Vagina

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Vagina
Scheme female reproductive system-en.svg
Vagina in the female human reproductive system.
Vaginal opening - english description.jpg
Vulva with vaginal opening
Latin Vagina
Gray's p.1264
Artery superior part to uterine artery, middle and inferior parts to vaginal artery
Vein uterovaginal venous plexus, vaginal vein
Nerve Sympathetic: lumbar splanchnic plexus
Parasympathetic: pelvic splanchnic plexus
Lymph upper part to internal iliac lymph nodes, lower part to superficial inguinal lymph nodes
Precursor urogenital sinus and paramesonephric ducts
MeSH Vagina
Dorlands
/Elsevier
Vagina

The vagina is a fibromuscular elastic tubular tract (passage) which is a sex organ and mainly functions for the facilitation of sexual intercourse and childbirth. In mammals (especially primates), menstruation, which is the periodic discharge of blood and mucosal tissue (the endometrium) from the uterus and vagina, is another primary function of the vagina and usually signals fertility. The location and size of the vagina varies among species, and may vary in size within the same species. In humans, the vagina leads from the opening of the vulva to the uterus (womb), but the vaginal tract ends at the cervix.

Unlike mammalian males, who usually have only one genital orifice, mammalian females usually have two, the urethra and the vagina. The vaginal opening is much larger than the urethral opening, and both openings are protected by the labia.[1][2] During sexual arousal, vaginal moisture increases (vaginal lubrication) to reduce friction and allow for smoother penetration of the vagina during sexual activity. In the absence or presence of sufficient vaginal lubrication, the vagina's inner mould has a foldy texture, which can create friction for[citation needed] the penis during sexual intercourse and stimulate it toward ejaculation, enabling fertilization.

The female reproductive tract may also be termed the vagina with regard to amphibians, birds, reptiles and monotremes, but is generally known among zoologists as the cloaca in these species and may additionally serve as intestinal and urinary tracts.

Etymology and definition

The term vagina is from Latin vāgīna, literally "sheath" or "scabbard." It is often referred to as the birth canal in the context of pregnancy and childbirth, though the term is, by definition, the area between the outside of the vagina and the fully dilated uterus.[3]

The Latinate plural "vaginae" is rarely used in English. Colloquially, the word vagina is often used to refer to the vulva or to the female genitals in general.[4] However, by its dictionary and anatomical definitions, vagina refers exclusively to the specific internal structure.

Structure

General structure

The vagina is derived from the embryonic Mullerian duct.[5] The human vagina is an elastic muscular canal that extends from the cervix to the vulva.[6] It, along with the inside of the vulva, is reddish pink in color, and it connects the superficial vulva to the cervix of the deep uterus. The vagina, posterior to the urethra and bladder, reaches across the perineum superiorly and posteriorly toward the cervix; at approximately a 90 degree angle, the cervix protrudes into the vagina.[7]

The vagina's internal lining consists of stratified squamous epithelium.[8] Beneath this lining is a layer of smooth muscle, which may contract during vaginal intercourse and when giving birth. Beneath the muscle is a layer of connective tissue called adventitia.[9]

Regions and layers

Where the vaginal lumen surrounds the cervix of the uterus, it is divided into four regions of fornices (the vaginal fornix); these are the anterior, posterior, and the right lateral and left lateral (the lateral fornix).[6] Supporting the vagina are its upper third, middle third and lower third. The upper third concerns the levator ani muscles (transcervical, pubocervical) and the sacrocervical ligaments; these areas are also described as the cardinal ligaments laterally and uterosacral ligaments posterolaterally. The middle third of the vagina concerns the urogenital diaphragm (also described as the paracolpos and pelvic diaphragm). The lower third concerns the perineal body; it is sometimes described as containing the perineal body, pelvic diaphragm and urogenital diaphragm.[6][10]

Three layers, sometimes categorized as four layers, compose the vaginal walls.[7][8][9][11] With regard to three layers, the first layer is made up of a stratified squamous non-keratinized epithelium and is an underlying lamina propria of connective tissue (a layer of connective tissue that is highly vascular under the base area lining the epithelium). The second layer is the muscular layer, which is composed of smooth muscle fibers and situated longitudinally and circularly. The third layer is the adventitia, which is a dense connective tissue that blends with the fascia surrounding the area.[7][9]

