|Schematic frontal view of female anatomy|
|1: Fallopian tube, 2: bladder, 3: pubic bone, 4: vagina–G-Spot, 5: clitoris, 6: urethra, 7: vagina, 8: ovary, 9: sigmoid colon, 10: uterus, 11: fornix, 12: cervix, 13: rectum, 14: anus|
|Gray's||subject #268 1259|
|Artery||Vaginal artery and uterine artery|
The cervix (Latin for neck) is the narrow neck-like passage that forms the lower end of the uterus (womb), as the neck of the uterus which joins to the upper part of the vagina. The cervix forms as a cavity which connects the uterus to the vagina and is also termed the cervical canal. It is between two and three centimetres long. The shape of the cervix can change, particularly after normal childbirth.
The cervix is structured as a cavity that is also referred to as the cervical canal and is continuous with the rest of the uterus and the vagina. It has a narrow opening (orifice) at each end; the upper internal orifice opens into the body of the uterus and the lower external orifice opens out to the vagina. (These can also be referred to as the internal os and the external os, of the uterus). The portion of the cervix that extends beyond the external orifice to join with the vagina is called the ectocervix. The cervix is about 2–3 cm long. On average, the ectocervix is 3 cm long and 2.5 cm wide. It has a convex, elliptical surface and is divided into anterior and posterior labia (lip–shaped structures). The size and shape of the external os and the ectocervix can vary according to age, hormonal state, and whether natural, or normal childbirth has taken place. Where no natural childbirth has taken place, the external orifice appears as a small, circular opening of about 8mm. In women who have given birth naturally (vaginal childbirth), the ectocervix appears bulkier and the external os appears a lot wider. The cervical canal (also called canal of cervix or endocervical canal) varies greatly in length and width. Flattened anterior to posterior, the endocervical canal measures 7 to 8 mm at its widest in pre-menopausal adults.
After menstruation and directly under the influence of estrogen, the cervix undergoes a series of changes in position and texture. During most of the menstrual cycle, the cervix remains firm, and is positioned low and closed. However, with approaching ovulation, the cervix becomes softer, and rises to open, in response to the higher levels of oestrogen present. These changes are accompanied by the production of cervical mucus, supportive of the survival and movement of sperm.
The lymphatic drainage of the cervix is along the uterine arteries and cardinal ligaments to the parametrial, external iliac vein, internal iliac vein, and obturator and presacral lymph nodes. From these pelvic lymph nodes, drainage proceeds to the paraaortic lymph nodes. In some instances, the lymphatics drain directly to the paraaortic nodes.
Nerves supplying the cervix pass through uterosacral ligaments.
As a component of the female reproductive system, the cervix is derived from the two paramesonephric ducts, which develop around the sixth week of embryogenesis. During development, the outer parts of the two ducts fuse, forming a single urogenital canal that will become the vagina, cervix and uterus.
The epithelium of the cervix varies along its parts. The transformation zone (also referred to as the squamocolumnar junction) is adjacent to the borders of the ectocervix and the endocervix of the canal; and refers to the area where the change occurs between the squamous epithelium lining the ectocervix to the columnar epithelium that lines the endocervix. The squamous epithelium of the ectocervix is without keratin, and is continuous with the adjacent vagina. Underlying both types of epithelium is a tough layer of collagen. The transformation zone undergoes physiological changes at different times. At puberty, the columnar epithelium extends outwards, and cover portions of the ectocervix. The cervix also undergoes changes related to the menstrual cycle and pregnancy[clarification needed]. Additionally, the transformation zone may retreat post-menopause, such that the ectocervix is again covered with stratified squamous epithelium. Also, when the endocervix is exposed to the harsh acidic environment of the vagina it undergoes metaplasia to squamous epithelium.
During menstruation, the cervix stretches open slightly to allow the endometrium to be shed. This stretching is believed to be part of the cramping pain that many women experience. Evidence for this is given by the fact that some women's cramps subside or disappear after their first vaginal birth because the cervical opening has widened.
Cervical mucus is produced by glands in the endocervix and is composed 90% of water. Depending on the water content which varies during the menstrual cycle, the mucus functions as a barrier or a transport medium to spermatozoa: during the proliferative phase, the mucus is thin and serous to allow sperm to enter the uterus while during the secretory phase, the mucus is thick to prevent sperm from interfering with the already fertilized egg. Thick mucus also prevents pathogens from interfering with a nascent pregnancy. Cervical mucus also contains electrolytes (calcium, sodium and potassium), organic components such as glucose, amino acids and soluble proteins.
