the body and the part of the cervix that is above the vagina and is prolonged onto the posterior vaginal wall; from there it is folded back to the rectum. The middle layer of tissue, the myometrium, is muscular and makes up the majority of the organ. It is very firm and consists of densely packed, unstriped, smooth muscle fibres with blood vessels, lymphatics, and nerves. The muscle is arranged in three layers of fibres running in different directions. The outermost fibres are arranged longitudinally. Those of the middle layer, which is the thickest, run in all directions without any orderly arrangement. The innermost fibres are longitudinal and circular in their arrangement.
The innermost layer of tissue in the uterus is the mucous membrane, or endometrium. It lines the uterine cavity as far as the internal os, where it becomes continuous with the lining of the cervical canal. The endometrium contains numerous uterine glands that open into the uterine cavity and that are embedded in the cellular framework or stroma of the endometrium. Numerous blood vessels and lymphatic spaces are also present. The appearance of the endometrium varies considerably at the different stages in reproductive life. It begins to reach full development at puberty and thereafter exhibits dramatic changes during each menstrual cycle. It undergoes further changes before, during, and after pregnancy; during the menopause; and in old age. These changes are for the most part hormonally induced and controlled by the activity of the ovaries. The endometrium is divided into three layers, the stratum compactum, the stratum spongiosum, and the stratum basale, which are functionally distinct, but blend together. The stratum compactum is nearest to the uterine cavity and contains the lining cells and the necks of the uterine glands; its stroma is relatively dense. Superficial blood vessels lie beneath the lining cells. The stratum spongiosum is the large middle layer and it contains the main portions of uterine glands and accompanying blood vessels; the stromal cells are more loosely arranged and larger than in the stratum compactum. The stratum basale lies against the uterine muscle; it contains blood vessels and the bases of the uterine glands. Its stroma remains relatively unaltered during the menstrual cycle.
The blood supply to the vagina is derived from several adjacent vessels, a vaginal artery extends from the internal iliac artery, uterine, middle rectal, and internal pudendal arteries. As the vagina is not structurally related to the gastrointestinal system, the drainage avoids the liver and hence is not subject to first pass metabolism. The blood vessels are located close to the basal epithelial layer, but pores are present in the endothelial cells lining the capillaries through which an interchange of blood and vaginal fluid constiuents can occur. The channels in the basal, parabasal and intermediate levels change width depending upon hormonal levels, expanding to their maximum width during ovulatory and luteal phases. Molecules as large as albumin and immunoglobulins are able to pass from the blood to the lumen.
The nerve supply to the lower part of the vagina is from the pudendal nerve and from the inferior hypogastric and uterovaginal plexuses.
The uterus is supplied with blood by the two uterine arteries, which are branches of the internal iliac arteries, and by ovarian arteries, which connect with the ends of the uterine arteries and send branches to supply the uterus. The nerves to the uterus include the sympathetic nerve fibres, which produce contraction of uterine muscle and constriction of vessels, and parasympathetic (sacral) fibres, which inhibit muscle activity and cause dilation of blood vessels.
The vagina does not possess any glands except Bartholin's and Skene's glands, but these are not believed to contribute significantly to the vaginal fluid. The fluid consists mainly of cervical secretions and transudation from the blood vessels with desquamated vaginal cells and leucocytes. The fluid will also contain secretions from the endometrium and fallopian rubes. The amount and composition of fluid will vary with the menstrual cycle, but women of reproductive age produce about 1 g.h-1, but post-menopausal women produce only about half this much.
Cervical secretions originate both in the uterine cavity and the cervix and they flow constantly towards the vagina. During ovulation, the secretions are watery to facilitate the movement of spermatozoa. In response to increased levels of progesterone during the luteal phase of the menstrual cycle, or during pregnancy, the secretions become more viscous to prevent the passage of microorganisms and sperm into the body of the uterus.
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