When categorized as four layers, the vaginal walls may be described in significantly more detail: The first layer is an inner layer of non-keratinized squamous epithelium, which forms the folds or rugae and facilitate the vagina's ability to expand large enough for child birth. The rugae are a series of ridges produced by folding of the wall of the outer third of the vagina; they are transverse epithelial ridges and their function is to provide the vagina with increased surface area for extension and stretching. The second layer of the vagina is connective tissue, which contains blood vessels. The third layer is the muscle layer; it is an outer layer of longitudinal muscle, as well as the inner layer of circular muscle. The fourth layer is an outer layer of connective tissue; it is continuous with the other pelvic organs and is made up of blood and lymphatic vessels and fibers.[8][12]

The vaginal opening is at the caudal end of the vulva, behind the opening of the urethra. The upper one-fourth of the vagina is separated from the rectum by the recto-uterine pouch. Above the vagina is a cushion of fat called the mons pubis which surrounds the pubic bone and provides protective support during vaginal intercourse.

Lubrication

The Bartholin's glands, located near the vaginal opening and cervix, were originally thought to be the primary source for vaginal lubrication, but they provide only a few drops of mucus for vaginal lubrication;[13] the significant majority of vaginal lubrication is generally believed to be provided by plasma seepage from the vaginal walls, which is called vaginal transudation. Vaginal transudation, which initially forms as sweat-like droplets, is caused by vascular engorgement of the vagina (vasocongestion); this results in the pressure inside the capillaries increasing the transudation of plasma through the vaginal epithelium.[13][14][15]

Before and during ovulation, the cervix's mucus glands secrete different variations of mucus, which provides an alkaline environment in the vaginal canal that is favorable to the survival of sperm. "Vaginal lubrication typically decreases as women age, but this is a natural physical change that does not normally mean there is any physical or psychological problem. After menopause, the body produces less estrogen, which, unless compensated for with estrogen replacement therapy, causes the vaginal walls to thin out significantly."[16]

Variations and size

In its normal state, there is anatomical variation in the length of the vagina of a woman of child-bearing age. The length is approximately 7.5 cm (2.5 to 3 in) across the anterior wall (front), and 9 cm (3.5 in) long across the posterior wall (rear), making the posterior fornix deeper than the anterior.[8] During sexual arousal, the vagina expands in both length and width.

If a woman stands upright, the vaginal tube points in an upward-backward direction and forms an angle of slightly more than 45 degrees with the uterus and of about 60 degrees to horizon.[10]

Organs of the female reproductive system, with vagina seen in center

Hymen

The hymen is a membrane of tissue that surrounds or partially covers the external vaginal opening. The effects of vaginal intercourse and childbirth on the hymen are variable. If the hymen is sufficiently elastic, it may return to nearly its original condition. In other cases, there may be remnants (carunculae myrtiformes), or it may appear completely absent after repeated penetration.[17][18] Additionally, the hymen may be lacerated by disease, injury, medical examination, masturbation or physical exercise. For these reasons, it is not possible to definitively determine whether or not a girl or woman is a virgin by examining her hymen.[17][18][19][20]

Function

The vagina has several biological functions.

Uterine secretions

The vagina provides a path for menstrual blood and tissue to leave the body. In industrial societies, tampons, menstrual cups and sanitary napkins may be used to absorb or capture these fluids.

Sexual activity

The concentration of the nerve endings that lie close to the entrance of a woman's vagina (the lower third) can provide pleasurable sensations during sexual activity when stimulated in a way that a woman enjoys, and many women additionally derive pleasure from a feeling of closeness and fullness during vaginal penetration.[21][22] The vagina as a whole, however, lacks nerve endings, which generally hinders a woman's ability to receive sufficient sexual stimulation, including orgasm, solely from vaginal sexual activity.[21][23][24][25] The outer one-third of the vagina, especially near the opening, contains the majority of the vaginal nerve endings, making it more sensitive to touch than the inner two-thirds of the vaginal barrel.[21][24][25] This factor is considered to make the process of child birth significantly less painful, because an increased number of nerve endings means that there is an increased possibility for pain as well as pleasure.[24][26][27]

The clitoris contains an abundance of nerve endings and is a sex organ of multiplanar structure with a broad attachment to the pubic arch and via extensive supporting tissue to the mons pubis and labia, centrally attached to the urethra and vagina. Clitoral tissue forms a tissue cluster with the vagina. The tissue is more extensive in some women than in others, which may contribute to orgasms experienced vaginally.[23][25][28] During sexual arousal, and particularly the stimulation of the clitoris, the walls of the vagina lubricate. This reduces friction that can be caused by various sexual activities.[16]

With sexual arousal, the human vagina lengthens rapidly,[16] to an average of about 4 in.(10 cm), but can continue to lengthen in response to pressure. As the woman becomes fully aroused, the vagina tents (last ²⁄₃) expands in length and width, while the cervix retracts. The elastic walls of the vagina stretch or contract, with support from the pelvic muscles, to the size of the inserted penis or other object,[16] stimulating the penis and helping to cause the male to experience orgasm and ejaculation; this in turn enables fertilization.

