Genital canal in females. Together with the cavity of the uterus, it forms the birth canal. In most virgins, its external opening is partially closed by a thin fold of tissue (hymen), which has various forms, not all of which significantly obstruct the opening. The vagina's lining thickens and thins during the menstrual cycle (see menstruation) in response to estrogen from the ovaries, being thickest and most elastic during ovulation and pregnancy. Its normally slightly acid environment discourages disease-causing microorganisms. Thick, elastic muscle walls accommodate movement of the penis during intercourse and passage of a child during delivery. A mucuslike fluid seeps through them for lubrication during sexual arousal. Vaginal disorders include bacterial and fungal infections (e.g., sexually transmitted diseases, candida), vaginitis, sores (see ulcer), and prolapse.
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