Internal Anatomy

Vagina (Vuh-JEYE-nuh)
The vagina, also known as the birth canal, leads to the internal reproductive system. It is a narrow tunnel that measures between three and five inches in length. The vaginal opening is called the introitus. The vagina is surrounded by muscle and supporting tissue that can expand enough to allow passage of a baby. The natural vaginal secretions provide lubrication and keep a healthy balance of bacteria in the vagina to resist infection.

A certain degree of vaginal discharge is normal, and may change in consistency depending on the hormones present at different stages of the menstrual cycle. For example, during mid-cycle, called ovulation, when estrogen levels are high, the cervix produces a large amount of watery secretions that you may perceive as vaginal discharge. Normally, it looks clear and stretchy, and feels slippery -- like mucus. Nature's aim during ovulation is to enhance reproduction: the mucus is abundant and slippery, giving sperm the best chance possible of surviving in the cervix. In contrast, cervical mucus becomes thicker or disappears entirely at other times of the menstrual cycle.

After menopause, and even the few years leading up to menopause and thereafter, vaginal discharge may decrease. This reduction in lubrication, which is caused by lower levels of estrogen, may cause dryness, irritation or even infection.

Cervix (SER-viks)
At the top of the vagina is the cervix, or the connection between the vagina and the uterus. The cervix itself is only about an inch in diameter, small and pink. The opening to the cervix, called the os , is a very small hole in the middle of the cervix. After pregnancy, it appears as a 1/4-inch slit. The os opening is big enough to allow the flow of fluids, such as menstrual blood, from the uterus. During the labor of pregnancy, the os opens to 9–10 centimeters to allow delivery of the baby. In non-pregnant women, the os is only open a few millimeters. Cervical mucus acts as a barrier to bacteria by preventing bacteria from entering the uterus and the fallopian tubes.

Many women notice changes in the consistency of the cervical mucus during their cycle. It is often thin and mucus-like during ovulation when estrogen levels are high, and thicker and more sticky -- or seemingly nonexistent -- at any other time of the menstrual cycle.

Uterus (YEW-ter-us)
The uterus is a rose-hued, pear-shaped, muscular organ that can expand and stretch enough to accommodate the development of a fetus. The inner lining of the uterus is called the endometrium, which is a lining made up of blood vessels, specialized glands and supporting tissue. This is the part of the uterus that is shed during a period.

There are three openings to the uterus. The cervix, which is the lower part of the uterus that opens to the vagina, and each fallopian tube which enters the uterus towards the top, one on either side.

The main function of the uterus is to create a nurturing environment for the growing fetus. During pregnancy, this small mass of muscle starts out about the size of a pear, and grows to become the largest muscle in the body, larger even than thigh muscles.

During menstruation, the uterus may contract in response to a series of hormonal changes: the shedding endometrium releases prostaglandins, which trigger contractions. Furthermore, the decline in progesterone that occurs just prior to menses may also contribute to uterine contractions. During labor, uterine contractions thin the lower segment of the uterus and cervix, a process called effacement, and expand the cervical os to prepare for delivery of the baby (dilation). Uterine contractions also assist in the actual delivery of the baby.

At menopause, as estrogen levels fall off, the endometrial lining no longer sheds and menstruation comes to an end.


Fallopian tubes
Besides the cervix, there are two other openings in the uterus leading to two fallopian tubes. These soft, limp tubes extend about five inches from the uterus to the ovaries. There are four components that make up the fallopian tubes: the first is the intramural component, which is the segment that goes through the uterine muscle. The second component is the isthmus , or the first part of the tube after exiting the uterus. The next component is the ampulla . This is where fertilization occurs. Finally, the fallopian tubes end at the fimbria , which are fringed and trumpet-shaped with minuscule feather-like tissue at the end which grasp eggs (ova) that are released from the ovaries.

From a reproductive standpoint, the fallopian tubes are designed to perform four related functions: connect the ovary and the uterus; transport sperm in the right direction (from the fallopian tube toward the uterus); provide a meeting place where conception happens; and, help propel the fertilized egg by producing gentle, continuous contractions that move the egg toward the uterus.


Ovaries (OH-ver-eez)
The fallopian tubes lead to the ovaries which are oval-shaped organs that secrete hormones and house eggs, or ova. Measuring about an inch and a half wide and an inch long, the two ovaries sit on either side of the uterus, attached to the uterus by a ligament.

The ovaries can be smooth, or during ovulation, they become marked by clusters of rounded bumps, or follicles , which house and nurture eggs. The number of eggs that are contained in the ovaries depends on the age of the woman. The highest number is actually found before a girl is born. While still in the mother's womb, a 20-week-old female fetus has approximately 7 million eggs. At birth, the number has decreased to 2 million. By the time a girl enters puberty, she has between 300,000 and 500,000 eggs. This decline in number is the process called atresia , a natural and continuous process that is uninterruptable. Only between four and five hundred will ripen into mature eggs during a lifetime.

During the first half of the menstrual cycle, the follicles are growing and secreting estrogen and the egg is undergoing the maturation process. The egg continues to grow until it is released from the follicle and picked up by the fimbria and transported to the fallopian tube. Meanwhile, the empty follicle cells coalesce into a yellow mass called the corpus luteum , which secretes estrogen and progesterone. Progesterone is produced to support the gestation (or nurturing) of an egg in the event that it is fertilized and implanted in the uterus. If pregnancy does not occur, the estrogen and progesterone secretion from the corpus luteum will cease 11-16 days after ovulation. Without the support of the hormones, the endometrium will shed. Over time, the corpus luteum becomes incorporated back into the ovarian tissue.

As menopause approaches, ovarian estrogen begins to decline. Estrogen levels become very low once there are no remaining follicles in the ovaries. Without ovarian production of estrogen and progesterone, the endometrium is not stimulated to grow and shed. This eventually leads to the end of menstruation.

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