Semen is the fluid containing spermatozoa, which is secreted by the sexual glands of males. It is discharged by the body through the process known as ejaculation and used to fertilize the female ova.
The human semen is composed mainly of seminal plasma, which is the fluid secreted by the accessory male reproductive organs (the seminal vesicles and the prostate gland). The bulbourethral organs glands also contribute with a small amount of viscous mucus, which gives semen its texture and cohesion.
The purpose of the seminal plasma is to protect and feed the spermatozoa during their journey to the ova through the female reproductive tract. The normal environment of the vagina is too acidic for sperm cells, aside from the presence of local cells which are part of the immune system and which view sperm cells as foreign entities. This is why the sperm contains a series of basic amines designed to counter the vagina acidity and to protect the DNA inside the sperm cell from acidic destruction.
Finally, the 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 through, and preventing their diffusion out of the semen.