Human Reproductive System

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Human Reproductive System

Female Reproductive System
The organs of the human female reproductive system. Encyclopaedia Britannica/UIG/Getty Images

Reproductive System Function

The reproductive system is necessary for the production of new living organisms. The ability to reproduce is a basic characteristic of life. In sexual reproduction, two individuals produce offspring that have genetic characteristics from both parents. The primary function of the reproductive system is to produce male and female sex cells and to ensure the growth and development of offspring. The reproductive system is comprised of male and female reproductive organs and structures. The growth and activity of these organs and structures is regulated by hormones. The reproductive system is closely associated with other organ systems, particularly the endocrine system and urinary system.

Male and Female Reproductive Organs

Both male and female reproductive organs have internal and external structures. Reproductive organs are considered to be either primary or secondary organs. The primary reproductive organs are the gonads (ovaries and testes), which are responsible for gamete (sperm and egg cell) and hormone production. The other reproductive structures and organs are considered secondary reproductive structures. Secondary organs aid in the growth and maturation of gametes and developing offspring.

Female Reproductive System Organs

Structures of the female reproductive system include:

  • Labia majora - Larger lip-like external structures that cover and protect sexual structures.
     
  • Labia minora - Smaller lip-like external structures found inside the labia majora. They provide protection for the clitoris and for the urethra and vaginal openings.
     
  • Clitoris - Very sensitive sexual organ located in front of the vaginal opening. It contains thousands of sensory nerve endings and responds to sexual stimulation.
     
  • Vagina - Fibrous, muscular canal leading from the cervix (opening of the uterus) to the external portion of the genital canal.
     
  • Uterus - Muscular internal organ that houses and nurtures female gametes after fertilization. Also called the womb, the uterus is where a developing fetus resides during pregnancy.
     
  • Fallopian tubes - Uterine tubes which transport egg cells from the ovaries to the uterus. Fertilization typically occurs in these tubes.
     
  • Ovaries - Female primary reproductive structures that produce gametes and sex hormones. There is one ovary on each side of the uterus.

The male reproductive system consists of sexual organs, accessory glands, and a series of duct systems that provide a pathway for fertile sperm cells to exit the body. Male reproductive structures include the penis, testes, epididymis, seminal vesicles, and prostate gland.

Reproductive System and Disease

The reproductive system can be impacted by a number of diseases and disorders. This includes cancer that may develop in reproductive organs such as the uterus, ovaries, testicles, or prostate. Disorders of the female reproductive system include endometriosis (endometrial tissue develops outside of the uterus), ovarian cysts, uterine polyps, and prolapse of the uterus. Disorders of the male reproductive system include testicular torsion (twisting of the testes), hypogonadism (testicular under-activity resulting in low testosterone production), enlarged prostate gland, hydrocele (swelling in the scrotum), and inflammation of the epididymis.

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Male Reproductive System

Male Reproductive System
The organs of the human male reproductive system. Encyclopaedia Britannica/UIG/Getty Images

Male Reproductive System Organs

The male reproductive system consists of sexual organs, accessory glands, and a series of duct systems that provide a pathway for fertile sperm cells to exit the body.

  • Penis - Main organ involved in sexual intercourse. This organ is composed of erectile tissue, connective tissue, and skin. The urethra extends through the length of the penis, allowing urine and sperm to pass.
     
  • Testes - Male primary reproductive structures that produce male gametes (sperm) and sex hormones.
     
  • 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. Its function is to develop immature sperm and to 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.
     
  • Ejaculatory Duct - Duct formed from the union of the ductus deferens and seminal vesicles. Each ejaculatory duct empties into 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 for sperm cells. Tubes leading from the seminal vesicles join the ductus deferens to form the ejaculatory duct.
     
  • Prostate Gland - Gland that produces a milky, alkaline fluid which 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 urine in the urethra and acidity in the vagina.

Similarly, the female reproductive system contains organs and structures that promote the production, support, growth, and development of female gametes (egg cells) and a growing fetus.

Reproductive System: Gamete Production

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

The production of sperm cells is known as spermatogenesis. 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 (ovum development) occurs in the female ovaries. In meiosis I of oogenesis, daughter cells are divided 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 and fertilization begins. Once fertilization is initiated, the secondary oocyte completes meiosis II and is then called an ovum. The ovum fuses with the sperm cell, and fertilization is complete. The fertilized ovum is called a zygote.

Sources:

  • SEER Training Modules, Introduction to the Reproductive System. U. S. National Institutes of Health, National Cancer Institute. Accessed 24 January 2014 (http://training.seer.cancer.gov/anatomy/reproductive/)
  • The Reproductive System. On-Line Biology Book. Updated 03/12/07 (http://www2.estrellamountain.edu/faculty/farabee/BIOBK/BioBookREPROD.html)