Internal Genital Organs

The internal genital organs form a pathway (the genital tract). This pathway consists of the following:

the vagina (part of the birth canal), where sperm are deposited and from which a baby can emerge
the uterus, where an embryo can develop into a fetus
the fallopian tubes (oviducts), where a sperm can fertilize an egg
the ovaries, which produce and release eggs.

Sperm can travel up the tract, and eggs down the tract.

See the figure Internal Female Genital Organs.

At the beginning of the tract, just inside the vaginal opening is the hymen, a mucous membrane. In virgins, the hymen usually encircles the opening like a tight ring, but it may completely cover the opening. The hymen helps protect the genital tract but is not necessary for health. It may tear at the first attempt at sexual intercourse, or it may be so soft and pliable that no tearing occurs. The hymen may also be torn during exercise or insertion of a tampon or diaphragm. Tearing usually causes slight bleeding. In women who have had intercourse, the hymen may be unnoticeable or may form small tags of tissue around the vaginal opening.

The vagina is a narrow, muscular but elastic organ about 4 to 5 inches long in an adult woman. It connects the external genital organs with the uterus. The vagina is the main female organ of sexual intercourse, into which the penis is inserted. It is the passageway for sperm to the egg and for menstrual bleeding or a baby to the outside.

Usually, there is no space inside the vagina unless it is stretched open--for example, during an examination or sexual intercourse. The lower third of the vagina is surrounded by elastic muscles that control the diameter of its opening. These muscles contract rhythmically during sexual intercourse and can be toned by Kegel exercises.

The vagina is lined with a mucous membrane, kept moist by fluids oozing from cells on its surface and by secretions from glands in the cervix (the lower part of the uterus). These fluids may pass to the outside as a vaginal discharge, which is normal. During a woman's reproductive years, the lining of the vagina has folds and wrinkles. Before puberty and after menopause (if the woman is not taking estrogen), the lining is smooth.

The uterus is a thick-walled, muscular, pear-shaped organ located in the middle of the pelvis, behind the bladder, and in front of the rectum. The uterus is anchored in position by several ligaments. The main function of the uterus is to sustain a developing fetus. The uterus consists of the cervix and the main body (the corpus).

The cervix, the lower part of the uterus, protrudes into the upper end of the vagina and can be seen during a pelvic examination. Like the vagina, the cervix is lined with a mucous membrane, but the mucous membrane of the cervix is smooth.

Sperm can enter and menstrual blood can exit the uterus through a channel in the cervix. The channel is narrow. During labor, the channel widens to let the baby through. The cervix is usually a good barrier against bacteria, except during the menstrual period, around the time an egg is released by the ovaries (ovulation), or during labor. Bacteria can enter the uterus through the cervix during sexual intercourse. Unlike the bacteria that cause sexually transmitted diseases (see Section 17, Chapter 200), bacteria normally found in the vagina rarely cause problems.

The channel through the cervix is lined with glands that secrete mucus. This mucus is thick and impenetrable to sperm until just before ovulation. At ovulation, the consistency of the mucus changes so that sperm can swim through it and fertilization can occur. At this time, the mucus-secreting glands of the cervix can store live sperm for 2 or 3 days. These sperm can later move up through the corpus and into the fallopian tubes to fertilize an egg. Thus, intercourse 1 or 2 days before ovulation can lead to pregnancy. For some women, the time between a menstrual period and ovulation varies from month to month. Consequently, pregnancy can occur at different times during a menstrual cycle.

The corpus of the uterus, which is highly muscular, can stretch to accommodate a growing fetus. Its muscular walls contract during labor to push the baby out through the cervix and the vagina. During the reproductive years, the corpus is twice as long as the cervix. After menopause, the reverse is true.

As part of a woman's reproductive cycle (which usually lasts about a month), the lining of the corpus (endometrium) thickens. If the woman does not become pregnant during that cycle, most of the endometrium is shed and bleeding occurs, resulting in the menstrual period.

The two fallopian tubes, which are about 2 to 3 inches long, extend from the upper edges of the uterus toward the ovaries. The tubes do not connect with the ovaries. Instead, the end of each tube flares into a funnel shape with fingerlike extensions (fimbriae). When an egg is released from an ovary, the fimbriae guide the egg into the relatively large opening of a fallopian tube.

The fallopian tubes are lined with tiny hairlike projections (cilia). The cilia and the muscles in the tube's wall propel an egg downward through the tube to the uterus. The egg may be fertilized by a sperm in the fallopian tube (see Section 22, Chapter 257).

The ovaries are usually pearl-colored, oblong, and somewhat smaller than a chicken egg. They are attached to the uterus by ligaments. In addition to producing female sex hormones (estrogen and progesterone) and male sex hormones, the ovaries produce and release eggs. The developing egg cells (oocytes) are contained in fluid-filled cavities (follicles) in the wall of the ovaries. Each follicle contains one oocyte.

See the sidebar How Many Eggs?