From Encyclopedia of Sex and Sexuality
One of the woman’s principal Female Sex Organs, the vagina is a hollow muscular tube extending from the Cervix at the entrance to the Uterus to the external opening between the woman’s labia (lips). It is the organ into which the erect male Penis is inserted during Sexual Intercourse and it receives the sperm ejaculated by the male’s Orgasm. Unless a barrier such as a condom or diaphragm prevents sperm from reaching the inner vagina, it is there they begin their voyage into the woman’s body—from the vagina into the uterus and one of the Fallopian Tubes, where fertilization of the woman’s egg may take place.
An adult woman’s vagina is about 3 to 4 inches (8 to 10 centimeters) long when it is at rest. Its walls lie against each other but it is extremely flexible. During intercourse the vagina expands to receive the male’s penis and during childbirth it expands to many times its normal size, becoming the birth canal through which the child about to be born passes from the uterus to the world outside.
The walls of the vagina have three layers: the vaginal lining (or mucosa), the muscular layer, and a layer of connective tissue (the advunticia). The mucosa is very thick and forms several longitudinal and circular folds. Its tissue changes in response to hormonal changes in the menstrual cycle, due primarily to estrogen levels. Under the stimulus of estrogen, glycogen (a storage form of carbohydrates) synthesis and accumulation take place. The bacteria normally present in the vagina metabolize the glycogen and form lactic acid, responsible for the vagina’s usually acidic environment. During medical treatment involving antibiotics, these bacteria are suppressed and the acidic vaginal environment is lost, allowing fungal or bacterial growth.
The vaginal mucosa is lubricated by the transudation (passage) of fluids through the vagina’s walls, especially during sexual excitation. Mucus from the cervix lubricates the vagina as well.
The muscular layer contains muscle bundles running longitudinally and circumferentially. These muscles are rich in blood and during sexual arousal vasocongestion of the vaginal wall occurs. Muscles of the pelvic floor blend with the vaginal wall at the middle (mid-length) of the vagina. These muscles are responsible for keeping the vagina elevated, tight, and firm. Repeated exercise of these muscles prevents their loosening (see Kegel Exercises).
In prepubertal girls who lack adult hormonal development, the vaginal wall is thin, glycogen deficient, and predisposed to bacterial infections. During the reproductive years, poor hygiene, contraceptive agents, sexual intercourse, and use of tight, nonabsorbent underwear may cause vulvo-vaginal infections. In postmenopausal women, due to low estrogen levels, the cells of the vaginal mucosa lose glycogen and vaginal acidity declines, resulting in vaginal tissue that is fragile and susceptible to trauma and infection. The hormone replacement therapy taken by many women after menopause reverses the thinning of the vaginal mucosa and restores the acidic environment of the vagina.
The lack of estrogen common in postmenopausal women affects the pelvic blood supply and soft tissue of the vagina, compromises the elasticity of pelvic structures, and may result in internal slippage of the uterus, bladder, rectum, and intestine. The most common symptom of vulvo-vaginal disease is an intense itching, which can be caused by either infection, or rarely, by precancerous or cancerous diseases. Any such complaint should be carefully investigated by a physician.