The vagina, or birth canal, is a sexual organ that is part of the female reproductive system and plays multiple roles. It facilitates menstruation, childbirth, and sexual intercourse. The vagina also plays a significant role in human female sexuality and sexual pleasure.
The vagina is a fibromuscular tube roughly 8 to 10 centimeters (3 to 4 inches) long across the posterior wall (rear), and about 7.5 centimeters (2.5 to 3 inches) long across the anterior wall (front), and extends from the vulva to the uterus. The vulva includes all of the external organs seen outside of the body, which includes the mons pubis (Latin for “pubic mound”), labia majora (outer lips), labia minora (inner lips), clitoris, and the external openings of the urethra and vagina.
When the labia minora are spread and meet the underside of the clitoris when the hood of the clitoris is pulled back, the urinary opening can be seen as a small dot or slit. This is the outer opening of the urethra, which is short (about four centimeters, or an inch and a half) thin tube that leads to the bladder. The vagina tilts posteriorly between the urethra and rectum, with the urethra bound to its anterior wall. If standing, the vaginal tube will point in an upward-backward direction to form a 45-degree angle with the uterus and an about 60-degree angle to the horizontal. However, the exact angles are variable depending on individual anatomy and with contents of the bladder and colon.
At the vulva, the vaginal orifice (or opening, or introitus) may be partially covered by a membrane formed by inward folding mucosa known as the hymen. Internally, the cervix (neck of the uterus) bulges through the anterior wall at the end of the vaginal canal. The wall of the vagina is thin and distensible, consisting of an outer adventitia, middle muscularis, and inner mucosa. Histologically, the vagina has a stratified squamous, non-keratinizing epithelium, with a thick and vascular lamina propria, and a muscular layer.
During menstruation, the vagina allows passage and expulsion of the endometrium, which is shed on a monthly basis at the end of the follicular phase. Throughout the menstrual cycle, the epithelium will undergo subtle changes, where the rate of desquamation will be higher during the progesterone phase than during the estrogen phase.
Vasculature of the vagina is primarily supplied by the vaginal artery, which is a branch of the anterior division of the internal iliac artery. Some of these arteries may be found on either side of the pelvis to and help to richly supply the vagina. The hymen is a structure that stretches across the vaginal opening and possesses one or more openings to allow for the discharge of menstrual fluid, and is usually ruptured to allow for sexual intercourse. Although, the hymen is commonly already ruptured by tampons, medical examinations, or strenuous exercise. The hymen is often crescent-shaped in children, but many shapes are possible.
During sexual intercourse (coitus) and sexual arousal, the vagina will expand in both length and width. While the upper two-thirds of the vagina expands and lengthens, the uterus will rise into the greater pelvis, with the cervix being elevated above the vaginal floor. At the lower end of the vagina are vaginal rugae, which are transverse friction ridges that can contribute to the sexual stimulation of both male and female partners during intercourse. While the vagina itself does not have glands, it is lubricated by serous fluid through transudation by mucus from the cervical glands found above it. In other words, there is leakage of fluid from the vasculature of the lamina propria. The vagina extends a bit beyond the cervix and forms blind-ended spaces called fornices that surround it. Vaginal secretions can also come from the uterus, or a minuscule amount from the Bartholin’s glands upon sexual arousal.
There is a higher concentration of nerve endings near the entrance of the vagina (lower third) that provides sexual pleasure when stimulated, and women may also derive pleasure from a feeling of closeness and fullness during sexual intercourse. The majority of nerve endings are derived from the autonomic nervous system, and sensory fibers arise from the pudendal nerve, while pain fibers arise from sacral nerve roots. However, the vagina as a whole lacks nerve endings, which commonly will hinder a woman's ability to receive sufficient sexual stimulation, including orgasm, solely from penetration of the vagina. Despite the scientific examinations of vaginal wall innervation showing no consistent or single areas with greater nerve ending densities, some women do have a greater density of nerve endings in the anterior vaginal wall that can lead to heightened sensitivity and pleasure in when the anterior wall is stimulated. Therefore, it is in the anterior wall that the erogenous zone is the G-spot (also known as the Gräfenberg spot) is found in women. In these examples, the outer third of the vagina near the opening will have more nerve endings, making it more sensitive to touch that the deeper and inner two-thirds of the vagina. Functionally, this is most likely the case so that childbirth will be significantly less painful, since increased nerve endings throughout would otherwise equate to more pain.
During childbirth, the vagina expands to allow the baby to pass through and provides pulsatile contractions to assist the downward passage. A healthy vagina of a woman of childbearing age is acidic with a pH range of 3.8 to 4.5 due to the degradation of glycogen to lactic acid by enzymes secreted by the Döderlein's bacillus, which is part of the normal flora of the vagina. The acidity functions to retard and prevent growth of various strains of pathogenic microbes. An increased acidity can be caused by bacterial overgrowth, as in bacterial vaginosis, trichomoniasis, or the rupturing of membranes during pregnancy. The ecosystem of a healthy vaginal microbiota consists majorly of the Lactobacillus species, but under stress or disturbances (hormonal or physical), changes can occur that upset the healthy balance of microorganisms that protect its host and result in undesirable outcomes, such as vaginosis or yeast infections.