Structure of the wall of vagina

An area in the vagina that may be an erogenous zone is the G-spot (also known as the Gräfenberg Spot); it is typically defined as being located at the anterior wall of the vagina, about five centimeters in from the entrance. Some women experience intense pleasure, and sometimes an orgasm, if this area is stimulated during sexual activity. A G-spot orgasm may be responsible for female ejaculation, leading some doctors and researchers to believe that G-spot pleasure comes from the Skene's glands, a female homologue of the prostate, rather than any particular spot on the vaginal wall.[29][30][31] Other researchers consider the connection between the Skene's glands and the G-spot to be weak.[32][33][34] They contend that the Skene's glands do not appear to have receptors for touch stimulation, and that there is no direct evidence for their involvement.[34] The G-spot's existence, and existence as a distinct structure, is still under dispute, as its location can vary from woman to woman, appears to be nonexistent in some women,[26][28][32] and it is hypothesized to be an extension of the clitoris and therefore the reason for orgasms experienced vaginally.[23][28][35]

Childbirth

During childbirth, the vagina provides the channel to deliver the newborn from the uterus to its independent life outside the body of the mother. During birth, the elasticity of the vagina allows it to stretch to many times its normal diameter.

Vaginal ecosystem and acidity

The vagina is a nutrient rich environment that harbors a unique and complex microflora. It is a dynamic ecosystem that undergoes long term changes, from neonate to puberty and from the reproductive period (menarche) to menopause. Moreover, under the influence of hormones, such as estrogen (estradiol), progesterone and follicle stimulating hormone (FSH), the vaginal ecosystem undergoes cyclic or periodic changes, i.e. during menses and pregnancy.[36][37][38][39]

One important variable parameter is the vaginal pH, which varies significantly during a woman‘s lifespan, from 7.0 in premenarchal girls, to 3.8-4.4 in women of reproductive age to 6.5-7.0 during menopause without hormone therapy and 4.5-5.0 with hormone replacement therapy.[36] Estrogen, glycogen and lactobacilli are important factors in this variation.[36]

Microflora and possible role in acidification

The vagina of a newborn is affected by the residual maternal estrogen still present. At birth, the vaginal mucosa is rich in glycogen and the vagina becomes colonized by lactic-acid producing bacteria, such as Lactobacillus spp., within the first day after birth.[40] These estrogen effects will slowly disappear by the fourth week after birth and the glycogen content will diminish. The vaginal pH becomes neutral or alkaline, likely due to the almost absence of lactic-acid producing microorganisms.[37][41] In premenarchal girls, the vaginal microflora is composed of anaerobic and aerobic cocci and rods with low loads of lactobacilli, Gardnerella vaginalis and Mobiluncus spp.[36][41] During puberty, the estrogen levels rise until they reach the concentrations found in adult women. The estrogen causes the vaginal epithelium to thicken and the intracellular glycogen production to rise,[42] which consequently causes a shift in the composition of the vaginal microflora.[41] At this moment, lactobacilli, probably from the rectum,[43][44] become the most important inhabitants colonizing the vaginal econiche, utilizing the available glycogen, which is converted to lactic acid, resulting in the acidification (pH < 4.4) of the vagina.[36][43][44][45] In addition, besides lactic acid, the principal vaginal acidifier, other organic acids, i.e. acetic and linoleic acid, are also normally found in vaginal fluid.[46]

Estrogen vs. lactobacilli

The source of the lactic acid has been subject to debate. Some studies suggest that the presence of estrogen and not Lactobacillus is primarily related to acidification of the vagina,[47] and even that lactobacilli cannot produce lactic acid from glycogen. The overall vaginal pH can be attributed to the total of estrogen mediated lactic acid production by the epithelial cells (producing only the L-isomer of lactate) and the lactic acid contribution by the endogenous vaginal bacteria (producing both D- and L-isomers of lactate).[46][48] Boskey et al.,[49] studying vaginal lactate production in 11 women, found that D-lactate (bacterial origin) in vaginal secretions ranged from 6% to 75%, with a mean of 55%. They concluded that these findings supported the role of Lactobacillus as the primary source for vaginal acidification, although the wide range found was not indicative for a dominant bacterial source of lactate in all women. Moreover, the midportion of the vagina has a higher pH than the vaginal fornix, although the lactobacilli concentration is uniform throughout the vaginal canal.[46] This is indicative for the fact that the vaginal mucosal metabolism may be more dominant in determining the final pH.[46]

