Semen
When sperm cells are ejaculated, they are swimming in a thick, grayish-white fluid called semen. Following coitus, the semen is deposited close to the entrance of the uterus. The total volume of semen thus discharged at one time is less than a teaspoonful, but it contains between 150 and 500 million sperm cells, which now try to move into the uterus and onward up into the Fallopian tubes. However, unless an egg has been released by one of the ovaries around that time, the opening of the uterus will be plugged by an impenetrable, thick mucus. Only during ovulation does this mucus become thin enough to be penetrated by sperm cells. Even then, only about 1 percent of them succeed. The others die within a few hours in the slightly acidic, and therefore hostile, environment of the vagina. Those sperm cells that enter the uterus find themselves in a slightly alkaline, and therefore friendly, environment. Furthermore, muscular movements of the uterus and Fallopian tubes help them along on their way. Nevertheless, only a few hundred or a few thousand ever reach that upper part of the tubes where a union of egg and sperm can occur. (Naturally, the sperm cells enter both tubes whether an egg is present or not.)