The vagina is the muscular tube that connects the outer part of the vulva with the cervix of the uterus (which is almost a closed end). It's involved into sex, menstruation and giving birth. This tube is normally compressed, to avoid anything unwanted to get in, but it had amazing stretching capabilities. The vagina is normally 7-10cm long, but when you are aroused it expands is all directions, length- and widthwise. Normally the vagina is a strong muscular tube, thanks to the support of pelvic floor muscles (that's why we have to keep them in good shape), and it gets lubricated through its mucous wall. The fluids amounts gets higher when you are stimulated or aroused to avoid friction in case of penetration. In addition to this fluids, there are also the ones produced by the Bartholin's Glands, located behind the vaginal opening.