Most of human seminal fluid comes from glands, not the testes
Sperm cells make up only part of semen. About 70 percent of the human fluid comes from the seminal vesicles, a sugary secretion rich in fructose, with more added by the prostate. Its alkaline chemistry exists to protect sperm from the acidic, immune-patrolled environment they must cross to reach an egg.
Semen, or seminal fluid, carries spermatozoa from the male gonads. In humans and other placental mammals it contains enzymes and fructose that keep sperm alive and give them a medium to swim through toward the uterus and fallopian tubes, where fertilisation can form a zygote. Farmers and conservationists collect it from animals for artificial insemination and for cryoconservation, freezing genetic material to preserve particular breeds.
Fertilisation happens in different places across the animal kingdom. Many fish fertilise externally, the female releasing eggs into the water for the male's semen to reach. Internal fertilisation occurs inside the female after mating. Amphibians, reptiles, birds and egg-laying monotremes transfer sperm through the cloaca, while marsupials and placental mammals use the vagina. In kangaroos and their relatives, semen coagulates afterward into a mating plug.
The fluid is assembled on the way out. Sertoli cells in the testes release fluid that helps move sperm along, cells in the ducts reabsorb some of it, and the epididymis secretes a compound that probably prevents sperm from maturing too early. During ejaculation, sperm mix with the yellowish, viscous output of the seminal vesicles, the thin whitish secretion of the prostate containing citric acid, enzymes and lipids, and a clear lubricant from the bulbourethral glands.
Its chemistry serves a purpose. The vagina is acidic because of lactic acid from resident microbes, viscous, and watched by immune cells. Alkaline amines such as spermine, putrescine and cadaverine, which also give semen its characteristic smell, buffer that acidity and shield sperm DNA from damage. A 1992 World Health Organization report defined normal human semen as at least 2 millilitres, with a pH of 7.2 to 8.0, at least 20 million sperm per millilitre, and at least half of them motile.
Source: Semen
