam. It is the only exocrine organ located in the midline in humans and similar animals. Within the prostate, the urethra coming from the bladder is called the prostatic urethra and merges with the two ejaculatory ducts. The prostate is sheathed in the muscles of the pelvic floor, which contract during the ejaculatory process.
Bulbourethral glands are located posterior and lateral to the membranous portion of the urethra at the base of the penis, between the two layers of the fascia of the urogenital diaphragm, in the deep perineal pouch. They are enclosed by transverse fibers of the sphincter urethrae membranaceae muscle. The bulbourethral glands are compound tubulo-alveolar glands, each approximately the size of a pea. They are composed of several lobules held together by a fibrous covering. Each lobule opens into a duct that joins with the ducts of other lobules to form a single excretory duct. This duct is approximately 2.5 cm long and opens into the urethra at the base of the penis. The glands gradually diminish in size with advancing age.
During sexual arousal, each gland produces a clear, salty, viscous secretion known as pre-ejaculate. This fluid helps to lubricate the urethra for spermatozoa to pass through, neutralizes traces of acidic urine in the urethra, and helps flush out any residual urine or foreign matter. It is possible for this fluid to pick up sperm remaining in the urethral bulb from previous ejaculations and carry them out prior to the next ejaculation.
Source: Boundless. “Accessory Sex Glands.” Boundless Anatomy and Physiology. Boundless, 08 Aug. 2016. Retrieved 24 Aug. 2016 from https://www.boundless.com/physiology/textbooks/boundless-anatomy-and-physiology-textbook/the-reproductive-system-27/the-male-reproductive-system-252/accessory-sex-glands-1231-8610/