From Encyclopedia of Sex and Sexuality
Microscopic cells produced in the testicles that carry a man’s genetic potential. When sperm meet and combine with a woman’s egg, together they begin the process leading to pregnancy and the creation of a new life. In the head of the sperm are chromosomes containing one half the genetic content of all other body cells (the other half of the genetic material in the cells of the fetus will come from the mother’s egg). Half of a man’s sperm cells contains the Y chromosome, which will produce a male child when combined with the X chromosome from the egg, and half carry the X chromosome, which specifies femaleness.
The sperm is the only cell in the human body that has a tail, enabling it to swim from the vagina, through the cervix and uterus, and to the fallopian tubes, where it will meet the woman’s egg. Each sperm has a cap (acrosome) containing powerful enzymes that enable the sperm to penetrate the protective coating around a woman’s egg. A single sperm is a bit longer than one-thousandth of an inch and can swim one to two inches an hour.
A man’s testicles begin to manufacture sperm at the time of puberty and produce 50,000 sperm per minute, every minute of every hour, well into old age. Sperm production inside the testicles takes about seventy-seven days. It then takes another four to five days for the sperm to learn how to swim, in the duct called the epididymis. The vas deferens is the powerful muscular tube that propels the sperm from the epididymis to the penis. A man will normally ejaculate 80 million to 300 million sperm at the time of orgasm. Only a few of the millions of sperm deposited in the female vagina will survive the arduous journey to the fallopian tube, where one may meet and fertilize the woman’s egg. Sperm can live from two to seventy-two hours inside the female reproductive tract.
Because sperm cells are so tiny, all 80 to 300 million can fit inside a space the size of a pinhead. Most of what comes out when a man ejaculates is the fluid that nourishes the sperm, not the sperm itself. The sperm and fluid from the testicles make up only 3 to 5 percent of the ejaculate. The rest is nourishing fluid made in the seminal vesicles (65 percent) and prostate (30 percent).