However, in a pyrosequencing study by Ravel et al.[50] of the vaginal microflora of 396 asymptomatic North American women, representing four ethnic groups (white, black, Hispanic, and Asian), the higher median pH values in Hispanic (pH 5.0 ± 0.59) and black (pH 4.7 ± 1.04) women reflected a higher prevalence of communities not dominated by Lactobacillus spp. in these two ethnic groups, when compared with Asian (pH 4.4 ± 0.59) and white (pH 4.2 ± 0.3) women. Ravel et al.[50] found that the lowest pH values were associated with vaginal ecosystems dominated by L. iners and L. crispatus, and the highest pH values were associated with vaginal ecosystems not dominated by species of Lactobacillus.[50][51] The results of Ravel may be controversial for L.iners, which was strongly associated with Bacterial vaginosis-like vaginal microflora, which is characterized by alkaline pH.[52][53] Hummelen et al.[51] stated in their pyrosequencing study that L. crispatus is associated with a low pH, but when L. crispatus is not present, a large fraction of L. iners is required to predict a low pH.[51] Nevertheless, these results indicate that bacteria play an important role in the acidification of the vagina and more research, e.g. regarding the resource of glycogen, used to produce lactic acid and regarding the bacterial species that can metabolize glycogen, is needed to understand the various factors that govern vaginal pH.

Immune system

Next to the protective effects of pH and the endogenous vaginal microflora, pathogen colonization is also prevented by the local components of the innate and acquired immune systems.[54] The innate immune system recognizes pathogen-associated molecular patterns (PAMPs) on microbial pathogens and includes soluble factors (e.g. mannose-binding lectin, complement components, defensins, secretory leukocyte protease inhibitor (SLPI) and nitric oxide), membrane-associated components (e.g. Toll-like receptors) and tissue-associated phagocytes (macrophages and neutrophils).[46][54][55][56] The vaginal adaptive (acquired) immunity produces locally IgG and IgA, which will recognize and bind to specific antigens on microorganisms in the vagina.[57] For instance, an IgA response against G. vaginalis vaginolysin has been reported and a correlation between this response and BV has been shown.[58][59]

Finally, after the reproductive years, women enter the menopause, which is characterized by the cessation of menstruation because of the loss of follicular activity.[38] The circulating estrogen levels are low and the glycogen content is low to absent, resulting in a rise of the vaginal pH, and consequently the vagina is colonized by bacteria associated with intermediate/disturbed vaginal microflora.[37][38][60][61] It has been shown that estrogen therapy can augment the loads of lactobacilli, which in turn can diminish the increased risk for BV and urinary tract infections.[60][62] However, the absence of lactobacilli by itself is not related to an elevated prevalence of BV,[63] and estrogen replacement may merely potentiate the effect of lactobacilli present on the vaginal pH.[64]

The vaginal ecosystem is under the influence of many factors, endogenous (hormonal, innate immunity, ethnicity), as well as exogenous (behavioral factors, such as sexual activity, cigarette smoking, douching and antibiotic treatment in general),[65][66][67] that are determinative for the vaginal health. The endogenous susceptibility for disturbance of the vaginal microflora and for infections is determined by the genetic characteristics of each individual woman,[50][54] the specific composition of her vaginal microflora at a certain point in time and the genotypic differences between the vaginal lactobacilli present (at species and strain level).[68][69]

Clinical significance

An ultrasound showing the urinary bladder (1), uterus (2), and vagina (3)

The vagina is self-cleansing and therefore usually needs no special treatment. Doctors generally discourage the practice of douching.[70] Since a healthy vagina is colonized by a mutually symbiotic flora of microorganisms that protect its host from disease-causing microbes, any attempt to upset this balance may cause many undesirable outcomes, including but not limited to abnormal discharge and yeast infection.

