Human Reproductive System

A doctor painting ovaries with light.
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The human reproductive system and the ability to reproduce make life possible. In sexual reproduction, two individuals produce offspring that have some of the genetic characteristics of both parents. The primary function of the human reproductive system is to produce sex cells. When a male and female sex cell unite, an offspring grows and develops.

The reproductive system is usually comprised of either male or female reproductive organs and structures. The growth and activity of these parts are regulated by hormones. The reproductive system is closely associated with other organ systems, particularly the endocrine system and urinary system. 

Gamete Production

Gametes are produced by a two-part cell division process called meiosis. Through a sequence of steps, replicated DNA in a parent cell is distributed among four daughter cells. Meiosis produces gametes that are considered haploid because they have half the number of chromosomes as the parent cell. Human sex cells contain one complete set of 23 chromosomes. When sex cells unite during fertilization, the two haploid sex cells become one diploid cell that contains all 46 chromosomes.

Spermatogenesis

The production of sperm cells is known as spermatogenesis. Stem cells develop into mature sperm cells by first dividing mitotically to produce identical copies of themselves and then meiotically to create unique daughter cells called spermatids. Spermatids then transform into mature spermatozoa through spermiogenesis. This process occurs continuously and takes place within the male testes. Hundreds of millions of sperm must be released in order for fertilization to take place.

Oogenesis

Oogenesis (ovum development) occurs in the female ovaries. In meiosis I of oogenesis, daughter cells divide asymmetrically. This asymmetrical cytokinesis results in one large egg cell (oocyte) and smaller cells called polar bodies. The polar bodies degrade and are not fertilized. After meiosis I is complete, the egg cell is called a secondary oocyte. The haploid secondary oocyte will only complete the second meiotic stage if it encounters a sperm cell. Once fertilization is initiated, the secondary oocyte completes meiosis II and becomes an ovum. The ovum fuses with the sperm cell and fertilization completes while embryonic development begins. A fertilized ovum is called a zygote.

Reproductive System Disease

The reproductive system is susceptible to a number of diseases and disorders. These are of varying degrees of detriment to the body. This includes cancer that can develop in reproductive organs such as the uterus, ovaries, testicles, and prostate.

Disorders of the female reproductive system include endometriosis—a painful condition in which endometrial tissue develops outside of the uterus—ovarian cysts, uterine polyps, and uterine prolapse.

Disorders of the male reproductive system include testicular torsion—twisting of the testes—testicular under-activity resulting in low testosterone production called hypogonadism, enlarged prostate gland, swelling of the scrotum called hydrocele, and inflammation of the epididymis.

Reproductive Organs

Both male and female reproductive systems have internal and external structures. Reproductive organs are considered to be either primary or secondary organs based on their role. The primary reproductive organs of either system are called gonads (ovaries and testes) and these are responsible for gamete (sperm and egg cell) and hormone production. Other reproductive structures and organs are considered secondary reproductive structures and they aid in the growth and maturation of gametes and offspring.

Female Reproductive System

Illustration of the Female Reproductive System Encyclopaedia Britannica/UIG/Getty Images

The female reproductive system is comprised of both internal and external reproductive organs that both enable fertilization and support embryonic development. Structures of the female reproductive system include:

  • Labia majora: Larger lip-like external structures that cover and protect other reproductive structures.
  • Labia minora: Smaller lip-like external structures found inside the labia majora. They provide protection to the clitoris, urethra, and vaginal openings.
  • Clitoris: Sensitive sexual organ located in the uppermost section of the vaginal opening. The clitoris contains thousands of sensory nerve endings that respond to sexual stimulation and promote vaginal lubrication.
  • Vagina: Fibrous, muscular canal leading from the cervix to the external portion of the genital canal. The penis enters the vagina during sexual intercourse.
  • Cervix: Opening of the uterus. This strong, narrow structure expands to allow sperm to flow from the vagina into the uterus.
  • Uterus: Internal organ that houses and nurtures female gametes after fertilization, commonly called the womb. A placenta, which encases a growing embryo, develops and attaches itself to the uterine wall during pregnancy. An umbilical cord stretches from the fetus to its placenta to provide nutrients from a mother to an unborn baby.
  • Fallopian tubes: Uterine tubes that transport egg cells from the ovaries to the uterus. Fertile eggs are released from ovaries into fallopian tubes during ovulation and typically fertilized from there.
  • Ovaries: Primary reproductive structures that produce female gametes (eggs) and sex hormones. There is one ovary on either side of the uterus.

Male Reproductive System

Illustration of the Male Reproductive System
Encyclopaedia Britannica/UIG/Getty Images

The male reproductive system consists of sexual organs, accessory glands, and a series of duct systems that provide a pathway for sperm cells to exit the body and fertilize an egg. Male genitalia only equips an organism to initiate fertilization and does not support the development of a growing fetus. Male sex organs include:

  • Penis: The main organ involved in sexual intercourse. This organ is composed of erectile tissue, connective tissue, and skin. The urethra stretches the length of the penis and allows either urine or sperm to pass through its external opening.
  • Testes: Male primary reproductive structures that produce male gametes (sperm) and sex hormones. Testes are also called testicles.
  • Scrotum: External pouch of skin that contains the testes. Because the scrotum is located outside of the abdomen, it can reach temperatures that are lower than that of internal body structures. Lower temperatures are necessary for proper sperm development.
  • Epididymis: System of ducts that receive immature sperm from the testes. The epididymis functions to develop immature sperm and house mature sperm.
  • Ductus Deferens or Vas Deferens: Fibrous, muscular tubes that are continuous with the epididymis and provide a pathway for sperm to travel from the epididymis to the urethra
  • Urethra: Tube that extends from the urinary bladder through the penis. This canal allows for the excretion of reproductive fluids (semen) and urine from the body. Sphincters prevent urine from entering the urethra while semen is passing through.
  • Seminal Vesicles: Glands that produce fluid to nurture and provide energy to sperm cells. Tubes leading from the seminal vesicles join the ductus deferens to form the ejaculatory duct.
  • Ejaculatory Duct: Duct formed from the union of the ductus deferens and seminal vesicles. Each ejaculatory duct empties into the urethra.
  • Prostate Gland: Gland that produces a milky, alkaline fluid that increases sperm motility. The contents of the prostate empty into the urethra.
  • Bulbourethral or Cowper's Glands: Small glands located at the base of the penis. In response to sexual stimulation, these glands secrete an alkaline fluid which helps to neutralize acidity from the vagina and urine in the urethra.

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