Reproductive system

The reproductive system (or genital system) is the set of body parts involved in making offspring, whether through sexual or asexual reproduction. Alongside the organs, non-living materials like fluids, hormones, and pheromones play key supporting roles. Unlike most other organ systems, this one often looks very different between the sexes of a species. Those differences allow two individuals to mix their genetic material, which can lead to offspring with better genetic fitness.

Animals

In animals, mammals have main reproductive parts that include external genitals (the penis and vulva) and internal organs, such as the gonads that produce gametes (testicles and ovaries). Diseases of the human reproductive system are very common, especially sexually transmitted infections. Most other vertebrates have a similar setup—gonads, ducts, and openings—but there is a huge variety of physical adaptations and reproductive strategies across different vertebrate groups.

Vertebrates

All vertebrates share key reproductive features: they have gonads that make gametes. In females, these gonads connect via oviducts to an external opening, usually a cloaca or vagina.

For humans, reproduction happens through internal fertilization. During sex, semen is ejaculated from the male’s erect penis into the female’s vagina. Sperm then travel through the vagina and cervix into the uterus or fallopian tubes to fertilize an egg.

After fertilization and implantation, the fetus grows in the uterus for about nine months (pregnancy). Pregnancy ends with childbirth, when uterine muscles contract, the cervix opens, and the baby exits through the vagina. Human babies need a lot of parental care, including milk from the female’s mammary glands in the breasts.

The female reproductive system has two jobs: producing egg cells and protecting and nourishing the offspring until birth. The male reproductive system has one job: producing and depositing sperm. Humans show a high level of sexual differentiation, with differences in nearly every reproductive organ and in many secondary sexual characteristics.

The male reproductive system is a series of organs mostly outside the body and around the pelvic area. Its main job is to supply sperm for fertilizing an egg. The major male organs fall into three groups.

First, sperm production and storage: sperm are made in the testicles, which sit in the temperature-regulating scrotum; immature sperm then move to the epididymides to develop and be stored. Second, the glands that make ejaculatory fluid: the seminal vesicles, prostate, and vasa deferentia. Third, the parts used for copulation and depositing sperm: the penis, urethra, vas deferens, and Cowper’s gland.

Major secondary sex characteristics in males include a larger, more muscular build, a deeper voice, facial and body hair, broad shoulders, and an Adam’s apple. A key male sex hormone is androgen, especially testosterone. The testes release a hormone that controls sperm development and also drives traits like facial hair and a deep voice.

The human female reproductive system is a series of organs mostly inside the body and around the pelvic area. It has three main parts: the vulva (which leads to the vagina and vaginal opening), the uterus (which holds the developing fetus), and the ovaries (which produce the female’s eggs). The breasts are involved in parenting but are usually not classified as part of the reproductive system. The vagina meets the outside at the vulva, which includes the labia, clitoris, and urethra; during sex, this area is lubricated by mucus from the Bartholin’s glands.

The vagina connects to the uterus through the cervix, and the uterus connects to the ovaries via the fallopian tubes. Each ovary holds hundreds of eggs. About every 28 days, the pituitary gland releases a hormone that stimulates some eggs to develop. One egg is released, travels through the fallopian tube into the uterus.

Hormones from the ovaries prepare the uterus to receive the egg. The egg moves through the fallopian tubes and waits for sperm to fertilize it. If no sperm arrives, the uterine lining (endometrium) and the unfertilized egg are shed each cycle in menstruation. If the egg is fertilized, it attaches to the endometrium and embryonic development begins.

Most mammal reproductive systems are similar to humans, but there are notable differences. For example, most male mammals keep their penis stored internally until it becomes erect, and many have a penis bone (baculum). Also, males and females of most species are not continuously fertile like humans, and females of most mammalian species do not have permanent mammaries like human females do. Like humans, most mammal groups have descended testicles located within a scrotum.