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, very viscous, containing water and small amounts of salt, protein, and fructose.
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 semen is composed of seminal plasma, the fluid portion of semen.This fluid is contributed by the accessory male reproductive organs . Some 60% of the volume of ejaculate is produced by the seminal vesicles, and most of the remainder is generated by the prostate. A small amount of viscous mucus secreted by the bulbourethral glands contributes to the cohesive jelly-like texture of semen.
Seminal plasma of humans contains a complex range of organic and inorganic constituents. They include metal and salt ions, sugars, lipids, steroid hormones, enzymes, prostaglandin hormones, amino acids and basic amines. The purpose of the seminal plasma is to provide a nutritive and protective medium for the spermatozoa during their journey through the female reproductive tract. The normal enviroment of the vagina is a hostile one for sperm cells, it is very acidic (from the native microflora producing lactic acid), viscous, and patrolled by immune cells. The components in the seminal plasma attempt to compensate for this hostile enviroment.
Basic amines such as putrescine, spermine , spermidine and cadaverine are responsible for the smell and flavor of semen. These alkaline bases counteract the acidic environment of the vaginal canal (which is harmful to sperm), and protect DNA inside the sperm from acidic denaturation. Salts and metal ions in the semen help to create a more hospitable enviroment for the sperm in the vaginal canal. A typical ejaculation can contain up to 5 mg of zinc. Zinc serves to help to stabilize the DNA containing chromatin in the sperm cells. A zinc deficiency may result in lowered fertility because of increased sperm fragility. Zinc deficiency can also adversely affect spermatogenesis.
The simple sugar fructose is the main energy source of sperm cells, which rely entirely on sugars from the seminal plasma for energy. Other components of semen (mucus and texturizing proteins) serve to increase the mobility of sperm cells in the vagina and cervix by creating a less viscous channel for the sperm cells to swim though, and preventing their diffusion out of the semen. Prostaglandin hormones are involved in supressing an immune response by the female against the foreign semen.
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. 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.
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.