Why does the sperm last up 2 4 hours?

Answer 1

Sperm are thought to retain fertilizing capability for up to 48 hours once they have entered the female genital tract, but it's possible their lifespan is even longer.

Sperm production by the testicle takes about 90 days; the immature sperm are then deposited in a receptacle called the epididymis. They remain there until the time of ejaculation, when they undergo further maturation and develop their swimming ability. We do not know exactly how long they live once they reach maturity. Optimal semen specimens occur with ejaculation every two or three days. As the length of time between ejaculation increases, the percentage of normal, motile sperm in a semen sample progressively decreases.

Repeated ejaculations at intervals of 24 hours or less may result in sperm depletion and a drop in numbers of 30 percent or more. Muscular contractions usually diminish with subsequent orgasms, which also contributes to lower sperm volumes and counts.

There is no reason to believe that consecutive episodes are beneficial to achieve pregnancy. There is considerable variation in sperm count among individuals from specimens taken from repeated "back-to-back" ejaculations. For those with normal semen analysis, a sufficient number of motile sperm are usually ejaculated during the first episode. Unfortunately, those with male fertility problems usually have very limited number of sperm in a second specimen.

Source: http://health.ivillage.com/infertility/infertmen/0,,5ll4,00.html

Answer 2

Sperm can stay alive in the female reproductive tract for up to 4-5 days, not hours, due to the cervical mucus providing a nourishing environment that helps sperm survive. This extended survival time increases the chances of fertilization occurring if ovulation happens during that window.