Semen

The fluid in the lumen of the seminiferous tubules is quite different from plasma; it contains very little protein and glucose but is rich in androgens, estrogens, K+, inositol, and glutamic and aspartic acids.
Maintenance of its composition depends on the blood-testis barrier. The barrier also protects the germ cells from bloodborne noxious agents, prevents antigenic products of germ cell division and maturation from entering the circulation and generating an autoimmune response, and may help establish an osmotic gradient that facilitates movement of fluid into the tubular lumen.

Semen
The fluid that is ejaculated at the time of orgasm, the semen, contains sperm and the secretions of the seminal vesicles, prostate, Cowper glands, and, probably, the urethral glands. An average volume per ejaculate is 2.5-3.5 mL after several days of abstinence from sexual activity. The volume of semen and the sperm count decrease rapidly with repeated ejaculation.
Even though it takes only one sperm to fertilize the ovum, each milliliter of semen normally contains about 100 million sperm. Reduction in sperm production is associated with infertility: 50% of men with counts of 20-40 million/mL and essentially all of those with counts under 20 million/mL are sterile. The presence of many morphologically abnormal or immotile spermatozoa also correlates with infertility.
The prostaglandins in semen, which come from the seminal vesicles, are at high concentrations, but their function in semen is unknown. The causes of male infertility, as well as the underlying mechanisms of sperm in fertilization, are used as clues in developing male contraception .