The vagina is examined during gynecological exams, often using a speculum, which holds the vagina open for visual inspection of the cervix or taking of samples (see pap smear). Medical activities involving the vagina, including examinations, administration of medicine, and inspection of discharges, are also referred to as being per vaginam (or p.v.).[71]

pH

The healthy vagina of a woman of child-bearing age is acidic, with a pH normally ranging between 3.8 and 4.5.[72] This is due to the degradation of glycogen to the lactic acid by enzymes secreted by the Döderlein's bacillus. This is a normal commensal of the vagina. The acidity retards the growth of many strains of pathogenic microbes.[73]

An increased pH of the vagina (with a commonly used cut-off of pH 4.5 or higher), can be caused by bacterial overgrowth, as occurs in bacterial vaginosis and trichomoniasis, or rupture of membranes in pregnancy.[72]

Vaginismus

Vaginismus, not to be confused with vaginitis (an inflammation of the vagina), refers to an involuntary tightening of the vagina due to a conditioned reflex of the muscles in the area. It can affect any form of vaginal penetration, including sexual intercourse, insertion of tampons and menstrual cups, and the penetration involved in gynecological examinations. Various psychological and physical treatments are possible to help alleviate it.

Signs of disease

Lumps

The presence of unusual lumps in the wall or base of the vagina is always abnormal. The most common of these is Bartholin's cyst.[74] The cyst, which can feel like a pea, is formed by a blockage in glands which normally supply the opening of the vagina. This condition is easily treated with minor surgery or silver nitrate. Other less common causes of small lumps or vesicles are herpes simplex. They are usually multiple and very painful with a clear fluid leaving a crust. They may be associated with generalized swelling and are very tender. Lumps associated with cancer of the vaginal wall are very rare and the average age of onset is seventy years.[75] The most common form is squamous cell carcinoma, then cancer of the glands or adenocarcinoma and finally, and even more rarely, melanoma.

Discharge

Most vaginal discharges occur due to normal bodily functions such as menstruation or sexual arousal. Abnormal discharges, however, can indicate disease.

Normal vaginal discharges include blood or menses (from the uterus), the most common, and clear fluid either as a result of sexual arousal or secretions from the cervix. Other non-infective causes include dermatitis. Non-sexually transmitted discharges occur from bacterial vaginosis and thrush or candidiasis. The final group of discharges include the sexually transmitted diseases gonorrhea, chlamydia, and trichomoniasis. The discharge from thrush is slightly pungent and white, that from trichomoniasis more foul and greenish, and that from foreign bodies resembling the discharge of gonorrhea, greyish or yellow and purulent (pus-like).[76]

Sores

All sores involve a breakdown in the walls of the fine membrane of the vaginal wall. The most common of these are abrasions and small ulcers caused by trauma. While these can be inflicted during rape most are actually caused by excessive rubbing from clothing or improper insertion of a sanitary tampon. The typical ulcer or sore caused by syphilis is painless with raised edges. These are often undetected because they occur mostly inside the vagina. The sores of herpes which occur with vesicles are extremely tender and may cause such swelling that passing urine is difficult. In the developing world a group of parasitic diseases also cause vaginal ulceration such as Leishmaniasis but these are rarely encountered in the west. HIV/AIDS can be contracted through the vagina during intercourse but is not associated with any local vaginal or vulval disease.[77] All the above local vulvovaginal diseases are easily treated. Often only shame prevents patients from presenting for treatment.[78]

Route of administration

Intravaginal administration is a route of administration where the substance is applied to the inside of the vagina. Pharmacologically, it has the potential advantage to result in effects primarily in the vagina or nearby structures (such as the vaginal portion of cervix) with limited systemic adverse effects compared to other routes of administration.

Other animals

The vagina (along with the penis) is a general feature of animals in which the female is internally fertilised (other than by traumatic insemination). The shape of the vagina varies among different animals.

In placental mammals and marsupials, the vagina leads from the uterus to the exterior of the female body. Female marsupials have two lateral vaginas, which lead to separate uteri, but both open externally through the same orifice.[79] The urethra and vagina of the female spotted hyena exits through the clitoris, allowing the females to urinate, copulate and give birth through the clitoris.[80]

In birds, monotremes, and some reptiles, a homologous part of the oviduct leads from the shell gland to the cloaca.[citation needed] In some jawless fish, there is neither oviduct nor vagina and instead the egg travels directly through the body cavity (and is fertilised externally as in most fish and amphibians). In insects and other invertebrates, the vagina is part of the oviduct (see insect reproductive system).[citation needed]

See also

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External links

  • Media related to Vaginas at Wikimedia Commons
  • The dictionary definition of vagina at Wiktionary