Female Sex Organs
From Encyclopedia of Sex and Sexuality
Women’s sex organs are either internal, and can be neither seen nor felt outside the women’s bodies, or they are external. Most of the female sex organs—and specifically those involved in reproduction—are part of the internal genitalia.
 The External Sex Organs
The external and visible female sex organs are usually referred to collectively as the vulva. The vulva consists of the mons pubis (or mons veneris), the clitoris, the paired labia majora, the paired labia minora, and the hymen.
The mons pubis is the uppermost part of the vulva. It is the fatty area overlying the pubic bone and it is covered by coarse pubic hair. Immediately below the mons pubis is the clitoris, an erectile organ analogous to the male penis. It is approximately two-thirds of an inch long and less than one-third of an inch wide. The clitoris is exquisitely sensitive, becomes enlarged with sexual excitation, and is the most sexually responsive portion of the vulva. It is the primary source of women’s sensations of sexual pleasure.
Beginning on either side of the clitoris are the labia minora (minor and inner lips), which surround the vaginal opening, extending from the clitoris to just above the anus (opening of the rectum). Outside the labia minora are the labia majora (major and outer lips), which, like the mons pubis, are covered with thick pubic hair. Between the labia minora is the vaginal opening. In girls and women who have not had sexual relations or used tampons, the opening is partially covered by a thin membrane called the hymen. In rare instances the hymen may completely obstruct the vaginal opening and prevent menstrual periods. Between the vaginal opening and the clitoris is the opening of the urethra, the tube from the bladder that permits the passage of urine.
 The Internal Sex Organs
The tubular-shaped vagina extends from the vulva to the uterine cervix. The vagina is approximately four inches long, although it is enlarged during sexual relations and childbirth. It is located between the rectum and the urethra and bladder. If there is nothing in it, the vagina is collapsed like an empty balloon. The vagina functions as the internal female sexual organ, accepting the penis during sexual intercourse and receiving the male’s sperm when he ejaculates, and it also functions as a portion of the birth canal, providing passage for the baby from the uterus to outside the body.
The cervix, or mouth of the uterus, is actually the lowest portion of the uterus. In a nonpregnant woman it is approximately one inch long and protrudes about one-third of an inch into the upper end of the vagina. In the center of the cervix is the opening (or os) of the cervical canal. In women who have never been pregnant, the cervical opening is generally circular, while in women who have had one or more pregnancies it tends to be more elliptical and somewhat irregular in shape.
The uterus is approximately three inches long and is the size and approximate shape of a pear, with the stem of the pear representing the cervical opening. It is a solid, muscular organ that contains a triangular-shaped cavity (the endometrial cavity). As with the vagina, the endometrial cavity is collapsed when empty. The endometrial cavity is lined by a tissue, called the endometrium, which develops and is shed (see menstruation) in response to ovarian hormones.
The fallopian tubes are paired structures approximately four inches long and one-third of an inch in diameter. They are attached to the uterus at one end, with the other end open and lying adjacent to the ovaries. The open ends of the fallopian tubes (called the fimbriated ends) are covered with a fringe of tiny fingers (fimbria) that assist in picking up the egg from the ovary as it is released (ovulated) and in transporting it into the fallopian tube.
The fallopian tubes have a thick, muscular wall in the part of the tube closest to the uterus and a relatively thin wall in the part of the tube closest to the ovary. The opening of the fallopian tube is relatively large at the ovarian end (the ampullary portion) but narrows abruptly as it reaches the portion of the tube closest to the uterus (the isthmic portion). This narrow portion of the fallopian tube is connected to a similar narrow channel that passes through the muscular wall of the uterus and connects with the endometrial cavity.
The ovaries are paired structures measuring approximately one by one and one-half inches. They are a glistening white and their surface is generally irregular and convoluted. Depending upon the stage of the menstrual cycle, the ovaries may contain one or more developing follicles—the fluid-filled cysts in which an egg is developing—or a corpus luteum, the remains of a follicle from which the egg has been released. The corpus luteum is responsible for producting hormones to prepare the lining of the uterus for implantation of a fertilized egg and maintenance of the new pregnancy.
 Physiology and Function
Many of the female reproductive organs exhibit dramatic changes in structure and function as a result of cyclic hormonal activity and pregnancy. The most obvious physiological changes can be observed in the cervix, endometrium (lining of the uterine cavity), and ovaries.
