Vagina is the Latin word for ‘sheath’, which makes an interesting comparison with the alternative word for a condom. From this same root, the term ‘invaginated’ means ensheathed, or turned in on itself, like the finger of an empty glove introverted into the space for the hand. Comparably, the vagina is a tube which can be considered as part of the outer surface of the body that has been introverted, forming a passage to and from the uterus and lined with an epithelium continuous with that of the vulva, which is in turn continuous with the skin.
The opening of the vagina at the vulva is initially partly covered by a thin fold of membrane known as the hymen. This normally has a central perforation, which is extended when torn by tampon use or during the first sexual intercourse. Once torn the hymen becomes an irregular ring of tissue round the vaginal opening. From the opening, the vagina extends upwards and backwards for about 10 cm and joins the cervix of the uterus at right angles so that the front wall is shorter than the back wall. The passage is roughly H-shaped in cross section, with the walls normally in close contact with each other. Around the epithelial cell lining the wall is formed of connective tissue and muscle. The muscle coat is rather thin but is nevertheless very strong. The wall is folded; this allows for expansion and stretching during sexual intercourse, and much more so in childbirth. The vagina is also richly supplied with blood vessels which become engorged during sexual arousal, assisting the opening of the passage.
While the vagina helps to support the uterus it also provides a receptacle for the penis, an entrance for sperm, and an exit for menstrual flow and for the products of conception. Because the vagina is in continuity with the inside of the uterus and in turn with the Fallopian tubes, which lead to the peritoneal cavity, an important function is the prevention of infection from the outside world. Living in the vagina are Döderlein's bacilli (named after a German gynaecologist). These are quite normal inhabitants and ferment the glycogen (which provides glucose) in the vaginal wall to lactic acid. Thus the vaginal environment is acidic, a pH of 3.8-4.5 compared with the normal pH of 7.4 found in most body tissues. This highly acidic environment helps to prevent the growth of many microorganisms which could otherwise invade the upper parts of the reproductive tract. As a consequence, when the number of these bacilli is reduced, or the availability of glucose changes, a woman becomes susceptible to vaginal infections which cause inflammation and discharge — vaginitis. For example, antibiotics can kill off the friendly bacteria in the vagina so that it becomes an ideal environment for the fungus, candida albicans, to flourish. Candidiasis (thrush) develops. Diabetes and sex hormones can alter the amount of glucose available in the vagina and so disturb the acidic environment. Thus oral contraceptives and pregnancy, for example, can make women prone to vaginal infections.
Sex hormones not only alter the acidic environment of the vagina but also alter the activity of the vaginal epithelium. When oestrogen levels are high the epithelial cells become keratinized or cornified, an effect reversed by progesterone. The same hormones can alter the proportions of the organic aliphatic acids produced by the vaginal wall which are responsible for the characteristic smells of normal vaginal secretions; in many animals, and maybe even in humans, the odours (pheromones) produced by vaginal secretions can increase the sexual attractiveness of the female around the time of ovulation.
— Saffron Whitehead
See also coitus; uterus; vulva.
The Oxford Companion to the Body. Copyright © 2001, 2003 by Oxford University Press. All rights reserved.
