|Schematic frontal view of female anatomy|
|1: Fallopian tube, 2: bladder, 3: pubic bone, 4: posterior fornix erogenous zone, 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 (from the Latin cervix uteri, meaning "neck of the womb") is the inferior, narrow portion of the uterus where it joins with the superior end of the vagina. It is cylindrical or conical in shape and protrudes through the upper anterior vaginal wall. Approximately half its length is visible with appropriate medical equipment; the remainder lies above the vagina beyond view. The cervix has an opening to allow semen and menstrual fluid to move through.
The cervix is an anatomical structure that is continuous with the vagina and the uterus. The external opening of the cervix (Latin: external os) faces towards the vagina, and the internal opening (Latin: internal os) faces towards the uterus. The cervical canal is the portion between these two openings, and the portion of the cervix within the vagina outside of the external opening is known as the ectocervix. :415 The cervix is about 2-3cm long.  In greater detail:
- Ectocervix The portion projecting into the vagina is referred to as the portio vaginalis or ectocervix.  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 lips.
- External os The ectocervix's opening is called the external os. The size and shape of the external os and the ectocervix varies widely with age, hormonal state, and whether the woman has had a vaginal birth. In women who have not had a vaginal birth the external os appears as a small, circular opening of about 8mm.  In women who have had a vaginal birth, the ectocervix appears bulkier and the external os appears wider, more slit-like and gaping.
- Cervical canal The passageway between the external os and the uterine cavity is referred to as the endocervical canal.  It varies greatly in length and width, along with the cervix overall. Flattened anterior to posterior, the endocervical canal measures 7 to 8 mm at its widest in reproductive-aged women.
- Internal os The endocervical canal terminates at the internal os which is the opening of the cervix inside the uterine cavity. 
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, like the tip of the nose, and is positioned low and closed. However, as a woman approaches ovulation, the cervix becomes softer, more similar to the lips, and rises and opens in response to the high levels of estrogen present at ovulation. These changes, accompanied by the production of fertile types of cervical mucus, support 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 then proceeds to the paraaortic lymph nodes. In some women, the lymphatics drain directly to the paraaortic nodes.
Nerves supplying the cervix pass through uterosacral ligaments. :1-16
The epithelium of the cervix is varied. The ectocervix is composed of of stratified squamous epithelium without keratin, which is continuous with the adjacent vagina. The endocervix is composed of simple columnar epithelium. Underlying the epithelium is a tough layer of collagen. :376
The area adjacent to the border of the endocervix and ectocervix is known as the transformation zone, or squamocolumnar junction. This point represents a change between the types of epithelia that line the cervix, with the area outside of the canal having stratified squamous epithelia, and the area within the canal having colummnar epithelia. :1-16 The transformation zone undergoes numerous physiological changes times during a woman's life. At puberty, the columnar epithelium extends outwards, and cover portions of the ectocervix. The cervix also experiences numerous 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. Additionally, when the endocervix is exposed to the harsh acidic environment of the vagina it undergoes metaplasia to squamous epithelium.  :350-351 Due to this metaplasia, glands underlying the columnar epithelia may become blocked, leading to the formation of Nabothian cysts. :227–228 This metaplasia also increases the risk of cancer in the area, and the transformation zone is the most common area for cervical cancer to occur. :350-351
- The cervix acts as a sphincter of the uterus. 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. During childbirth, contractions of the uterus will dilate the cervix up to 10 cm in diameter to allow the child to pass through. Cervical incompetence is where there is shortening of the cervix before term pregnancy, and is the strongest predictor of preterm birth. Some treatments to prevent cervical cancer, such as LEEP, cold-knife conization, or cryotherapy may shorten the cervix.
- Sperm upsuck and storage: During orgasm, the cervix convulses and the external os dilates. Robin Baker and Mark A. Bellis, both at the University of Manchester, first proposed that this behavior would tend to draw semen in the vagina into the uterus, increasing the likelihood of conception. This explanation has been called the "upsuck theory of female orgasm." Komisaruk, Whipple, and Beyer-Flores, in their book, The Science of Orgasm, claimed there is evidence in support of the upsuck theory. Science historian Elisabeth Lloyd, author of The Case of the Female Orgasm, questioned the logic of this theory and the quality of the experimental data used to back it, commenting in 2005: "[The upsuck theory] has been widely accepted in the community of scientists for the past 12 years... But unfortunately the evidence for it is really badly flawed[vague]. In one of their tables 73% of the data came from one woman. It's really quite shocking that for 12 years this research has been taught as "fact" all across the US, Canada and the UK." Cervix acts as a reservoir of sperms after coitus. :1-16
- Barrier: Cervix also acts as a barrier for infection, preventing ascending infection of uterus. :1-16
- Lubrication: Cervical mucus lubricates the vagina.
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.
Human papillomavirus (HPV) infection is a necessary factor in the development of nearly all cases of cervical cancer. HPV vaccines can reduce the chance of developing cervical cancer, if administered before initiation of sexual activity. Potentially pre-cancerous changes in the cervix can be detected by a Pap 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.  :276 With appropriate treatment of detected abnormalities, cervical cancer can be prevented. Most women who develop cervical cancer have never had a Pap smear, 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 smear screening has greatly reduced cervical cancer incidence and mortality in nations with regular screening programs.
During a vaginal delivery, the cervix must dilate to a diameter of more than 10cm to accomodate the head of the foetus as it is pushed from the uterus to the vagina. As well as becoming wider, the cervix also becomes shorter, known as effacement. Along with other factors, cervical dilation is used to divide delivery into stages. Generally, the active first stage of labor is defined with a cervical dilation of more than 3-4cm, with the second phase of labor defined when the cervix is dilated to more than 10cm, regarded as its fullest dilation.  :157-160
Reporting systems such as the Bishop score, used to recommend interventions, also incorporate cervical dilation and effacement as a factor. The time taken for the cervix to efface and dilate in labour may affect the likelihood of an intervention, such as a forceps delivery, induction, or Caesarean section is carried out. Whether a woman has had past vaginal deliveries is a strong factor influencing how rapidly the cervix is able to dilate in labor.:537-539
- Health and Wellness for Life
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