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Semen or ejaculate is the fluid discharged from the penis during ejaculation, usually at the time of orgasm. Like blood, semen consists of two compartments, the cellular compartment (spermatozoa) and noncellular compartment (seminal plasma). It contains the sperm, which sometimes results in pregnancy following vaginal sex with a female. Semen is a whitish, milky fluid, slightly viscous, containing water and small amounts of salt, protein, and fructose.
Composition
Around 200 million to 500 million spermatozoa (or 'sperm', for short), produced in the testes, are released per ejaculation. However, they make up only about 2–5% of the volume of semen. The bulk of the seminal plasma, the fluid portion of semen, is contributed by the male accessory organs of reproduction. Some 60% of semen is produced by the seminal vesicles, and most of the remainder is generated by the prostate. Prostatic secretions in humans contain high levels of citric acid, acid phosphatase and zinc. A small amount of viscous mucus comes from the bulbourethral glands. Overall, the seminal plasma contains a very complex range of organic and inorganic constituents. These include metal ions, sugars, lipids, steroid hormones, prostaglandins, amino acids and nitrogenous bases. Strongly basic (cationic or positively charged) amines such as putrescine, spermine, spermidine and cadaverine are responsible for the smell in semen. However, an offensive and putrid smell indicates a urinary tract or other urogenital infection. The cationic bases counteract the acidic nature of vaginal secretion, and also buffer the DNA inside the sperm.
Fructose is the main energy source of sperm cells. Some of the other components of semen serve to increase the mobility of sperm cells. Sperm function best in a slightly alkaline environment, whereas the vagina is usually slightly acidic. The fluid excreted by the seminal vesicles is alkaline. Prostaglandins are unsaturated fatty acids used for hormonal actions.
A typical ejaculation can contain up to 5 mg of zinc. Men that ejaculate frequently might become deficient in zinc. Zinc deficiency can also cause reproductive problems and problems with spermatogenesis.
Semen and transmission of disease
Semen is in itself harmless on the skin or if swallowed. However, semen can be the vehicle for many sexually transmitted diseases, including HIV, the virus that causes AIDS. It is also hypothesized that components of semen, such as the spermatozoa as well as the seminal plasma, can cause immunosuppression in the body when introduced to the bloodstream or lymph. Evidence for this dates back to 1898, when Elie Metchnikoff injected a guinea pig with its own and foreign guinea pig sperm, finding that an antibody was produced in response; however the antibody was inactive, pointing to a suppression response by the immune system. Further research, such as that by S. Mathur and J.M. Goust, demonstrated that non-preexisting antibodies were produced in humans in response to the sperm. These antibodies mistakenly recognized native T lymphocytes as foreign antigens, and consequently the T lymphyocytes would fall under attack by the body's B lymphocytes.
Other semen components shown to spur an immunosuppressive effect are seminal plasma and seminal lymphocytes. These findings provide sustainance to views that AIDS, an immunodeficiency disease, may have causes other than HIV.
Note that any kind of sexual or other skin contact with the semen of a person infected with HIV should be avoided, even by persons already infected with the virus, as this may cause harmful re-infection.
Cultural views
In some cultures, semen is attributed with special properties of masculinity. For instance, among the Etoro people of Papua New Guinea, it is believed that young boys must fellate their elders and ingest their sperm to achieve proper sexual maturation. Other cultures believe semen to have beneficial qualities when applied to the skin, mainly for cosmetic purposes.
See also
References
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