Human reproduction is essential for the continuance of the human species. Humans reproduce sexually by the uniting of the female and male sex cells. Although the reproductive systems of the male and female are different, they are structured to function together to achieve internal fertilization.
It is a characteristic of all living things on Earth that they reproduce or produce offspring, and humans are no different. If humans are examined as large, complex land mammals, then we can say from a strictly biological point of view that the male and female have the same role as other mammals. The male's job is to produce sperm cells and deliver them into the female reproductive tract. The female's job is to produce ova (eggs), receive the sperm, and nourish the embryo that grows inside her. She must also give birth and produce milk for the offspring during its early years.
Unlike other land mammals, humans are not simply physical creatures who must follow the course nature has planned for them. People have intelligence and the ability to do what they want. People have invented culture and civilization, and have made up rules and values. A good example of how different humans are from all other living things is reproduction itself. Humans are the only ones who can choose not to reproduce, for whatever reason. So, despite the millions of years of evolution and adaptation that thousands of generations went through to produce a certain human being, that person can decide not to reproduce with another human and therefore not to pass on its genetic inheritance. Although humans are greatly influenced by their biological makeup, they are not compelled by it they way other animals are.
MALE REPRODUCTIVE SYSTEM
Among humans who do choose to reproduce (and this is by far the greater part of the human species) reproduction is basically a biological process. As mammals, humans practice internal fertilization, which means that the sperm and egg come together inside the female's body. In order for this to happen, both male and female need a set of organs and systems that work together. The male reproductive system produces, stores, and releases its gametes, or sex cells, known as spermatozoa. Sperm are produced in the testes, two oval-shaped organs contained in the scrotum, which is a like a pouch of skin. Located outside the body, it keeps sperm a few degrees lower than 98.6°F (37°C), since normal body temperature would kill most sperm. The testes are made of tightly coiled tubes in which sperm cells are formed. They are stored internally in a liquid called seminal fluid that keeps them nourished. The penis is the external part of the male reproductive system and contains a central channel called the urethra. Sperm flow out of the urethra at the proper time. As the specialized organ through which sperm is introduced into the female reproductive tract, the penis is made of spongy tissue that lengthens and stiffens when excited. At this point, it is ready and able to be inserted into the female's vagina.
FEMALE REPRODUCTIVE SYSTEM
The female gametes, or sex cells, are known as eggs, or ova, and are produced and stored in the ovaries. An egg is 75,000 times larger than an individual sperm cell. Females are born with about 40,000 immature eggs and do not produce any more during their lifetime. In most females, only about 400 of these eggs actually mature. From the onset of puberty until sometime in their forties, females release one mature ovum approximately every month. This monthly release of an egg that is ready to be fertilized is part of the female's menstrual cycle. The term menstrual comes from the Latin word mensis, which means "month." Therefore, every twenty-eight days an egg matures and is positioned to meet with a sperm cell in the Fallopian tubes. These two, 3-inch (7.6-centimeter) tubes connect the ovaries with the uterus. It is in the Fallopian tubes that fertilization takes place.
The uterus is a muscular structure that houses the developing egg if it is fertilized. The vagina is the muscular tube leading from the uterus to the outside of the body and it is the entrance or canal through which the male deposits his sperm. The act of human sexual reproduction is called sexual intercourse or coitus, and for it to work properly, both partners must usually be excited or experience what is called sexual arousal. This sexual stimulation results in body changes, such as the male's erect penis and the female's lubricated vagina. Nature has arranged it so that sexual reproduction is pleasurable or feels good to most organisms, and for humans, the inward and outward movement of the penis in the vagina
causes friction that leads the male to an orgasm. In the male, orgasm causes very strong, involuntary contractions of muscles at the base of the penis. These contractions forcefully expel semen, which contains sperm, from the penis. This release of sperm is called ejaculation. The female may or may not experience a similarly intense feeling, but even if she does not, she can still become pregnant.
Hundreds of millions of sperm cells are released during an ejaculation, and they swim through the uterus and into the Fallopian tubes. Many sperm attach themselves to the egg but only one actually enters it. Once a sperm enters the egg, the egg prevents any others from doing so. If fertilization occurs—and for many reasons it often does not—the zygote (fertilized egg) begins to divide and grow. It will then implant itself in the uterus where it will be nourished and grow into a baby. If the egg was not fertilized, it is eventually discharged out of the vagina with other uterine tissues and blood. This is sometimes called a woman's "period."
A fertilized egg that successfully attaches to the uterus will take about 270 days to grow into a fully developed fetus or baby. When it is ready to be born, the baby's adrenal glands secrete a hormone that signals the mother's pituitary gland to secrete a hormone called oxytocin. This causes the uterine muscles to contract rhythmically, and eventually the baby is born, or expelled, from the uterus.