The vagina is a muscular canal passing inferiorly and anteriorly from the cervix of the uterus, through the pelvic and urogenital diaphragms, into the perineum. It lies posterior to the lower part of the bladder and the urethra, and anterior to the recto-uterine pouch, rectum, perineal body and anal canal. At its upper end, the lumen of the vagina is circular, except where the cervix projects into it. Here, the lumen is reduced to a shallow groove around the margins of the cervix.
The anterior part of the groove is termed the anterior fornix, the posterior part, the posterior fornix and there are two lateral fornices. The posterior fornix is separated from the recto-uterine pouch by the wall of the vagina and the peritoneum lining the pouch. The vagina opens at the vestibule of the vagina, located between two folds of skin and fascia, known as the labia minora. In young females, the vaginal orifice is narrowed by the presence of a membranous fold, the hymen. Later, the hymen becomes torn or stretched (e.g., during exercise, or intercourse). Lateral to the labium minora on each side is a second fold, known as the labium majora. The external genitalia in the female (labia majora and minora, clitoris, vestibule of the vagina) are collectively termed the vulva.
During sexual arousal, vasocongestion of the genitalia also results to fluid leaking through the wall of the vagina, causing it to become moist (vaginal lubrication). In addition, the upper end of the vagina dilates (tenting). The bulbs of the vestibule also become congested with blood and cause the vagina to tighten around the penis. The greater vestibular glands open into the vestibule and lower vagina. This keeps the vulva moist and lubricated during sexual arousal.
The vaginal wall consists of three layers: mucosa, muscularis and adventitia.
The mucosa layer consists of stratified squamous epithelium, similar to the exocervix of the cervix. Before menarche and after menopause, the epithelium of this layer is thin and fragile, with no superficial cells.
The muscularis layer consists of smooth muscle that is has poorly defined inner circular and outer longitudinal bundles.
The adventitia is a thin coat of dense connective tissue.