Semen undergoes extreme chemical transformations after ejaculation, metamorphosing from a viscous liquid to a semi-solid and back again. Usually about five minutes after ejaculation, semen coagulates into a gel-like substance; then, within about fifteen minutes it becomes a sticky liquid. In most animals, the seminal vesicles and prostate act on semen as a one-two punch: The coagulation is due to a substance made by the seminal vesicles. Then PSA, an enzyme made by the prostate, causes the coagulum to break down. The character of semen varies greatly among species. For example, in bulls and dogs (which don’t have seminal vesicles), semen does not coagulate at all. But in rats and rabbits, semen quickly coagulates to form a pellet; for these animals, PSA is crucial in helping the sperm reach their destination.
Because semen is a body fluid, like blood, it is acted on by drugs, carcinogens, and pathogens (any agent that causes disease) — including sexually transmitted pathogens such as the HIV virus, which causes AIDS—to which the body is exposed. These have been proven to find their way into semen—and, via semen, into sexual partners.
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