Cervical mucus contains trace elements including zinc, copper, iron, mangenese and selenium, the levels of which vary dependant on cyclical hormone variation during different phases of the menstrual cycle. Various enzymes have been identified in human cervical mucus. Glycerol is a natural ingredient of human cervical fluid. Studies have shown that the amount of glycerol in cervical fluid increases during sexual excitement. This increase in glycerol has been postulated to be responsible for the lubricating quality of this fertile cervical fluid and may be biologically relevant during the early phase of reproductive events.
After a menstrual period ends, the external os is blocked by mucus that is thick and acidic. This "infertile" mucus blocks spermatozoa from entering the uterus. For several days around the time of ovulation, "fertile" types of mucus are produced; they have a higher water content, and are less acidic and higher in electrolytes. These electrolytes cause the 'ferning' pattern that can be observed in drying mucus under low magnification; as the mucus dries, the salts crystallize, resembling the leaves of a fern.
Some methods of fertility awareness such as the Creighton Model and the Billings Method involve estimating a woman's periods of fertility and infertility by observing changes in her body. Among these changes are several involving the quality of her cervical mucus: the sensation it causes at the vulva, its elasticity (Spinnbarkeit), its transparency, and the presence of ferning.
Most methods of hormonal contraception work primarily by preventing ovulation, but their effectiveness is increased because they prevent the fertile types of cervical mucus from being produced. Conversely, methods of thinning the mucus may help to achieve pregnancy. One suggested method is to take guaifenesin in the few days before ovulation.
During pregnancy, the cervix is blocked by a special antibacterial mucosal plug, which prevents infection, somewhat similar to its state during the infertile portion of the menstrual cycle. The mucus plug comes out as the cervix dilates in labor or shortly before.
A main function of the cervix is its role in childbirth. The cervix is shaped to guide the foetus into position for birth. As full term approaches, the foetus moves to align itself into a headfirst position where its head becomes rested on and supported by the cervix. This support begins to give way as the cervix starts to dilate when the uterus begins its contractions. The foetus has also turned itself to face away from the mother towards the rectum. During childbirth, the cervix must dilate to a diameter of more than 10 cm to accommodate the head of the fetus as it is pushed from the uterus to the vagina. In becoming wider, the cervix also becomes shorter,(effacement). When this happens an area between the cervix and the vagina becomes exposed and is known as the birth canal. Along with other factors, cervical dilation is used to divide delivery into stages. Generally, the active first stage of labour is defined with a cervical dilation of more than 3–4 cm, with the second phase of labor defined when the cervix is dilated to more than 10 cm, regarded as its fullest dilation.
Cervix as reservoir
The cervix acts as a reservoir of sperms after coitus. This one statement of fact counters a weakly founded theory put forward that the convulsing cervix, during orgasm, draws semen into the uterus, (as a function of the cervix). The theory was given the name of the upsuck theory and was generally accepted for some years. In 2005, science historian Elisabeth Lloyd, author of The Case of the Female Orgasm queried the logic of this theory and the methodology used to support it. Her findings were that over 70% of the data came from one woman. Masters and Johnson in November 2008 set out to disprove this upsuck theory and it is accepted that they succeeded.
Cervical incompetence is a condition in which there is shortening of the cervix due to dilation and thinning, before term pregnancy, and is the strongest predictor of preterm birth. Some treatments to prevent the spread of cervical cancer, such as LEEP, cold-knife conization, or cryotherapy may shorten the cervix.
Cervical cancer nearly always implicates papillomavirus infection. Vaccines against this virus, greatly reduce the chances of developing cervical cancer, especially if administered before a female becomes sexually active. Potentially pre-cancerous changes in the cervix can be detected by a Pap test, (a cervical smear) in which epithelial cells are scraped from the surface of the cervix and examined under a microscope. Most cervical cancer is detected in this way, and without any other symptoms. When symptoms occur, they may include vaginal bleeding, discharge, or discomfort. With appropriate treatment of detected abnormalities, cervical cancer can be prevented. Most women who develop cervical cancer have never had a Pap test, or have not had one within the last ten years. Worldwide, cervical cancer is the fifth most deadly cancer in women. It affects about 16 per 100,000 women per year and kills about 9 per 100,000 per year. Pap test screening has greatly reduced cervical cancer incidence and mortality in nations with regular screening programs.
Metaplasia, which can take place in the transformation zone, can cause the glands underlying the columnar epithelia to become blocked, leading to the formation of Nabothian cysts. So that metaplasia increases the risk of cancer forming in the transformation zone, which is the most common area for cervical cancer to occur.
Labour difficulties can necessitate varying techniques of intervention. Reporting systems such as the Bishop score, (also known as a cervix score), used to recommend interventions, also incorporate cervical dilation and effacement as a factor. The time taken for the cervix to dilate and efface in labour may affect a decision to intervene, such as a forceps delivery, induction, or Caesarean section (C-section). A history of past vaginal deliveries, parity, is a strong factor in influencing how rapidly the cervix ought to be able to dilate in labour.
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