The cervix contains a tubular opening (the cervical canal) lined by glands that produce a mucus-like secretion. The quantity and quality of the cervical mucus varies in response to the hormones produced by the ovaries. During the first half of the menstrual cycle, particularly as the time of ovulation approaches, the cervical mucus is abundant, clear, and watery. At this point, it is very receptive to sperm penetration and survival. When sperm are deposited in the upper vagina during sexual intercourse, the moving (motile) sperm rapidly swim into the cervical mucus, where they may maintain their motility and fertilizing ability for several days. Once ovulation occurs (at about the fourteenth day of a twenty-eight-day menstrual cycle), the cervical mucus changes dramatically in character. It becomes thick, semi-opaque, and viscous, and is virtually impenetrable to sperm. This change is brought about by the release of progesterone from the ovaries after ovulation. Similarly, a woman taking birth control pills will have cervical mucus which is impenetrable by sperm. This is one of the ways birth control pills prevent pregnancy.
During labor, as a result of the uterine contractions (labor pains), the cervix expands dramatically, with the diameter of the cervical canal reaching four inches at the end of the first stage of labor. When fully dilated, the cervix forms the upper part of the birth canal, with the vagina forming the lower part.
As noted, the uterine cavity is lined by endometrium, a tissue that grows and is shed in response to ovarian hormones. These changes in the endometrial lining, part of the menstrual cycle, prepare the uterus for the possible establishment of a pregnancy. In the first half of the menstrual cycle (the follicular, or proliferative phase), the lining of the uterus progressively grows and develops. In the second half of the cycle (the luteal, or secretory phase), the endometrium matures as a result of progesterone produced by the corpus luteum, and is prepared for implantation of a fertilized egg. As part of this process, the endometrium releases secretions into the uterine cavity that nourish the early embryo before it attaches to the wall of the uterus. If fertilization and implantation do not occur, the developed lining of the uterus is shed and results in menstrual bleeding. The cramps most women experience at this time are a result of the contractions of the muscular uterus, which aid in the expulsion of menstrual blood and endometrium from the uterine cavity. The same type of contractions during labor produces dilatation of the cervix and expulsion of the child through the vagina.
The outer portion of the fallopian tubes is the normal site of fertilization (egg/sperm interaction). The fimbria aid in the process of egg pickup and transportation into the ampullary region of the fallopian tube at the time of ovulation (egg release from the ovary). At the same time, if sperm have been deposited in the upper vagina, they enter the mucus in the cervical canal and make their way to the upper region of the fallopian tube. Although hundreds of millions of sperm may be deposited into the vagina, typically only a few hundred reach the site of fertilization. If fertilization is successful, the early embryo spends about three days in the upper region of the fallopian tube before it is transported into the uterine cavity. It then spends another three days floating in the uterine cavity before it attaches to the wall of the uterus.
The ovaries have two main functions: production of the woman’s eggs and of the female hormones estrogen and progesterone. The female hormones are necessary first, for development of the normal secondary sex characteristics (for example, breast development and growth and maturation of the reproductive organs) and second, for the preparation of the uterine lining in each menstrual cycle for possible attachment of a fertilized egg. If a fertilized egg successfully attaches to the uterine wall, the ovaries continue to produce these same hormones in large amounts to sustain the early pregnancy during the first eight to twelve weeks.
The ovaries respond to the pituitary hormones, follicle stimulating hormone (FSH), and luteinizing hormone (LH). A woman’s ovarian cycle is continuous and is generally thought of as beginning with the onset of menstrual bleeding. At that time the production of hormones from the ovaries is at its lowest level. In response to the low levels of ovarian hormones, the pituitary releases FSH which stimulates the growth of a follicle—a fluid-filled cyst containing an immature egg. As the follicle grows and develops, the egg matures and the cells surrounding the egg begin to make estrogen. The estrogen thus produced develops the lining of the uterus, results in production of favorable cervical mucus, and causes the pituitary to decrease the amount of FSH produced. When the ovary has produced a critical amount of estrogen for a certain period of time, the pituitary is triggered to release a burst of LH (the LH surge). The LH surge initiates the final maturation of the egg, preparing it for possible fertilization, causes the follicle to rupture and release the egg (ovulation), and finally, changes what had been the follicle into the corpus luteum. In addition to producing estrogen, the corpus luteum produces progestrone, necessary for the preparation of the lining of the uterus for possible implantation of a fertilized egg. If pregnancy occurs, the hormone human chorionic gonadotropin (hCG) is produced by the early pregnancy and causes the corpus luteum to maintain its production of estrogen and progesterone and thus sustains the early pregnancy, If there is no implantation (and hCG secretion) the corpus luteum spontaneously stops producing hormones. This leads to a shedding of the lining of the uterus and a menstrual period, beginning the entire cycle anew.