During mating, human males produce semen which is produced by the male reproductive system, the penis when he ejaculates. Each drop of semen contains thousands of sperms which are microscopic in nature. When the male ejaculates in the woman’s genitals or the vagina, sperm enter through the cervix into the uterus. Approximately 300 million sperm are present in each human ejaculation, but less than 1% of it make their way into the cervix(mouth of the womb) as most of it is killed by the acidic fluids near the vagina, fail to make it due to flow back, prevented from entering by mucus or move towards the walls of the cervix into dead-end channels.
The rest(10000 sperm cells approximately) of the sperms approach towards the oviduct where ovaries release eggs towards the far end of the uterus, half of which move towards the oviduct. Out of this, due to the mucus lining in the unterobeal junction, only 1000 make it to the fallopian tube. At this point, 25% of the remaining sperm furthermore is either died out or gets stuck to the oviduct lining.
Finally, one sperm manages to enter the egg for the event of fertilization, the rest of which are propelled by the zona reaction causing it to become impermeable to any sperms further.
For detailed information on the human reproductive system, please register at BYJU’S.