Fertilization (fecundation) of the human being is internal, meaning, the cells that will form the new individual bond and develop within the body. This, because the body of a woman is the one prepared to receive the male gametes deposited in the vagina and then house the embryo in her uterus. This important transfer (from spermatozoa to the female interior) is carried out through coitus or sexual intercourse. During vaginal coitus, a series of voluntary and involuntary actions take place. Among the first, the conscious desire of the couple to be joined physically stands out, while in the involuntary aspect there are all of the processes that prepare both bodies for an effective coupling. While the vaginal walls lubricate, the clitoris increases size (due to an increase in blood irrigation) and the vagina expands in the woman. In men, changes take place like the erection of the penis (increase in size) and the appearance of a lubricating liquid on its tip.
Penetration takes place when the man introduces his penis into the woman’s vagina. At that moment, the tissue that surrounds the exterior part of the vagina swells, the clitoris elevates and the penis erects completely. The arousal of the man and woman is ever greater, steadily increasing their heart and respiratory rate, as well as their blood pressure. When both have reached the threshold of their arousal, they are close to what is called orgasm. This is the culminating point of the sexual act and only lasts a few seconds. In both men and women, the muscles tense, the vagina goes through a series of contractions and the man ultimately ejaculates. Ejaculation is the expulsion of semen into the woman’s vagina, process in which between 120 and 600 million spermatozoa are deposited into the proximities of the cervix. After this process, the penis exits the vagina and shrinks, while the vagina returns to its original size. Not so the cervix, which remains open in order to ease the race that the spermatozoa have undertaken.