The Female Reproductive System

Sure, all women know the basics about their menstrual cycle – once a month your body releases an egg that travels along the fallopian tube to the uterus. If it doesn’t get fertilized, you have your period. But properly understanding just what happens to get that egg released by the fallopian tubes can give you insight as to just how delicate and sensitive the reproductive system really is.

The Organs

  • External Genitals: This is made up of the fleshy tissue surrounding the vaginal and uterine opening, called the labia minor (the inner, hairless folds, closest to the body), the labia major (the outer folds which often have hair and sit on top of the labia minor) and the clitoris. The Bartholin’s glands are found just by the opening of the vagina and are responsible for producing lubricating fluid.
  • Vagina: This part of your body connects the external genitals with the internal reproductive organs. The vagina is a tube-like structure that ends at the cervix. It acts as the point of entry for the penis and sperm and as an exit when a child is born.
  • Cervix: Sometimes referred to as the "neck" of the uterus, the cervix contains mucus that can help or hinder sperm from fertilizing an egg. When estrogen levels are high, the cervical mucus tends to be thin and slippery, thereby making it easier for sperm to travel further into the uterus. When estrogen levels are low, cervical mucus is more likely to be thick, making it difficult for sperm to navigate their way through. The cervix is positioned between the vagina and the uterus.
  • Uterus: This is the part of your reproductive organs that acts as a home to your baby for nine months. It is a muscular organ made up of three layers: the outer layer is called the peritoneum; the middle, muscular layer is the myometrium while the inner lining of the uterus is referred to as the endometrium. The endometrium thickens every month in preparation for implantation of a fertilized egg. The cervix begins at the bottom, narrower part of the uterus, while the fallopian tubes are positioned near the top of the uterus, one on each side.
  • Fallopian Tubes: Most women have two fallopian tubes, one on the left side and one on the right side. The fallopian tubes act as a passageway for the mature egg to travel down every month. They are also the site of fertilization. When an egg is fertilized, it stays in the fallopian tubes until it reaches the blastocyst stage, at which point it continues on to the uterus and implants itself. The fallopian tubes are connected to the uterus on one end and hover just over the ovaries on the other. This end of the fallopian tubes have 20 to 25 finger-like structures called fimbraie that help collect the mature egg into the fallopian tube each month.
  • Ovaries: This is the site of your egg production. Females usually have two ovaries, one on each side of the body, and are found just outside of the uterus. The ovaries contain follicles that house immature eggs. Each month, roughly 20 follicles are stimulated and begin developing an egg. Usually, only one egg will get released although in rare instances, two or more may mature. The ovaries are also responsible for producing estrogen and progesterone, two important hormones necessary for normal reproductive function.

The Hormones

  • Estrogen: One of the better-known hormones, there are actually different types of estrogen that your body produces. Two of the most important ones for reproduction are estradiol and androgen. Estradiol helps to encourage the thickening of the endometrium every month as well as making the vaginal and cervical mucus more ideal for sperm. Although it does not start out as estrogen, androgen gets converted into extra estrogen by the ovaries, causing the follicles that will not produce an egg during the monthly cycle to disappear.
  • Progesterone: Produced by the corpus luteum (the remaining follicle from which a mature egg has been released), progesterone is responsible for making the endometrium lining more receptive to implantation by a fertilized egg. At the same time, it prevents further growth by the follicles.
  • Gonadotropin Releasing Hormone (GnRH): Unlike estrogen and progesterone, which are produced by the reproductive organs, GnRH is produced by the hypothalamus in the brain. GnRH controls the production and levels of estrogen in your body. When your levels of estrogen begin to decline at the end of your cycle, GnRH kicks in. High levels of progesterone, though, stop the production of GnRH.
  • Follicle Stimulating Hormone (FSH) and Lutenizing Hormone (LH): These hormones are produced by the pituitary gland and are a response to the hypothalamus’ production of GnRH. Increased levels of FSH help to stimulate the follicles, causing egg production as well as estradiol production. LH, on the other hand, works with the follicles to produce androgen.

Your Menstrual Cycle
Every woman is born with immature eggs in her ovaries. Just before birth, the number of eggs in the ovaries is roughly a few million but by the time puberty begins, this number has diminished to about 400,000. Of these, only three hundred or so will ever be released. As you age, the number of eggs in your ovaries continue to diminish, which isn’t so surprising when you remember that about 20 or so immature eggs begin to develop and then die off when the one dominant egg is released every month.

The reproductive years begin with the onset of menstruation during puberty and end when you enter menopause around age 45 or 50. A normal menstrual cycle can last for 25 to 34 days and have three to five days of bleeding. However, when discussing menstrual cycles, a 28-day cycle is the standard most professionals use.

The first day of your menstrual cycle begins on the first day of your period. This is known as Day 1. Each menstrual cycle is divided into two phases. The first phase is marked by the development and maturation of an egg while the second phase is known as the luteal phase because of the spike in LH production. These phases are separated by ovulation, which occurs about mid-way through your cycle.

Having It All Come Together
During the first few days of your cycle, your levels of estrogen decrease, signaling your body to increase its production of FSH. This helps promote the maturation of the follicles that contain the eggs. From the 20 or so eggs that are stimulated every month, one egg will manage to attract the most attention from FSH. This helps the follicle to produce more estradiol, which in turns tells the pituitary gland to stop producing so much FSH. As a result, the other follicles die off when they stop receiving enough FSH to survive.

The increase in estrogen also sends a signal to the endometrium, causing it to start thickening. Your cervical mucus will begin to change in consistency, from thick to thin and slippery. Additionally, more mucus will be produced, thereby aiding any sperm that enters your vagina in its quest to fertilize an egg.

About a day and a half before ovulation occurs, your estrogen levels will climax, causing the pituitary to release a surge of LH. Once the follicle releases the egg, the follicle itself shrinks and gathers fatty substances known as lipids. This then becomes the corpus luteum, which is responsible for the production of progesterone. As progesterone levels in your body increase, the endometrium begins to prepare itself for implantation by a fertilized egg.

Once the ovaries have released an egg, it has approximately 72 hours to be fertilized. Since sperm can survive in your body for several days, it is usually recommended that you start to have unprotected sex before the onset of ovulation to increase your chances of fertilization.

If the egg is fertilized, your body begins to produce human Chorionic Gonadotropin (hCG), which is often referred to as the "pregnancy hormone." HCG helps ensure that your body will continue to produce the correct amounts of estrogen and progesterone to provide the ideal conditions for implantation. If pregnancy does result, the corpus luteum will be stimulated by the placenta and continue to produce progesterone.

If the egg is not fertilized, the corpus luteum will dry up after about 14 days and become scar tissue. Your endometrium lining will shed itself as hormone levels fall, resulting in your period. Because of the drop in hormone levels, more GnRH will start to be produced and your monthly menstrual cycle will begin again.

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