Human Reproduction

Reproduction is how a species continues. In asexual reproduction a single parent makes copies of itself with no gametes, so the offspring are genetically identical (for example fission in Amoeba, budding in Hydra, spore formation in Rhizopus, and vegetative propagation in plants). In sexual reproduction two parents each contribute a gamete; the gametes fuse, so the offspring get a new mixture of genes. This variation is the great advantage of sexual reproduction: it gives a population the differences it needs to survive changing conditions. Humans reproduce only sexually.

The two testes lie outside the body in a skin sac called the scrotum, which keeps them slightly cooler than body temperature so sperm can form. Inside the testes, coiled seminiferous tubules make millions of sperms, while Leydig (interstitial) cells make the hormone testosterone that drives male secondary sexual characters. Mature sperms travel from the testis to the epididymis (where they are stored), then up the vas deferens. Glands such as the seminal vesicles and prostate add fluid to make semen, which leaves the body through the urethra in the penis.

The two ovaries make the eggs and the hormones oestrogen and progesterone. At birth a girl already has thousands of immature eggs; from puberty onward, usually one egg matures and is released each cycle (ovulation). The released egg is swept into the funnel of the oviduct (Fallopian tube), where fertilisation can happen. The oviduct carries it on to the uterus, a thick muscular organ in which a fertilised egg implants and the baby develops. The uterus opens through the cervix into the vagina, which receives the sperm.

Fertilisation is the fusion of a sperm with an egg to form a single cell, the zygote, in the oviduct. The zygote divides again and again as it moves to the uterus, becoming a ball of cells that fixes into the thick uterine lining; this fixing is called implantation and marks the start of pregnancy. A special tissue, the placenta, then grows to connect the developing embryo to the mother's blood. Through it the embryo gets oxygen and nutrients and passes back carbon dioxide and wastes. After about nine months the fully formed baby is pushed out by powerful contractions of the uterus.

From puberty to menopause the uterus prepares for a possible pregnancy in a roughly 28-day rhythm. Its lining thickens with blood vessels; ovulation releases an egg around day 14. If the egg is not fertilised, the lining is no longer needed and is shed as blood and tissue through the vagina, which is menstruation (the period), lasting a few days; then the cycle begins again. Reproductive health means more than the cycle: it includes hygiene, avoiding sexually transmitted infections (STIs such as HIV-AIDS), and family planning using contraceptive methods (barrier, hormonal, surgical) so that a couple can choose when to have children.